Internet-Draft Cryptographic MIME Header Protection September 2023
Gillmor, et al. Expires 16 March 2024 [Page]
Workgroup:
LAMPS Working Group
Internet-Draft:
draft-ietf-lamps-header-protection-16
Published:
Intended Status:
Standards Track
Expires:
Authors:
D. K. Gillmor
American Civil Liberties Union
B. Hoeneisen
pEp Foundation
A. Melnikov
Isode Ltd

Header Protection for Cryptographically Protected E-mail

Abstract

S/MIME version 3.1 introduced a mechanism to provide end-to-end cryptographic protection of e-mail message headers. However, few implementations generate messages using this mechanism, and several legacy implementations have revealed rendering or security issues when handling such a message.

This document updates the S/MIME specification to offer a different mechanism that provides the same cryptographic protections but with fewer downsides when handled by legacy clients. The header protection schemes described here are also applicable to messages with PGP/MIME cryptographic protections. Furthermore, this document offers more explicit guidance for clients when generating or handling e-mail messages with cryptographic protection of message headers.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at https://dkg.gitlab.io/lamps-header-protection/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-lamps-header-protection/.

Discussion of this document takes place on the LAMPS Working Group mailing list (mailto:[email protected]), which is archived at https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at https://www.ietf.org/mailman/listinfo/spasm/.

Source for this draft and an issue tracker can be found at https://gitlab.com/dkg/lamps-header-protection.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 16 March 2024.

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Table of Contents

1. Introduction

Privacy and security issues regarding e-mail Header Protection in S/MIME and PGP/MIME have been identified for some time. Most current implementations of cryptographically-protected electronic mail protect only the body of the message, which leaves significant room for attacks against otherwise-protected messages. For example, lack of header protection allows an attacker to substitute the message subject and/or author.

This document describes two different schemes for how message headers can be cryptographically protected, and provides guidance for implementers of MUAs that generate and interpret such messages. It uses the term "legacy MUA" to refer to a MUA that does not implement either scheme. This document takes particular care to ensure that messages interact reasonably well with legacy MUAs.

1.1. Two Schemes of Header Protection

This document addresses two different schemes for cryptographically protecting e-mail header sections or fields and provides guidance to implementers. One scheme ("Injected Headers") is more interoperable with legacy clients, and is mandatory to implement and interpret. The other, older scheme ("Wrapped Message") is described here to enable interpretation of archived messages.

The older scheme was first specified in S/MIME 3.1 and later, and involves wrapping a message/rfc822 or message/global MIME object with a Cryptographic Envelope around the message to protect. This document calls this scheme "Wrapped Message", and it is documented in more detail in [RFC8551]. Experience has shown that this form does not interact well with some legacy MUAs (see Section 1.2).

The more interoperable scheme of header protection is introduced in this document. In this scheme, the protected header fields are placed directly on the Cryptographic Payload, without using an intervening message/* MIME object. This document calls this scheme "Injected Headers", and it is documented in more detail in this document, in Section 2.3.4 and Section 2.5.3.

1.2. Problems with Wrapped Messages

Several legacy MUAs have revealed rendering issues when dealing with a message that uses the Wrapped Message header protection scheme.

In some cases, some mail user agents cannot render message/rfc822 message subparts at all, in violation of baseline MIME requirements as described on page 5 of [RFC2049]. This leaves all wrapped messages unreadable by any recipient using such a MUA.

In other cases, the user sees an attachment suggesting a forwarded e-mail message, which -- in fact -- contains the protected e-mail message that should be rendered directly. In most of these cases, the user can click on the attachment to view the protected message.

However, viewing the protected message as an attachment in isolation may strip it of any security indications, leaving the user unable to assess the cryptographic properties of the message. Worse, for encrypted messages, interacting with the protected message in isolation may leak contents of the cleartext, for example, if the reply is not also encrypted.

1.3. Problems with Injected Headers

A legacy MUA dealing with an encrypted message that has some header fields obscured using the Injected Headers scheme will not render the obscured header fields to the user at all. A workaround "legacy display" mechanism is provided in this document, which most legacy MUAs should render to the user, albeit not in the same location that the header fields would normally be rendered.

1.4. Motivation

Users generally do not understand the distinction between message body and message header. When an e-mail message has cryptographic protections that cover the message body, but not the header fields, several attacks become possible.

For example, a legacy signed message has a signature that covers the body but not the header fields. An attacker can therefore modify the header fields (including the Subject header) without invalidating the signature. Since most readers consider a message body in the context of the message's Subject header, the meaning of the message itself could change drastically (under the attacker's control) while still retaining the same cryptographic indicator of authenticity.

In another example, a legacy encrypted message has its body effectively hidden from an adversary that snoops on the message. But if the header fields are not also encrypted, significant information about the message (such as the message Subject) will leak to the inspecting adversary.

However, if the sending and receiving MUAs ensure that cryptographic protections cover the message headers as well as the message body, these attacks are defeated.

1.4.1. Backward Compatibility

If the sending MUA is unwilling to generate such a fully-protected message due to the potential for rendering, usability, deliverability, or security issues, these defenses cannot be realized.

The sender cannot know what MUA (or MUAs) the recipient will use to handle the message. Thus, an outbound message format that is backward-compatible with as many legacy implementations as possible is a more effective vehicle for providing the whole-message cryptographic protections described above.

This document aims for backward compatibility with legacy clients to the extent possible. In some cases, like when a user-visible header like the Subject is cryptographically hidden, the message cannot behave entirely identically to a legacy client. But accommodations are described here that ensure a rough semantic equivalence for legacy clients even in these cases.

1.4.2. Deliverability

A message with perfect cryptographic protections that cannot be delivered is less useful than a message with imperfect cryptographic protections that can be delivered. Senders want their messages to reach the intended recipients.

Given the current state of the Internet mail ecosystem, encrypted messages in particular cannot shield all of their header fields from visibility and still be guaranteed delivery to their intended recipient.

This document accounts for this concern by providing a mechanism (Section 2.3.2) that prioritizes initial deliverability (at the cost of some header leakage) while facilitating future message variants that shield more header metadata from casual inspection.

1.5. Other Protocols to Protect E-Mail Header Fields

A separate pair of protocols also provides some cryptographic protection for the e-mail message header integrity: DomainKeys Identified Mail (DKIM) [RFC6376], as used in combination with Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489]. This pair of protocols provides a domain-based reputation mechanism that can be used to mitigate some forms of unsolicited e-mail (spam).

However, the DKIM+DMARC suite provides cryptographic protection at a different scope than the mechanisms described here. In particular, the message integrity and authentication signals provided by DKIM+DMARC correspond to the domain name of the sending e-mail address, not the sending address itself, so the DKIM+DMARC suite does not provide end-to-end protection. DKIM and DMARC are typically applied to messages by (and interpreted by) mail transfer agents, not mail user agents. The mechanisms in this document are typically applied to messages by (and interpreted by) mail user agents.

Furthermore, the DKIM+DMARC suite only provides cryptographic integrity and authentication, not encryption. So cryptographic confidentiality is not available from that suite.

The DKIM+DMARC suite can be used on any message, including messages formed as described in this document. There should be no conflict between these schemes.

Though not strictly e-mail, similar protections have been in use on Usenet for signing and verification of message headers for years. See ([PGPCONTROL] and [PGPVERIFY-FORMAT] for more details. Like DKIM, these Usenet control protections offer only integrity and authentication, not encryption.

1.6. Applicability to PGP/MIME

This document describes end-to-end cryptographic protections for e-mail messages in reference to S/MIME ([RFC8551]).

Comparable end-to-end cryptographic protections can also be provided by PGP/MIME ([RFC3156]).

The mechanisms in this document should be applicable in the PGP/MIME protections as well as S/MIME protections, but analysis and implementation in this document focuses on S/MIME.

To the extent that any divergence from the mechanism described here is necessary for PGP/MIME, that divergence is out of scope for this document.

1.7. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.8. Terms

The following terms are defined for the scope of this document:

  • S/MIME: Secure/Multipurpose Internet Mail Extensions (see [RFC8551])
  • PGP/MIME: MIME Security with OpenPGP (see [RFC3156])

  • Message: An E-Mail Message consisting of Header Fields (collectively called "the Header Section of the message") followed, optionally, by a Body; see [RFC5322].

    Note: To avoid ambiguity, this document avoids using the terms "Header" or "Headers" in isolation, but instead always uses "Header Field" to refer to the individual field and "Header Section" to refer to the entire collection.

  • Header Field: A Header Field is a line beginning with a field name, followed by a colon (":"), followed by a field body (value), and terminated by CRLF; see [RFC5322].
  • Header Section: The Header Section is a sequence of lines of characters with special syntax as defined in [RFC5322]. The Header Section of a Message contains the Header Fields associated with the Message itself. The Header Section of a MIME part (that is, a subpart of a message) typically contains Header Fields associated with that particular MIME part.
  • Body: The Body is the part of a Message that follows the Header Section and is separated from the Header Section by an empty line (i.e., a line with nothing preceding the CRLF); see [RFC5322]. It is the (bottom) section of Message containing the payload of a Message. Typically, the Body consists of a (possibly multipart) MIME [RFC2045] construct.
  • Header Protection (HP): cryptographic protection of e-mail Header Sections (or parts of it) for signatures and/or encryption
  • Cryptographic Layer, Cryptographic Payload, Cryptographic Envelope, Cryptographic Summary, Structural Headers, Main Body Part, User-Facing Headers, and MUA are all used as defined in [I-D.ietf-lamps-e2e-mail-guidance]
  • Legacy MUA: a MUA that does not understand header protection as described in this document. A Legacy Non-Crypto MUA is incapable of doing any end-to-end cryptographic operations. A Legacy Crypto MUA is capable of doing cryptographic operations, but does not understand or generate messages with header protection.
  • Wrapped Message: The header protection scheme that uses the mechanism described in [RFC8551], where the Cryptographic Payload is a message/rfc822 or message/global MIME object. (see Section 2.2).
  • Injected Headers: The header protection scheme that uses the mechanism described in this document (see Section 2.1), where the protected header fields are inserted on the Cryptographic Payload directly.
  • Header Confidentiality Policy: a functional specification of which header fields should be obscured when composing an encrypted message with header protection. See Section 2.3.2.

1.9. Document Scope

This document describes sensible, simple behavior for a program that generates an e-mail message with standard end-to-end cryptographic protections, following the guidance in [I-D.ietf-lamps-e2e-mail-guidance]. An implementation conformant to this draft will produce messages that have cryptographic protection that covers the message's headers as well as its body.

1.9.1. In Scope

This document also describes sensible, simple behavior for a program that interprets such a message, in a way that can take advantage of these protections covering the header fields as well as the body.

The message generation guidance aims to minimize negative interactions with any legacy receiving client while providing actionable cryptographic properties for modern receiving clients.

In particular, this document focuses on two standard types of cryptographic protection that cover the entire message:

  • A cleartext message with a single signature, and
  • An encrypted message that contains a single cryptographic signature.

1.9.2. Out of Scope

The message composition guidance in this document (in Section 2.3.4) aims to provide minimal disruption for any legacy client that receives such a message. However, a legacy client by definition does not implement any of the guidance here. Therefore, the document does not attempt to provide guidance for legacy clients directly.

Furthermore, this document does not explicitly contemplate other variants of cryptographic message protections, including any of these:

  • Encrypted-only message (without a cryptographic signature)
  • Triple-wrapped message
  • Signed message with multiple signatures
  • Encrypted message with a cryptographic signature outside the encryption.

All such messages are out of scope of this document.

2. Specification

As mentioned in Section 1.1, this document describes two ways to provide end-to-end cryptographic protection for an e-mail message that includes all header fields known to the sender at message composition time.

A receiving MUA MUST be able to handle both header protection schemes, as described in Section 2.5.

A sending MUA MUST be able to generate the Injected Headers scheme (Section 2.3.4), and MAY generate the Wrapped Message scheme (Section 2.3.5).

2.1. Injected Headers Scheme

A message that uses the Injected Headers scheme has protected header fields in the header section of the Cryptographic Payload.

For an encrypted message that has at least one user-visible header field omitted or obscured outside of the Cryptographic Payload, those header fields MAY also be duplicated into decorative copies in the Main Body MIME part of the Cryptographic Payload itself. These decorative copies within the message are known as "legacy display elements".

Such a legacy display element can be useful for a legacy receiving MUA that doesn't yet understand how to interpret or display a cryptographically-protected confidential header. See Section 3.1 for more details about how the ecosystem could shift so that a sending MUA could avoid the need to generate any legacy display element.

Composing a message with the Injected Headers scheme is described in Section 2.3.4. Rendering such a message is described in Section 2.5.3.

2.2. Wrapped Message Scheme

A message that uses the Wrapped Message scheme has a Cryptographic Payload of a single message/rfc822 (or message/global) MIME object, which itself contains the original message (including the protected header section).

Composing a message with the Wrapped Message scheme is described in Section 2.3.5. Rendering such a message is described in Section 2.5.4.

2.3. Sending Side

This section describes the process an MUA should use to apply cryptographic protection to an e-mail message with header protection. We start by describing the legacy message composition process as a baseline.

2.3.1. Composing a Cryptographically-Protected Message Without Header Protection

Section 5.1 of [I-D.ietf-lamps-e2e-mail-guidance] describes the typical process for a legacy crypto MUA to apply cryptographic protections to an e-mail message. That guidance and terminology is replicated here for reference:

  • origbody: the traditional unprotected message body as a well-formed MIME tree (possibly just a single MIME leaf part). As a well-formed MIME tree, origbody already has structural headers (Content-*) present.
  • origheaders: the intended non-structural headers for the message, represented here as a list of (h,v) pairs, where h is a header field name and v is the associated value. Note that these are header fields that the MUA intends to be visible to the recipient of the message. In particular, if the MUA uses the Bcc header during composition, but plans to omit it from the message (see section 3.6.3 of [RFC5322]), it will not be in origheaders.
  • crypto: The series of cryptographic protections to apply (for example, "sign with the secret key corresponding to X.509 certificate X, then encrypt to X.509 certificates X and Y"). This is a routine that accepts a MIME tree as input (the Cryptographic Payload), wraps the input in the appropriate Cryptographic Envelope, and returns the resultant MIME tree as output.

The algorithm returns a MIME object that is ready to be injected into the mail system:

  • Apply crypto to origbody, yielding MIME tree output

  • For each header name and value (h,v) in origheaders:

    • Add header h of output with value v
  • Return output

2.3.2. Header Confidentiality Policy

When composing an encrypted message with header protection, the composing MUA needs a Header Confidentiality Policy (HCP). In this document, we represent that Header Confidentiality Policy as a function hcp:

  • hcp(name, val_in) → val_out: this function takes a non-structural header field identified by name with initial value val_in as arguments, and returns a replacement header value val_out. If val_out is the special value null, it mean that the header field in question should be omitted from the set of header fields visible outside the Cryptographic Envelope.

Note that hcp is only applied to non-structural header fields. When composing a message, structural header fields are dealt with separately, as described in Section 2.3.4 and Section 2.3.5.

As an example, an MUA that obscures the Subject header field by replacing it with the literal string "[...]", hides all Cc'ed recipients, and does not offer confidentiality to any other header fields would be represented as (in pseudocode):

hcp_example(name, val_in) → val_out:
    if name is 'Subject':
        return '[...]'
    else if name is 'Cc':
        return null
    else:
        return val_in

Note that such a policy is only needed when the end-to-end protections include encryption (confidentiality). No comparable policy is needed for other end-to-end cryptographic protections (integrity and authenticity), as they are simply uniformly applied so that all header fields known by the sender have these protections.

This asymmetry is an unfortunate consequence of complexities in message delivery systems, some of which may reject, drop, or delay messages where all header fields are removed from the top-level MIME object.

This document does not mandate any particular Header Confidentiality Policy, though it offers guidance for MUA implementers in selecting one in Section 2.4. Future documents may recommend or mandate such a policy for an MUA with specific needs. Such a recommendation might be motivated by descriptions of metadata-derived attacks, or stem from research about message deliverability, or describe new signalling mechanisms, but these topics are out of scope for this document.

2.3.3. Definition of HP-Removed and HP-Obscured header fields

This document defines 2 new header fields used for conveying the effect of sender's Header Confidentiality Policy: HP-Removed and HP-Obscured. These header fields enable the MUA receiving an encrypted message to reliably identify whether the sending MUA intended to make a header field confidential (see Section 6.2.3).

An implementation that composes encrypted e-mail and hides any of the header fields as described in this document (for example, due to a non-null HCP) MUST include the appropriate HP-Removed or HP-Obscured headers in the Cryptographic Payload. These two MIME header fields should only ever appear directly within the header section of the Cryptographic Payload of a Cryptographic Envelope offering confidentiality. They MUST be ignored if they appear in other places.

HP-Removed includes a comma separated list of header field names that were omitted from the outer header when the message with header protection was generated. The HP-Removed header field can appear at most once in the header section of a Cryptographic Payload.

Each instance of HP-Obscured contains a header field name and the value that this header field was modified to in the outer header. The HP-Obscured header field can appear multiple times in the header section of a Cryptographic Payload.

If a header field name A doesn't appear in an HP-Obscured header field value, then the header field A was either removed (and thus would appear in the HP-Removed header field) or it was copied without any modifications to the outer header.

Syntax of these new header fields is defined using the following ABNF [RFC5234]:

hp-removed      =   "HP-Removed:" field-name-list CRLF

field-name-list =   [FWS] field-name
                    *([FWS] "," [FWS] field-name) [FWS]

hp-obscured     =   "HP-Obscured:" [FWS] field-name ": "
                    field-value CRLF

field-value     =   unstructured

2.3.4. Composing with "Injected Headers" Header Protection

The "Injected Headers" header protection scheme places the header fields to be protected directly on the Cryptographic Payload. Unlike in the "Wrapped Scheme" (see compose-wrapped-message), there is no wrapping of the message body in any additional message/* MIME part. This section describes how to generate such a message.

To compose a message using "Injected Headers" header protection, the composing MUA needs one additional input in addition to the Header Confidentiality Policy hcp defined in Section 2.3.2.

  • legacy: a boolean value, indicating whether any recipient of the message is believed to have a legacy client. If all recipients are known to implement this draft, legacy should be set to false. (How a MUA determines the value of legacy is out of scope for this document; an initial implementation can simply set it to true)

Enabling visibility of obscured header fields for decryption-capable legacy clients requires transforming a header list into a readable form and including it as a decorative "Legacy Display" element in specially-marked parts of the message. This document recommends two different mechanisms for such a decorative adjustment: one for a text/html Main Body part of the e-mail message, and one for a text/plain Main Body part. This document does not recommend adding a Legacy Display element to any other part.

Please see Section 7.1 of [I-D.ietf-lamps-e2e-mail-guidance] for guidance on identifying the parts of a message that are a Main Body Part.

To build such a message, we replace the algorithm described in Section 2.3.1 with a more sophisticated approach. The the algorithm for applying "Injected Headers" cryptographic protection to a message is as follows:

  • if crypto contains encryption, and legacy is true:

    • Create ldlist, an empty list of (header, value) pairs

    • For each header field name and value (h,v) in origheaders:

    • If ldlist is not empty:

      • Identify each leaf MIME part of origbody that represents the "main body" of the message.

      • For each "Main Body Part" bodypart of type text/plain or text/html:

        • Insert Legacy Display element header list ldlist into the content of bodypart (see Section 2.3.4.1 for text/plain and Section 2.3.4.2 for text/html)
        • Add Content-Type parameter hp-legacy-display with value 1 to bodypart

  • For each header field name and value (h,v) in origheaders:

    • Add header field h to MIME part origbody with value v
  • Set the protected-headers parameter on the Content-Type of origbody to v1

  • If crypto contains encryption:

    • Create new empty list of header field names and values newh
    • Let hpr be an empty comma-separated list of header field names

    • For header field name and value (h,v) in origheaders:

      • Let newval be hcp(h,v)

      • If newval is null:

        • Add the value h to hpr

      • Else (if newval is not null):

        • Add (h,newval) to newh

        • If newval is not v:

          • Let string record be the concatenation of h, a literal ": " (colon followed by space), and newval
          • Add header field "HP-Obscured" to MIME part origbody with value record

    • If hpr is not empty:

      • Add header field "HP-Removed" to MIME part origbody with value hpr
    • Set origheaders to newh
  • Apply crypto to origbody, producing MIME tree output

  • For each header field name and value (h,v) in origheaders:

    • Add header field h to output with value v
  • Return output

Note that both new parameters (hcp and legacy) are effectively ignored if crypto does not contain encryption. This is by design, because they are irrelevant for signed-only cryptographic protections.

2.3.4.1. Adding a Legacy Display Element to a text/plain Part

For a list of obscured header fields represented as (header, value) pairs, concatenate them as a set of lines, with one newline at the end of each pair. Add an additional trailing newline after the resultant text, and prepend the entire list to the body of the text/plain part.

For example, if the list of obscured header fields was [("Cc", "[email protected]"), ("Subject", "Thursday's meeting")], then a text/plain part that originally contained:

I think we should skip the meeting.

Would become:

Subject: Thursday's meeting
Cc: [email protected]

I think we should skip the meeting.
2.3.4.2. Adding a Legacy Display Element to a text/html Part

Adding a Legacy Display Element to a text/html part is similar to how it is added to a text/plain part (see Section 2.3.4.1). Instead of adding the obscured header fields to a block of text delimited by a blank line, the composing MUA injects them in an HTML <div> element annotated with a class attribute of header-protection-legacy-display.

The content and formatting of this decorative <div> have no strict requirements, but they SHOULD represent all the obscured header fields in a readable fashion. A simple approach is to assemble the text in the same way as Section 2.3.4.1, wrap it in a verbatim <pre> element, and put that element in the annotated <div>.

The annotated <div> should be placed as close to the start of the <body> as possible, where it will be visible when viewed with a standard HTML renderer.

For example, if the list of obscured header fields was [("Cc", "[email protected]"), ("Subject", "Thursday's meeting")], then a text/html part that originally contained:

<html><head><title></title></head><body>
<p>I think we should skip the meeting.</p>
</body></html>

Would become:

<html><head><title></title></head><body>
<div class="header-protection-legacy-display">
<pre>Subject: Thursday's meeting
Cc: [email protected]</pre></div>
<p>I think we should skip the meeting.</p>
</body></html>
2.3.4.2.1. Step-by-step Example for Inserting Legacy Display Element to text/html

A composing MUA MAY insert the Legacy Display Element anywhere reasonable within the message as long as it prioritizes visibility for the reader using a legacy decryption-capable MUA. This decision may take into account special message-specific HTML formatting expectations if the MUA is aware of them. However, some MUAs may not have any special insight into the user's preferred HTML formatting, and still want to insert a legacy display element. This section offers a non-normative, simple, and minimal step-by-step approach for a composing MUA that has no other information or preferences to fall back on.

The process below assumes that the MUA already has the full HTML object that it intends to send, including all of the text supplied by the user.

  • Assemble the text exactly as specified for text/plain (see Section 2.3.4.1).
  • Wrap that text in a verbatim <pre> element.
  • Wrap that <pre> element in a <div> element annotated with the class header-protection-legacy-display.
  • Find the <body> element of the full HTML object.
  • Insert the <div> element as the first child of the <body> element.
2.3.4.3. Only Add a Legacy Display Element to Main Body Parts

Some messages may contain a text/plain or text/html subpart that is not a main body part. For example, an e-mail message might contain an attached text file or a downloaded webpage. Attached documents need to be preserved as intended in the transmission, without modification.

The composing MUA MUST NOT add a Legacy Display element to any part of the message that is not a main body part. In particular, if a part is annotated with Content-Disposition: attachment, or if it does not descend via the first child of any of its multipart/mixed or multipart/related ancestors, it is not a main body part, and MUST NOT be modified.

See Section 7.1 of [I-D.ietf-lamps-e2e-mail-guidance] for more guidance about common ways to distinguish main body parts from other MIME parts in a message.

2.3.4.4. Do Not Add a Legacy Display Element to Other Content-Types

The purpose of injecting a Legacy Display element into each Main Body MIME part is to enable rendering of otherwise obscured header fields in legacy clients that are capable of message decryption, but don't know how to follow the rest of the guidance in this document.

The authors are unaware of any legacy client that would render any MIME part type other than text/plain and text/html as the Main Body. A generating MUA SHOULD NOT add a Legacy Display element to any MIME part with any other Content-Type.

2.3.5. Composing with "Wrapped Message" Header Protection

The Wrapped Message header protection scheme is briefly documented in Section 3.1 [RFC8551]. This section provides a more detailed explanation of how to build such a message, and augments it with the protected-headers parameter with the value wrapped. It also recommends marking such a wrapped message as Content-Disposition: inline to encourage legacy MUAs to render the inner message directly rather than treating it as an attachment.

To compose a message using "Wrapped Message" header protection, we use those inputs described in Section 2.3.1 plus the Header Confidentiality Policy hcp defined in Section 2.3.2. The new algorithm is:

  • For header field name and value (h,v) in origheaders:

    • Add header field h to origbody with value v

  • If crypto contains encryption:

    • Create new empty list of header field names and values newh
    • Let hpr be an empty comma-separated list of header field names

    • For header field name and value (h,v) in origheaders:

      • Let newval be hcp(h,v)

      • If newval is null:

        • Add the value h to hpr

      • Else (if newval is not null):

        • Append (h,newval) to newh

        • If newval is not v:

          • Let string record be the concatenation of h, a literal ": " (colon followed by a space), and newval
          • Add header field HP-Obscured to origbody with value record

    • If hpr is not empty:

      • Add header field "HP-Removed" to MIME part origbody with value hpr

  • If any of the header fields in origbody, including header fields in the nested internal MIME structure, contain any 8-bit UTF-8 characters (see section section 3.7 of [RFC6532]):

    • Let payload be a new MIME part with one header field: Content-Type: message/global; protected-headers=wrapped, and whose body is origbody.

  • Else:

    • Let payload be a new MIME part with one header field: Content-Type: message/rfc822; protected-headers=wrapped, and whose body is origbody.
  • Add a Content-Disposition header field to payload with value inline
  • Apply crypto to payload, yielding MIME tree output

  • If crypto contains encryption:

    • Set origheaders to newh

  • For header field name and value (h,v) in origheaders:

    • Add header field h to output with value v
  • Return output

Note that the Header Confidentiality Policy hcp is ignored if crypto does not contain encryption. This is by design.

2.3.6. Choosing Between Wrapped Message and Injected Headers

When composing a message with end-to-end cryptographic protections, an MUA SHOULD protect the header fields of that message as well as the body, using one of the formats described here.

A compatible MUA MUST be capable of generating a message with header protection using the Injected Headers Section 2.3.4 format.

2.4. Default Header Confidentiality Policy

An MUA SHOULD have a sensible default Header Confidentiality Policy, and SHOULD NOT require the user to select one. At the time this document was written, a good choice for default HCP is the conservative approach described by hcp_minimal (Section 2.4.1).

Any default Header Confidentiality Policy SHOULD provide confidentiality for the Subject header field by replacing it with the literal string "[...]". Most users treat the Subject of a message the same way that they treat the body, and they are surprised to find that the Subject of an encrypted message is visible.

2.4.1. Minimal Header Confidentiality Policy

The most conservative recommended Header Confidentiality Policy only protects the Subject header field:

hcp_minimal(name, val_in) → val_out:
    if name is 'Subject':
        return '[...]'
    else:
        return val_in

hcp_minimal is the recommended default HCP for a new implementation, as it provides meaningful confidentiality protections, and is unlikely to cause deliverability or usability problems.

2.4.2. Strong Header Confidentiality Policy

Alternately, a more aggressive (and therefore more privacy-preserving) Header Confidentiality Policy only leaks a handful of fields whose absence is known to increase rates of delivery failure, and simultaneously obscures the Message-ID behind a random new one:

hcp_strong(name, val_in) → val_out:
    if name in ['From', 'To', 'Cc', 'Date']:
        return val_in
    else if name is 'Subject':
        return '[...]'
    else if name is 'Message-ID':
        return generate_new_message_id()
    else:
        return null

The function generate_new_message_id() represents whatever process the MUA typically uses to generate a Message-ID for a new outbound message.

hcp_strong is known to cause usability problems with message threading for many legacy MUAs, and is not recommended as a default HCP for new implementations.

2.4.3. Null Header Confidentiality Policy

Legacy MUAs can be conceptualized as offering a null Header Confidentiality Policy, which offers no confidentiality protection to any header field:

hcp_null(name, val_in) → val_out:
    return val_in

A MUA offering header protection SHOULD NOT use hcp_null by default.

2.4.4. Offering Stronger Header Confidentiality

A MUA MAY offer even stronger confidentiality for header fields of an encrypted message than described in Section 2.4.2. For example, it might implement an HCP that obfuscates the From field, or omits the Cc field, or ensures Date is represented in UTC (obscuring the local timezone).

The authors of this document hope that implementers with deployment experience will document their chosen Header Confidentiality Policy and the rationale behind their choice.

This document defines hcp_null, hcp_minimal, hcp_example, and hcp_strong as a way to compare and contrast different possible behavioral choices for a composing MUA. Since the HCP is not strictly a protocol element, there is currently no explicit registry of named Header Confidentiality Policies. However, the first Internet Draft that attempts to define another HCP as a possible recommendation for the public should ask IANA to establish such a registry to avoid potential future namespace confusion.

2.5. Receiving Side

An MUA that receives a cryptographically-protected e-mail will render it for the user.

The receiving MUA will render the message body, a selected subset of header fields, and (as described in Section 3 of [I-D.ietf-lamps-e2e-mail-guidance]) provide a summary of the cryptographic properties of the message.

Most MUAs only render a subset of header fields by default. For example, few MUAs typically render Message-Id or Received header fields for the user, but most do render From, To, Cc, Date, and Subject.

A MUA that knows how to handle a message with header protection makes the following two changes to its behavior when rendering a message:

  • If it detects that an incoming message had protected header fields, it renders header fields for the message from the protected header fields, ignoring the external (unprotected) header fields.
  • It includes information in the message's cryptographic summary to indicate the types of protection that applied to each rendered header field (if any).

A MUA that handles a message with header protection does not need to render any new header fields that it did not render before.

2.5.1. Identifying that a Message has Header Protection

An incoming message can be identified as having header protection based on one of two signals:

  • The Cryptographic Payload has Content-Type: message/rfc822 or Content-Type: message/global and the parameter protected-headers has a value of wrapped. See Section 2.5.4 for rendering guidance.
  • The Cryptographic Payload has some other Content-Type and it has parameter protected-headers set to v1. See Section 2.5.3 for rendering guidance.

Messages of both types exist in the wild, and a compliant MUA MUST be able to handle them both. They provide the same semantics and the same meaning.

2.5.2. Updating the Cryptographic Summary

Regardless of whether a cryptographically-protected message has protected header fields, the cryptographic summary of the message should be modified to indicate what protections the header fields have. This field-by-field status is complex and isn't necessarily intended to be presented in full to the user. Rather, it represents the state of the message internally within the MUA, and may be used to influence behavior like replying to the message (see Section 2.5.8.1).

Each header field individually has exactly one the following protections:

  • unprotected (this is the case for all header fields in messages that have no header protection)
  • signed-only (bound into the same validated signature as the enclosing message, but also visible in transit)
  • encrypted-only (only appears within the Cryptographic Payload; the corresponding external header field was either omitted or obfuscated)
  • signed-and-encrypted (same as encrypted-only, but additionally is under a validated signature)

Note that while the message itself may be signed-and-encrypted, some header fields may be replicated on the outside of the message (e.g. Date). Those header fields would be signed-only, despite the message itself being signed-and-encrypted. Additionally, the data from some encrypted or signed-and-encrypted header fields may not be fully private (see Section 6.1 for more details).

Rendering the cryptographic status of each header field is likely to be complex and messy --- users may not understand it. It is beyond the scope of this document to suggest any specific graphical affordances or user experience. Future work should include examples of successful rendering of this information.

2.5.3. Rendering a Message with Injected Headers

When the Cryptographic Payload does not have a Content-Type of message/rfc822 or message/global, and the parameter protected-headers is set to v1, the values of the protected header fields are drawn from the header fields of the Cryptographic Payload, and the body that is rendered is the Cryptographic Payload itself.

2.5.3.1. Example Signed-only Message with Injected Headers 
A └─╴application/pkcs7-mime; smime-type="signed-data"
   ⇩ (unwraps to)
B  └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
C   ├─╴text/plain
D   └─╴text/html

The message body should be rendered the same way as this message:

B └┬╴multipart/alternative
C  ├─╴text/plain
D  └─╴text/html

It should render header fields taken from part B.

Its cryptographic summary should indicate that the message was signed and all rendered header fields were included in the signature.

The MUA should ignore header fields from part A for the purposes of rendering.

Because this message is signed-only, none of its parts will have a Legacy Display Element.

2.5.3.2. Example Signed-and-Encrypted Message with Injected Headers

Consider a message with this structure, where the MUA is able to validate the cryptographic signature:

E └─╴application/pkcs7-mime; smime-type="enveloped-data"
   ↧ (decrypts to)
F  └─╴application/pkcs7-mime; smime-type="signed-data"
    ⇩ (unwraps to)
G   └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
H    ├─╴text/plain
I    └─╴text/html

The message body should be rendered the same way as this message:

G └┬╴multipart/alternative
H  ├─╴text/plain
I  └─╴text/html

It should render header fields taken from part G.

Its cryptographic summary should indicate that the message was signed and encrypted. Each rendered header field found in G should be considered against any HP-Removed header field found in G and all HP-Obscured header fields found in G. If the field's name is found in the list of header field names in HP-Removed, or if one of the HP-Obscured fields refers to the field name, then the header field should be marked as signed-and-encrypted. Otherwise, the header field should be marked as signed-only.

If any of the user-facing headers are removed or obscured, the composer of this message MAY place Legacy Display Elements in parts H and I.

The MUA should ignore header fields from part E for the purposes of rendering.

2.5.3.3. Do Not Render Legacy Display Elements

As described in Section 2.1, a message with cryptographic confidentiality protection MAY include "Legacy Display" elements for backward-compatibility with legacy MUAs. These Legacy Display elements are strictly decorative, unambiguously identifiable, and will be discarded by compliant implementations.

The receiving MUA SHOULD avoid rendering the identified Legacy Display elements to the user at all, since it is aware of header protection and can render the actual protected header fields.

If a text/html or text/plain part within the Cryptographic Envelope is identified as containing Legacy Display elements, those elements SHOULD be hidden when rendering and SHOULD be dropped when generating a draft reply or inline forwarded message. Whenever a Message or MIME subtree is exported, downloaded or otherwise further processed, implementers should consider whether or not to drop the Legacy Display elements.

2.5.3.3.1. Identifying a Part with Legacy Display Elements

A receiving MUA acting on a message that contains an encrypting Cryptographic Layer identifies a MIME subpart within the Cryptographic Payload as containing Legacy Display elements based on the Content-Type of the subpart.

  • The subpart's Content-Type contains a parameter hp-legacy-display with value set to 1
  • The subpart's Content-Type is either text/html (see Section 2.5.3.3.3) or text/plain (see Section 2.5.3.3.2)

Note that the term "subpart" above is used in the general sense: if the Cryptographic Payload is a single part, that part itself may contain a Legacy Display element if it is marked with the hp-legacy-display=1 parameter.

2.5.3.3.2. Omitting Legacy Display Elements from text/plain

If a text/plain part within the Cryptographic Payload has the Content-Type parameter hp-legacy-display="1", it should be processed before rendering in the following fashion:

  • Discard the leading lines of the body of the part up to and including the first entirely blank line.

Note that implementing this strategy is dependent on the charset used by the MIME part.

See Appendix D.1 for an example.

2.5.3.3.3. Omitting Legacy Display Elements from text/html

If a text/html part within the Cryptographic Payload has the Content-Type parameter hp-legacy-display="1", it should be processed before rendering in the following fashion:

  • If any element of the HTML <body> is a <div> with class attribute header-protection-legacy-display, that entire element should be omitted.

This cleanup could be done, for example, as a custom rule in the MUA's HTML sanitizer, if one exists. Another implementation strategy for an HTML-capable MUA would b to add an entry to the [CSS] stylesheet for such a part:

body div.header-protection-legacy-display { display: none; }

2.5.4. Rendering a Wrapped Message

Some MUAs may compose and send a message with end-to-end cryptographic protections that offer header protection using the Wrapped Message scheme described in Section 3.1 of [RFC8551]. This section describes how a receiving MUA should identify and render such a message.

When the Cryptographic Payload has Content-Type of message/rfc822 or message/global, and the parameter protected-headers is set to wrapped, the values of the protected header fields are drawn from the header fields of the Cryptographic Payload, and the body that is rendered is the body of the Cryptographic Payload.

2.5.4.1. Example Signed-Only Wrapped Message

Consider a message with this structure, where the MUA is able to validate the cryptographic signature:

J └─╴application/pkcs7-mime; smime-type="signed-data"
   ⇩ (unwraps to)
K  └┬╴message/rfc822 [Cryptographic Payload]
L   └┬╴multipart/alternative [Rendered Body]
M    ├─╴text/plain
N    └─╴text/html

The message body should be rendered the same way as this message:

L └┬╴multipart/alternative
M  ├─╴text/plain
N  └─╴text/html

It should render header fields taken from part K.

Its cryptographic summary should indicate that the message was signed and all rendered header fields were included in the signature.

The MUA SHOULD ignore header fields from part J for the purposes of rendering.

2.5.4.2. Example Signed-and-Encrypted Wrapped Message

Consider a message with this structure, where the MUA is able to validate the cryptographic signature:

O └─╴application/pkcs7-mime; smime-type="enveloped-data"
   ↧ (decrypts to)
P  └─╴application/pkcs7-mime; smime-type="signed-data"
    ⇩ (unwraps to)
Q   └┬╴message/rfc822 [Cryptographic Payload]
R    └┬╴multipart/alternative [Rendered Body]
S     ├─╴text/plain
T     └─╴text/html

The message body should be rendered the same way as this message:

R └┬╴multipart/alternative
S  ├─╴text/plain
T  └─╴text/html

It should render header fields taken from part Q.

Its cryptographic summary should indicate that the message was signed and encrypted. As in Section 2.5.3.2, each rendered header field found in Q should be considered against any HP-Removed header field found in Q and all HP-Obscured header fields found in Q. If the field's name is found in the list of header field names in HP-Removed, or if one of the HP-Obscured fields refers to the field name, then the header field should be marked as signed-and-encrypted. Otherwise, the header field should be marked as signed-only.

2.5.5. Guidance for Automated Message Handling

Some automated systems have a control channel that is operated by e-mail. For example, an incoming e-mail message could subscribe someone to a mailing list, initiate the purchase of a specific product, approve another message for redistribution, or adjust the state of some shared object.

To the extent that such a system depends on end-to-end cryptographic guarantees about the e-mail control message, header protection as described in this document should improve the system's security. This section provides some specific guidance for systems that use e-mail messages as a control channel that want to benefit from these security improvements.

2.5.5.1. Interpret Only Protected Header Fields

Consider the situation where an e-mail-based control channel depends on the message's cryptographic signature and the action taken depends on some header field of the message.

In this case, the automated system MUST rely on information from the header field that is protected by the mechanism described in this document. It MUST NOT rely on any header field found outside the Cryptographic Payload.

For example, consider an administrative interface for a mailing list manager that only accepts control messages that are signed by one of its administrators. When an inbound message for the list arrives, it is queued (waiting for administrative approval) and the system generates and listens for two distinct e-mail addresses related to the queued message -- one that approves the message, and one that rejects it. If an administrator sends a signed control message to the approval address, the mailing list verifies that the protected To: header field of the signed control message contains the approval address before approving the queued message for redistribution. If the protected To: header field does not contain that address, or there is no protected To: header field, then the mailing list logs or reports the error, and does not act on that control message.

2.5.5.2. Ignore Legacy Display Elements

Consider the situation where an e-mail based control channel expects to receive an end-to-end encrypted message -- for example, where the control messages need confidentiality guarantees -- and where the action taken depends on the contents of some MIME part within message body.

In this case, the automated system that decrypts the incoming messages and scans the relevant MIME part SHOULD identify when the MIME part contains a legacy display element (see Section 2.5.3.3.1), and it SHOULD parse the relevant MIME part with the legacy display element removed.

For example, consider an administrative interface of a confidential issue tracking software. An authorized user can confidentially adjust the status of a tracked issue by a specially-formatted first line of the message body (for example, severity #183 serious). When the user's MUA encrypts a plain text control message to this issue tracker, depending on the MUA's HCP and its choice of legacy value, it may add a legacy display element. If it does so, then the first line of the message body will contain a decorative copy of the confidential Subject: header field. The issue tracking software decrypts the incoming control message, identifies that there is a legacy display element in the part (see Section 2.5.3.3.1), strips the legacy display lines (including the first blank line), and only then parses the remaining top line to look for the expected special formatting.

2.5.6. Affordances for Debugging and Troubleshooting

Note that advanced users of an MUA may need access to the original message, for example to troubleshoot problems with the rendering MUA itself, or problems with the SMTP transport path taken by the message.

A MUA that applies these rendering guidelines SHOULD ensure that the full original source of the message as it was received remains available to such a user for debugging and troubleshooting.

If a troubleshooting scenario demands information about the cryptographically-protected values of headers, and the message is encrypted, the debugging interface SHOULD also provide a "source" view of the Cryptographic Payload itself, alongside the full original source of the message as received.

2.5.7. Rendering Other Schemes

Other MUAs may have generated different structures of messages that aim to offer end-to-end cryptographic protections that include header protection.

While this document is not normative for those schemes, it offers guidance for how to identify and handle these other formats. In the following a list of systems that are known to generate e-mail messages with end-to-end cryptographic protections that include header protection using a different MIME scheme.

2.5.7.1. Pretty Easy Privacy (pEp)

The pEp (pretty Easy privacy) [I-D.pep-general] project specifies MIME schemes for Signed-and-Encrypted e-mail messages that also provide header protection [I-D.pep-email]. Similar to the "Wrapped Messages" scheme described in Section 2.3.5 and Section 2.5.4, pEp e-mail messages are fully encapsulated in the Cryptographic Payload.

More information can be found in [I-D.pep-email].

2.5.8. Composing a Reply to an Encrypted Message with Header Protection

When composing a reply to an encrypted message with header protection, the MUA is acting both as a receiving MUA and as a sending MUA. Special guidance applies here, as things can go wrong in at least two ways: leaking previously-confidential information, and replying to the wrong party.

2.5.8.1. Avoid Leaking Encrypted Header Fields in Reply

As noted in Section 5.4 of [I-D.ietf-lamps-e2e-mail-guidance], an MUA in this position MUST NOT leak previously-encrypted content in the clear in a follow-up message. The same is true for protected header fields.

Values from any header field that was identified as either encrypted-only or signed-and-encrypted based on the steps outlined above MUST NOT be placed in cleartext output when generating a message.

In particular, if Subject was encrypted, and it is copied into the draft encrypted reply, the replying MUA MUST obfuscate the unprotected (cleartext) Subject header field as described above.

When crafting the headers for a reply message, the composing MUA can make use of the HP-Removed and HP-Obscured headers from within the Cryptographic Envelope of the reference message to ensure that headers derived from the reference message do not leak in the reply.

Consider a header field in a reply message that is generated by derivation from a header field in the reference message. For example, the To header field is typically derived from the reference message's Reply-To or From header fields. When generating the outer copy of the header field, the composing MUA first applies its own header confidentiality policy. If the header field's value is changed by the HCP, then it is applied to the outside header and noted in the protected header section using HP-Removed or HP-Obscured as appropriate, as described in Section 2.3.3. Otherwise, if the header field's value is unchanged, the composing MUA re-generates the header field using the source header fields from the values within the Cryptographic Payload of the reference message, as modified by the HP-Obscured or HP-Removed headers. If that value is itself different than the protected value, then it is applied to the outside header and noted in the protected header section using HP-Obscured. If the value is the same as the protected value, then it is simply copied to the outside header directly.

See Appendix C.2 for a simple worked example of this process.

2.5.8.2. Avoid Misdirected Replies to Encrypted Messages with Header Protection

When replying to a message, the Composing MUA typically decides who to send the reply to based on:

  • the Reply-To, Mail-Followup-To, or From header fields
  • optionally, the other To or Cc header fields (if the user chose to "reply all")

When a message has header protection, the replying MUA MUST populate the destination fields of the draft message using the protected header fields, and ignore any unprotected header fields.

This mitigates against an attack where Mallory gets a copy of an encrypted message from Alice to Bob, and then replays the message to Bob with an additional Cc to Mallory's own e-mail address in the message's outer (unprotected) header section.

If Bob knows Mallory's certificate already, and he replies to such a message without following the guidance in this section, it's likely that his MUA will encrypt the cleartext of the message directly to Mallory.

2.5.9. Implicitly-rendered Header Fields

While From and To and Cc and Subject and Date are often explicitly rendered to the user, some header fields do affect message display, without being explicitly rendered.

For example, Message-Id, References, and In-Reply-To header fields may collectively be used to place a message in a "thread" or series of messages.

In another example, Section 2.5.8.2 observes that the value of the Reply-To field can influence the draft reply message. So while the user may never see the Reply-To header field directly, it is implicitly "rendered" when the user interacts with the message by replying to it.

An MUA that depends on any implicitly-rendered header field in a message with header protection SHOULD use the value from the protected header field, and SHOULD NOT use any value found outside the cryptographic protection.

2.5.10. Unprotected Header Fields Added in Transit

Some header fields are legitimately added in transit, and could not have been known to the sender at message composition time.

The most common of these header fields are Received and DKIM-Signature, neither of which are typically rendered, either explicitly or implicitly.

If a receiving MUA has specific knowledge about a given header field, including that:

  • the header field would not have been known to the original sender, and
  • the header field might be rendered explicitly or implicitly,

then the MUA MAY decide to operate on the value of that header field from the unprotected header section, even though the message has header protection.

The MUA MAY prefer to verify that the header fields in question have additional transit-derived cryptographic protections before rendering or acting on them. For example, the MUA could verify whether these header fields are covered by an appropriate and valid ARC-Authentication-Results (see [RFC8617]) or DKIM-Signature (see [RFC6376]) header field.

Specific examples of user-meaningful header fields commonly added by transport agents appear below.

2.5.10.1. Mailing list header fields: List-* and Archived-At

If the message arrives through a mailing list, the list manager itself may inject header fields (most of which start with List-) in the message:

  • List-Archive
  • List-Subscribe
  • List-Unsubscribe
  • List-Id
  • List-Help
  • List-Post
  • Archived-At

For some MUAs, these header fields are implicitly rendered, by providing buttons for actions like "Subscribe", "View Archived Version", "Reply List", "List Info", etc.

An MUA that receives a message with header protection that contains these header fields in the unprotected section, and that has reason to believe the message is coming through a mailing list MAY decide to render them to the user (explicitly or implicitly) even though they are not protected.

3. E-mail Ecosystem Evolution

This document is intended to offer tooling needed to improve the state of the e-mail ecosystem in a way that can be deployed without significant disruption. Some elements of this specification are present for transitional purposes, but would not exist if the system were designed from scratch.

This section describes these transitional mechanisms, as well as some suggestions for how they might eventually be phased out.

3.1. Dropping Legacy Display Elements

Any decorative Legacy Display element added to an encrypted message that uses the Injected Header scheme is present strictly for enabling header field visibility (most importantly, the Subject header field) when the message is viewed with a decryption-capable legacy client.

Eventually, the hope is that most decryption-capable MUAs will conform to this specification, and there will be no need for injection of Legacy Display elements in the message body. A survey of widely-used decryption-capable MUAs might be able to establish when most of them do support this specification.

At that point, a composing MUA could make the legacy parameter described in {#compose-injected-headers} to false by default, or could even hard-code it to false, yielding a much simpler message construction set.

Until that point, an end user might want to signal that their receiving MUAs are conformant to this draft so that a peer composing a message to them can set legacy to false. A signal indicating capability of handling messages with header protection might be placed in the user's cryptographic certificate, or in outbound messages.

This draft doesn't attempt to define the syntax or semantics of such a signal.

3.2. Stronger Default Header Confidentiality Policy

This draft defines two different forms of Header Confidentiality Policy. A MUA implementing an HCP for the first time SHOULD deploy hcp_minimal as recommended in Section 2.4. This HCP offers the most commonly-expected protection (obscuring the Subject header field) without risking deliverability or rendering issues.

The HCPs proposed in this draft are relatively conservative and still leak a significant amount of metadata for encrypted messages. This is largely done to ensure deliverability (see Section 1.4.2) and usability, as messages without some critical header fields are more likely to not reach their intended recipient.

In the future, some mail transport systems may accept and deliver messages with even less publicly-visible metadata. Many MTA operators today would ask for additional guarantees about such a message to limit the risks associated with abusive or spammy mail.

This specification offers the HCP formalism itself as a way for MUA developers and MTA operators to describe their expectations around message deliverability. MUA developers can propose a stronger default HCP, and ask MTA operators (or simply test) whether their MTAs would be likely to deliver or reject encrypted mail with that HCP applied. Proponents of a stronger HCP should explicitly document the HCP, and name it clearly and unambiguously to facilitate this kind of interoperability discussion.

Reaching widespread consensus around a stronger global default HCP is a challenging problem of coordinating many different actors. A piecemeal approach might be more feasible, where some signalling mechanism allows a message recipient, MTA operator, or third-party clearinghouse to announce what kinds of HCPs are likely to be deliverable for a given recipient. In such a situation, the default HCP for a MUA might involve consulting the signalled acceptable HCPs for all recipients, and combining them (along with a default for when no signal is present) in some way.

If such a signal were to reach widespread use, it could also be used to guide reasonable statistical default HCP choices for recipients with no signal.

This draft doesn't attempt to define the syntax or semantics of such a signal.

3.3. Deprecation of Messages Without Header Protection

At some point, when the majority of MUA clients that can generate cryptographically protected messages with header protection, it should be possible to deprecate any cryptographically protected message that does not have header protection.

For example, as noted in Section 4.1, it's possible for a MUA to decline to render a signed-only message that has no header protection the same as an unsigned message. And a signed-and-encrypted message without header protection could likewise be marked as not fully protected.

These stricter rules could be adopted immediately for all messages. Or a MUA developer could roll them out immediately for any new message, but still treat an old message (based on the Date header field and cryptographic signature timestamp) more leniently.

A decision like this by any popular receiving MUA could drive adoption of this standard for sending MUAs.

4. Usability Considerations

This section describes concerns for MUAs that are interested in easy adoption of header protection by normal users.

While they are not protocol-level artifacts, these concerns motivate the protocol features described in this document.

See also the Usability commentary in Section 2 of [I-D.ietf-lamps-e2e-mail-guidance].

4.1. Mixed Protections Within a Message Are Hard To Understand

When rendering a message to the user, the ideal circumstance is to present a single cryptographic status for any given message. However, when message headers are present, some message headers do not have the same cryptographic protections as the main message.

Representing such a mixed set of protection statuses is very difficult to do in a way that a normal user can understand without training. There are at least three scenarios that are likely to be common, and poorly understood:

  • A signed message with no header protection.
  • A signed-and-encrypted message with no header protection.
  • An signed-and-encrypted message with header protection as described in this document, where some user-facing headers have confidentiality but some do not.

A MUA should have a reasonable strategy for clearly communicating each of these scenarios to the user. For example, a MUA operating in an environment where it expects most cryptographically-protected messages to have header protection could use the following rendering strategy:

  • When rendering a message with signed-only cryptographic status but no header protection, an MUA may decline to indicate a positive security status overall, and only indicate the cryptographic status to a user in a message properties or diagnostic view. That is, the message may appear identical to an unsigned message except if a user verifies the properties through a menu option.
  • When rendering a message with signed-and-encrypted or encrypted-only cryptographic status but no header protection, overlay a warning flag on the typical cryptographic status indicator. That is, if a typical signed-and-encrypted message displays a lock icon, display a lock icon with a warning sign (e.g., an exclamation point in a triangle) overlaid. See, for example, the graphics in [chrome-indicators].
  • When rendering a message with signed-and-encrypted or encrypted-only cryptographic status, with header protection, but where the Subject header field has not been removed or obscured, place a warning sign on the on the Subject line.

Other simple rendering strategies could also be reasonable.

4.2. Users Should Not Have To Choose a Header Confidentiality Policy

This document defines the abstraction of a Header Confidentiality Policy object for the sake of communication between implementers and deployments.

Most e-mail users are unlikely to understand the tradeoffs between different policies. In particular, the potential negative side effects (e.g. poor deliverability) may not be easily attributable by a normal user to a particular HCP.

Therefore, MUA implementers should be conservative in their choice of default HCP, and should not require the ordinary user to make an incomprehensible choice that could cause unfixable, undiagnosable problems. The safest option is for the MUA developer to select a known, stable HCP (this document recommends hcp_minimal in Section 2.4) on the user's behalf. A MUA should not not expose the ordinary user to a configuration option where they are expected to manually select (let alone define) an HCP.

In the event that a MUA implementer gets user complaints about problems with removed or obscured header fields due to the MUA's defined HCP, the implementer may offer the user an option to drop header confidentiality altogether for freshly composed messages (thereby reverting to hcp_null). But when handling such a scenario for a reply to a message with some header confidentiality policy in place, note the guidance in Section 2.5.8.1 to avoid accidental leakage.

4.3. Users Should Not Have To Choose a Header Protection Scheme

This document also describes two different header protection schemes: Wrapped Messages in Section 2.2 and Injected Headers in Section 2.1.

These distinct schemes are described for the sake of implementers who may have to deal with messages found in the wild, but their intended semantics are identical. They represent different tradeoffs in terms of rendering and user experience on the recipient's side, things that a given user writing a message is not prepared to select.

When composing a message with cryptographic protections, the ordinary user should not be confronted with any choices about which header protection scheme to use. Rather, the MUA developer should use a single scheme for all outboud cryptographically-protected messages.

This document recommends the Injected Headers scheme for generating messages with cryptographic protections, as described in Section 2. A MUA should not expose the ordinary user to any configuration option where they are expected to manually select, enable, or disable header protections for new cryptographically-protected messages.

5. Security Considerations

This document describes a mechanism for improving the security of cryptographically-protected e-mail messages. Following the guidance in this document should improve security for users of these technologies by more directly aligning the underlying messages with user expectations about confidentiality, authenticity, and integrity.

However, many existing messages with cryptographic protections will not have these protections, and MUAs encountering these messages will need to handle older forms (without header protection) for quite some time. An implementation that deals with legacy message archives will need to deal with all the various formats forever. Helping the user distinguish between cryptographic protections of various messages is a difficult job for message renderers.

However, on the message generation side, the situation is much clearer: there is a standard form that a protected message can take, and an implementer can always generate the standard form. Generating the standard form also makes it more likely that any receiving implementation will be able to handle the generated message appropriately.

5.1. Caution about Composing with Legacy Display Elements

When composing a message, it's possible for a Legacy Display Element to contain risky data that could trigger errors in a rendering client.

For example, if the value for a header field to be included in a Legacy Display Element within a given body part contains folding whitespace, it should be "unfolded" before generating the Legacy Display Element: all contiguous folding whitespace should be replaced with a single space character. Likewise, if the header value was originally encoded with [RFC2047], it should be decoded first to a standard string and re-encoded using the charset appropriate to the target part.

When including a Legacy Display Element in a text/plain part (see Section 2.3.4.1), if the decoded Subject header field contains a pair of newlines (e.g., if it is broken across multiple lines by encoded newlines), any newline MUST be stripped from the Legacy Display Element. If the pair of newlines is not stripped, a receiving MUA that follows the guidance in Section 2.5.3.3.2 might leave the later part of the Legacy Display Element in the rendered message.

When including a Legacy Display Element in a text/html part (see Section 2.3.4.2), any material in the header values should be explicitly HTML escaped to avoid being rendered as part of the HTML. At a minimum, the characters <, >, and & should be escaped to &lt;, &gt;, and &amp;, respectively (see for example [HTML-ESCAPES]). If unescaped characters from removed or obscured header values end up in the Legacy Display element, a receiving MUA that follows the guidance in Section 2.5.3.3.3 might fail to identify the boundaries of the Legacy Display Element, cutting out more than it should, or leaving remnants visible. And a legacy client parsing such a message might misrender the entire HTML stream, depending on the content of the removed or obscured header values.

The Legacy Display Element is a decorative addition solely to enable visibility of obscured or removed header fields in legacy, decryption-capable MUAs. When it is produced, it should be generated conservatively and narrowly, to avoid damaging the rest of the message.

6. Privacy Considerations

6.1. Encrypted Header Fields Are Not Always Private

For encrypted messages, depending on the sender's HCP, some header fields may appear both within the Cryptographic Envelope and on the outside of the message. Section 2.5.2 identifies those messages as signed-only. These header fields are clearly not private at all, despite a copy being inside the Cryptographic Envelope.

A header field whose name can be found in the HP-Removed or in any HP-Obscured header field from the same part will have encrypted-only or signed-and-encrypted status. But even header fields with these stronger levels of cryptographic confidentiality protection might not be as private as the user would like.

6.2. Header Fields Can Leak Unwanted Information to the Recipient

For encrypted messages, even with a powerful HCP that successfully obscures most header fields from all transport agents, header fields will be ultimately visible to all intended recipients. This can be especially problematic for header fields that are not user-facing, which the sender may not expect to be injected by their MUA. Consider the three following examples:

  • The MUA may inject a User-Agent header field that describes itself to every recipient, even though the sender may not want the recipient to know the exact version of their OS, hardware platform, or MUA.
  • The MUA may have an idiosyncratic way of generating a Message-ID header, which could embed the choice of MUA, a timezone, a hostname, or other subtle information to a knowledgeable recipient.
  • The MUA may erroneously include a Bcc header field in the origheaders of a copy of a message sent to the named recipient, defeating the purpose of using Bcc instead of Cc (see Section 6.3 for more details about risks related to Bcc).

Clearly, no end-to-end cryptographic protection of any header field as described in this document will hide such a sensitive field from the intended recipient. Instead, the composing MUA MUST judiciously populate the origheaders list for any outbound message with only information that the user reasonably intends the recipient to have access to. This is true for messages without any cryptographic protection as well, of course, and it is even worse there: such a leak is exposed to the transport agents as well as the recipient. An encrypted message with header protection and a strong header confidentiality policy avoid these leaks exposing information to the transport agents, but cannot defend against such a leak to the recipient.

6.2.1. Encrypted Header Fields Can Be Inferred From External Metadata

For example, if the To: and Cc: header fields are omitted from the unprotected header section, the values in those fields might still be inferred with high probability by an adversary who looks at the message either in transit or at rest. If the message is found in, or being delivered to a mailbox for [email protected], it's likely that Bob was in either To: or Cc:. Additionally, an MTA that handles the message may add a Received: header field (or some other custom header field) that leaks some information about the nature of the delivery.

6.2.2. HCP May Not Mask All Data in an Encrypted Header Field

In another example, if the HCP modifies the Date: header to mask out high-resolution time stamps (e.g. rounding to the most recent hour) and to convert the local timezone to UTC, some information about the date of delivery will still be attached to the e-mail. At the very least, the low resolution, global version of the date will be present on the message. Additionally, headers like Received that are added during message delivery might include higher-resolution timestamps. And if the message lands in a mailbox that is ordered by time of receipt, even its placement in the mailbox and the non-obscured Date: header fields of the surrounding messages could leak this information.

Some fields like From: may be impossible to fully obscure, as many modern message delivery systems depend on at least domain information in the From: field for determining whether a message is coming from a domain with "good reputation" (that is, from a domain that is not known for leaking spam). So even if an aggressive HCP opts to remove the human-readable part from any From: header field, and to standardize/genericize the local part of the From: address, the domain will still leak.

6.2.3. A Naive Recipient May Overestimate the Cryptographic Status of a Header Field in an Encrypted Message

When an encrypted (or signed-and-encrypted) message is in transit, an active intermediary can strip or tamper with any header field that appears outside the Cryptographic Envelope. A receiving MUA that naively infers cryptographic status from differences between the external header fields and those found in the Cryptographic Envelope could be tricked into overestimating the protections afforded to some header fields.

For example, if the original sender's HCP passes through the Cc: header field unchanged, a cleanly-delivered message would indicate that the Cc: header field has a cryptographic status of signed. But if an intermediary attacker simply removes the header field from the unprotected header section before forwarding the message, then the naive recipient might believe that the field has a cryptographic status of signed-and-encrypted.

This draft offers protection against such an attack by way of the HP-Obscured and HP-Removed header fields that can be found on the Cryptographic Payload. If a header field appears to have been obscured, but no HP-Obscured header matches it; or if the header field appears to have been removed, but the HP-Removed header does not include its field name, the receiving MUA can indicate to the user that the header field in question may not have been confidential.

In such a case, a conservative MUA may render the header field in question as signed (because the sender did not hide it), but still treat it as signed-and-encrypted during reply, to avoid accidental leakage of the cleartext value in the reply message, as described in Section 2.5.8.1.

6.2.4. Summary and Implementation Guidance

In the abstract sense, the above concerns are of course also true for any encrypted data, including the body of the message: if the sender isn't careful, the message contents or session keys could leak in many different ways that are beyond the scope of this draft. The message recipient has no way in principle to tell whether the apparent confidentiality of any given piece of encrypted content has been broken via channels that they cannot perceive. And an active intermediary aware of the recipient's public key can always encrypt a cleartext message in transit to give the recipient a false sense of security.

A receiving MUA should be cautious about how it represents the cryptographic status of encrypted-only and signed-and-encrypted header fields to the user, to avoid overpromising. However, the MUA should also strive to avoid additional leakage of these header fields, as described in Section 2.5.8.1.

6.3. Privacy and Deliverability Risks with Bcc and Encrypted Messages

As noted in Section 9.3 of [I-D.ietf-lamps-e2e-mail-guidance], handling Bcc when generating an encrypted e-mail message can be particularly tricky. With header protection, there is an additional wrinkle. When an encrypted e-mail message with header protection has a Bcc'ed recipient, and the composing MUA explicitly includes the Bcc'ed recipient's address in their copy of the message (see the "second method" in Section 3.6.3 of [RFC5322]), that Bcc header field will always be visible to the Bcc'ed recipient.

In this scenario, though, the composing MUA has one additional choice: whether to hide the Bcc header field from intervening message transport agents, by returning null when the HCP is invoked for Bcc. If the composing MUA's rationale for including an explicit Bcc in the copy of the message sent to the Bcc recipient is to ensure deliverability via a message transport agent that inspects message headers, then stripping the Bcc field during encryption may cause the intervening transport agent to drop the message entirely. This is why Bcc is not explicitly stripped in hcp_minimal.

If, on the other hand, deliverability to a Bcc'ed recipient is not a concern, the most privacy-preserving option is to simply omit the Bcc header field from the protected header section in the first place. A MUA that is capable of receiving and processing such a message can infer that since their user's address was not mentioned in any To or Cc header field, they were likely a Bcc recipient.

Please also see Section 9.3 of [I-D.ietf-lamps-e2e-mail-guidance] for more discussion about Bcc and encrypted messages.

7. IANA Considerations

This document requests IANA to register the following two header fields in the "Permanent Message Header Field Names" registry within "Message Headers" in accordance with [RFC3864].

Table 1: Additions to 'Permanent Message Header Field Names' registry
Header Field Name Template Protocol Status Reference Author/Change Controller
HP-Removed   mail standard Section 2.3.3 of RFCXXXX IETF
HP-Obscured   mail standard Section 2.3.3 of RFCXXXX IETF

This document also defines the Content-Type parameter known as protected-headers. There does not appear to be an IANA registry for parameters for Content-Type. In the absence of such a registry, the Content-Type row in the "Permanent Message Header Field Names" registry should add a reference to this RFC to its "References" column.

That is, the current row:

Table 2: Existing row in 'Permanent Message Header Field Names' registry
Header Field Name Template Protocol Status Reference
Content-Type   MIME   [RFC4021]

Should be updated to have the following values:

Table 3: Replacement row in 'Permanent Message Header Field Names' registry
Header Field Name Template Protocol Status Reference
Content-Type   MIME   [RFC4021] [RFCXXXX]

If a registry of Content-Type parameters is created, the parameter protected-headers should refer to this document. Its possible values are v1 (meaning Injected Headers, see Section 2.1) and wrapped (meaning Wrapped Message, see Section 2.2).

8. Acknowledgments

The authors would like to thank the following people who have provided helpful comments and suggestions for this document: Berna Alp, Bernhard E. Reiter, Carl Wallace, Claudio Luck, David Wilson, Hernani Marques, juga, Krista Bennett, Kelly Bristol, Lars Rohwedder, Nicolas Lidzborski, Robert Williams, Russ Housley, Sofia Balicka, Steve Kille, Volker Birk, and Wei Chuang.

9. References

9.1. Normative References

[I-D.ietf-lamps-e2e-mail-guidance]
Gillmor, D. K., Hoeneisen, B., and A. Melnikov, "Guidance on End-to-End E-mail Security", Work in Progress, Internet-Draft, draft-ietf-lamps-e2e-mail-guidance-11, , <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-e2e-mail-guidance-11>.
[I-D.ietf-lamps-header-protection-requirements]
Melnikov, A. and B. Hoeneisen, "Problem Statement and Requirements for Header Protection", Work in Progress, Internet-Draft, draft-ietf-lamps-header-protection-requirements-01, , <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-header-protection-requirements-01>.
[RFC2045]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, , <https://www.rfc-editor.org/rfc/rfc2045>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3864]
Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, , <https://www.rfc-editor.org/rfc/rfc3864>.
[RFC5234]
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <https://www.rfc-editor.org/rfc/rfc5234>.
[RFC5322]
Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, , <https://www.rfc-editor.org/rfc/rfc5322>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8551]
Schaad, J., Ramsdell, B., and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification", RFC 8551, DOI 10.17487/RFC8551, , <https://www.rfc-editor.org/rfc/rfc8551>.

9.2. Informative References

[chrome-indicators]
Schechter, E., "Evolving Chrome's security indicators", , <https://blog.chromium.org/2018/05/evolving-chromes-security-indicators.html>.
[CSS]
World Wide Web Consortium, "Cascading Style Sheets Level 2 Revision 2 (CSS 2.2) Specification", , <https://www.w3.org/TR/2016/WD-CSS22-20160412/>.
[HTML-ESCAPES]
W3C, "Using character escapes in markup and CSS", n.d., <https://www.w3.org/International/questions/qa-escapes#use>.
[I-D.ietf-lamps-samples]
Gillmor, D. K., "S/MIME Example Keys and Certificates", Work in Progress, Internet-Draft, draft-ietf-lamps-samples-08, , <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-samples-08>.
[I-D.pep-email]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp): Email Formats and Protocols", Work in Progress, Internet-Draft, draft-pep-email-02, , <https://datatracker.ietf.org/doc/html/draft-pep-email-02>.
[I-D.pep-general]
Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy privacy (pEp): Privacy by Default", Work in Progress, Internet-Draft, draft-pep-general-02, , <https://datatracker.ietf.org/doc/html/draft-pep-general-02>.
[PGPCONTROL]
UUNET Technologies, Inc., "Authentication of Usenet Group Changes", , <https://ftp.isc.org/pub/pgpcontrol/>.
[PGPVERIFY-FORMAT]
Lawrence, D. C., "Signing Control Messages, Verifying Control Messages", n.d., <https://www.eyrie.org/~eagle/usefor/other/pgpverify>.
[RFC2047]
Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, DOI 10.17487/RFC2047, , <https://www.rfc-editor.org/rfc/rfc2047>.
[RFC2049]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, DOI 10.17487/RFC2049, , <https://www.rfc-editor.org/rfc/rfc2049>.
[RFC3156]
Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, , <https://www.rfc-editor.org/rfc/rfc3156>.
[RFC4021]
Klyne, G. and J. Palme, "Registration of Mail and MIME Header Fields", RFC 4021, DOI 10.17487/RFC4021, , <https://www.rfc-editor.org/rfc/rfc4021>.
[RFC6376]
Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, , <https://www.rfc-editor.org/rfc/rfc6376>.
[RFC6532]
Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, , <https://www.rfc-editor.org/rfc/rfc6532>.
[RFC7489]
Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, , <https://www.rfc-editor.org/rfc/rfc7489>.
[RFC8617]
Andersen, K., Long, B., Ed., Blank, S., Ed., and M. Kucherawy, Ed., "The Authenticated Received Chain (ARC) Protocol", RFC 8617, DOI 10.17487/RFC8617, , <https://www.rfc-editor.org/rfc/rfc8617>.

Appendix A. Possible Problems with some Legacy Clients

When an e-mail message with end-to-end cryptographic protection is received by a mail user agent, the user might experience many different possible problematic interactions. A message with header protection may introduce new forms of user experience failure.

In this section, the authors enumerate different kinds of failures we have observed when reviewing, rendering, and replying to messages with different forms of header protection in different legacy MUAs. Different legacy MUAs demonstrate different subsets of these problems.

Hopefully, a non-legacy MUA would not exhibit any of these problems. An implementer updating their legacy MUA to be compliant with this specification should consider these concerns and try to avoid them.

A.1. Problems Reviewing signed-and-encrypted Messages in List View

  • Unprotected Subject, Date, From, To are visible
  • Threading is not visible

A.2. Problems when Rendering a signed-and-encrypted Message

  • Unprotected Subject is visible
  • Protected subject (on its own) is visible in the body
  • Protected subject, date, from, to visible in the body
  • User interaction needed to view whole message
  • User interaction needed to view message body
  • User interaction needed to view protected subject
  • Impossible to view protected subject
  • Nuisance alarms during user interaction
  • Impossible to view message body
  • Appears as a forwarded message
  • Appears as an attachment
  • Security indicators not visible
  • User has multiple different methods to Reply: (e.g. reply to outer, reply to inner)
  • User sees English "Subject:" in body despite message itself being in non-English
  • Security indicators do not identify protection status of header fields
  • Header fields in body render with local header field names (e.g. showing "Betreff" instead of "Subject") and dates (TZ, locale)

A.3. Problems when Replying to a signed-and-encrypted Message

Note that the use case here is:

  • User views message, to the point where they can read it.
  • User then replies to message, and they are shown a message composition window, which has some UI elements
  • If the MUA has multiple different methods to Reply: to a message, each way may need to be evaluated separately

This section also uses the shorthand UI:x to mean "the UI element that the user can edit that they think of as x."

  • protected subject is in UI:subject (and will leak)
  • protected subject is quoted in UI:body
  • protected subject is not anywhere in UI
  • message body is not visible/quoted in UI:body
  • user cannot reply while viewing protected message
  • reply is not encrypted by default (but is for normal S/MIME sign+enc messages)
  • unprotected From: is in UI:To
  • User's locale (lang, TZ) leaks in quoted body
  • Header fields not protected (and in particular, Subject is not obscured) by default

A.4. Problems Reviewing signed-only Messages in List View

  • Unprotected Subject, Date, From, To are visible
  • Threading is not visible

A.5. Problems when Rendering a signed-only Message

  • Unprotected Subject is visible
  • Protected subject (on its own) is visible in the body
  • Protected subject, date, from, to visible in the body
  • User interaction needed to view whole message
  • User interaction needed to view message body
  • User interaction needed to view protected subject
  • Impossible to view protected subject
  • Nuisance alarms during user interaction
  • Impossible to view message body
  • Appears as a forwarded message
  • Appears as an attachment
  • Security indicators not visible
  • Security indicators do not identify protection status of header fields
  • User has multiple different methods to Reply: (e.g. reply to outer, reply to inner)
  • Header fields in body render with local header fields (e.g. showing "Betreff" instead of "Subject") and dates (TZ, locale)

A.6. Problems when Replying to a signed-only Message

This uses the same use case(s) and shorthand as Appendix A.3.

  • Unprotected Subject: is in UI:subject
  • Protected Subject: is quoted in UI:body
  • Protected Subject: is not anywhere in UI
  • Message body is not visible/quoted in UI:body
  • User cannot reply while viewing protected message
  • Unprotected From: is in UI:To
  • User's locale (lang, TZ) leaks in quoted body

Appendix B. Test Vectors

This section contains sample messages using the different schemes described in this document. Each sample contains a MIME object, a textual and diagrammatic view of its structure, and examples of how an MUA might render it.

The cryptographic protections used in this document use the S/MIME standard, and keying material and certificates come from [I-D.ietf-lamps-samples].

These messages should be accessible to any IMAP client at imap://[email protected]/ (any password should authenticate to this read-only IMAP mailbox).

You can also download copies of these test vectors separately at https://header-protection.cmrg.net.

If any of the messages downloaded differ from those offered here, this document is the canonical source.

B.1. Baseline Messages

These messages offer no header protection at all, and can be used as a baseline. They are provided in this document as a counterexample. An MUA implementer can use these messages to verify that the reported cryptographic summary of the message indicates no header protection.

B.1.1. No Cryptographic Protections Over a Simple Message

This message uses no cryptographic protection at all. Its body is a text/plain message.

It has the following structure:

└─╴text/plain 152 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Subject: no-crypto
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:00:02 -0500
User-Agent: Sample MUA Version 1.0

This is the no-crypto message.

This message uses no cryptographic protection at all.  Its body
is a text/plain message.

--
Alice
[email protected]

B.1.2. S/MIME Signed-only signedData Over a Simple Message, No Header Protection

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a text/plain message. It uses no header protection.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 3852 bytes
 ⇩ (unwraps to)
 └─╴text/plain 204 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:01:02 -0500
User-Agent: Sample MUA Version 1.0
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OA7IizkBD+GDEu8uKveERRXZncxGwy2MfbH1Ib3U8QzTjqB8+dz2AwYeMxODWq9o
pwtA/lTOkRg8uuivZfg/m5fFo/QshlHNaaTDVEXsU4Ps98Hm/3gznbvhdjFbZbi4
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MTkxMTIwMDY1NDE4WhgPMjA1MjA5MjcwNjU0MThaMDsxDTALBgNVBAoTBElFVEYx
ETAPBgNVBAsTCExBTVBTIFdHMRcwFQYDVQQDEw5BbGljZSBMb3ZlbGFjZTCCASIw
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xsVcF7q4WHWZWleYXFKlQHJD73nQwXP968+A/3rBX7PhO0DBbZnfitOLPgPEwjTt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B.1.3. S/MIME Signed-only multipart/signed Over a Simple Message, No Header Protection

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a text/plain message. It uses no header protection.

It has the following structure:

└┬╴multipart/signed 4191 bytes
 ├─╴text/plain 224 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="052";
 micalg="sha-256"
Subject: smime-multipart
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:02:02 -0500
User-Agent: Sample MUA Version 1.0

--052
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit

This is the smime-multipart message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a text/plain
message. It uses no header protection.

--
Alice
[email protected]

--052
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

MIIJ4AYJKoZIhvcNAQcCoIIJ0TCCCc0CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
hvcNAQcBoIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJ
KoZIhvcNAQENBQAwVTENMAsGA1UEChMESUVURjERMA8GA1UECxMITEFNUFMgV0cx
MTAvBgNVBAMTKFNhbXBsZSBMQU1QUyBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3Jp
dHkwIBcNMTkxMTIwMDY1NDE4WhgPMjA1MjA5MjcwNjU0MThaMDsxDTALBgNVBAoT
BElFVEYxETAPBgNVBAsTCExBTVBTIFdHMRcwFQYDVQQDEw5BbGljZSBMb3ZlbGFj
ZTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAJqVKfqLwaLjj+gBUCfk
acKTg8cc2OtJ9ZSed6U3jUoiZVpMLcP3MUKtLeLg9r1mAfIDlB/wlbdmadXPmrsz
yidmbuZmOpB5voVQfiLYYy3iOx7YOqzXrl6udP07k0sV+UdSNRFxrfKeoQEFXgOa
Gdmnx4OG/e3p1fIKM0dPzZLoOAJF5m5O0xzXPL74zFCWp2f1ZkuE4A6l41koaZXC
N5XL7wWTLMLeNf9Byb5ksKqUuqEHAMd1nmoNMgjY9VfVfcrv9w43GG8FtpSX+TWz
B2zNS2OF+XIVnzRG5DeoULq8v88Z5bLpIJ/nx26r8A4SSwIBaVv4wPxAf1iPsIVK
arUCAwEAAaOBrzCBrDAMBgNVHRMBAf8EAjAAMBcGA1UdIAQQMA4wDAYKYIZIAWUD
AgEwATAeBgNVHREEFzAVgRNhbGljZUBzbWltZS5leGFtcGxlMBMGA1UdJQQMMAoG
CCsGAQUFBwMEMA4GA1UdDwEB/wQEAwIFIDAdBgNVHQ4EFgQUolNB1UQ8gCkVfAEj
8OeOr83zdw8wHwYDVR0jBBgwFoAUkTCOfAcXDKfxCShlNhpnHGh29FkwDQYJKoZI
hvcNAQENBQADggEBAIFJeKCcsTKcFqQMpTryujRGzJdYA+R9eBAuDLsatbtKtl4F
zkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLvLir3hEUV2Z3MRsMt
jH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5UzpEYPLror2X4P5uXxaP0LIZR
zWmkw1RF7FOD7PfB5v94M5274XYxW2W4uKGd7QGnUZROSvSYkGiWDp1JhqXwfDz8
A0enITGXnoEkAFvvjiCqh64P1hIeMorj36pgL19oWZD6YrzSWHUz1F00juyuOfQs
qm6hvrDTqNpHNZ015fOURza1SkCvi9GFmNUPoVgwggPPMIICt6ADAgECAhM3QQV5
7XV/QqmiXDr0+GrOmqnXMA0GCSqGSIb3DQEBDQUAMFUxDTALBgNVBAoTBElFVEYx
ETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhTYW1wbGUgTEFNUFMgUlNBIENl
cnRpZmljYXRpb24gQXV0aG9yaXR5MCAXDTE5MTEyMDA2NTQxOFoYDzIwNTIwOTI3
MDY1NDE4WjA7MQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBXRzEXMBUG
A1UEAxMOQWxpY2UgTG92ZWxhY2UwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEK
AoIBAQC09InoWDgWPk2af0+StijSNOR8K/hN8D+l078oullsk4ASvSwjsCNo7sHU
a4xQUl5JO6VqY18LANwORjrc9BaX4MguzsbFXBe6uFh1mVpXmFxSpUByQ+950MFz
/evPgP96wV+z4TtAwW2Z34rTiz4DxMI07XYNFUEOls/gkUP2GxzymsO2kaYWTut3
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saqpo1d3f9jSkbtAV5w3vzfog8919MxKI9H6l4KuElnAtJ7BtZcsl7dUy9u9COgE
ykRiVokFQgqQ7XNDU+r3SeOWwks7AgMBAAGjga8wgawwDAYDVR0TAQH/BAIwADAX
BgNVHSAEEDAOMAwGCmCGSAFlAwIBMAEwHgYDVR0RBBcwFYETYWxpY2VAc21pbWUu
ZXhhbXBsZTATBgNVHSUEDDAKBggrBgEFBQcDBDAOBgNVHQ8BAf8EBAMCBsAwHQYD
VR0OBBYEFLv2zLItHQYSHJeuKWqQENMgZmZzMB8GA1UdIwQYMBaAFJEwjnwHFwyn
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G25nY/0eNARD6/oF0/sYonX2doizcGMk53riugAocCn5zbzhW/JVdYn30UxfyrZl
RAzEf7GHqgB/NyjOad3pdpVYeDh4ciNKjbs+aEoTWgAkoqENt1sRxlcvb7HVX524
bKZa1oPTUNlm6QpivtqDIdqGJdGf8L1zLfXBuo2zL3HR+M9CDr4Opq2JCkzP0Qhp
7poIccGE6I9Tsg+RrOA9iCQsPn1+Tg8YedjGzUWF07rNmT0TzPCVzUAuBlr+JJtz
OKypyQ3eoZ6EPazXqMyHAVcsm0GI364IOA0b8PSrJNtjh+AqJ5QfH+0e7NSzNnEm
MYICADCCAfwCAQEwbDBVMQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBX
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cml0eQITN0EFee11f0Kpolw69Phqzpqp1zALBglghkgBZQMEAgGgaTAYBgkqhkiG
9w0BCQMxCwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0yMTAyMjAxNTAyMDJa
MC8GCSqGSIb3DQEJBDEiBCDAkJYhqVAHhprkzEWP6PweksoYhj5ULTLbcfQ9Tu3C
zDANBgkqhkiG9w0BAQEFAASCAQCJe818STb4M4utvQsdcQEH0CZR7I38uL5TSZF3
llKmD9PuCDuV3GIkfdmZISKRuffBle1xaNc2av/0Qogr7OaFF485DAONVAEIQ7ah
t94pwgAE4yvXXWKmFQkKid1tnMXbnHADKWU0YC+BQkgd/5J3zg4ESeMwOUm0+b3C
GDaUBTIJhHfu9sqlt7jXa7PbzQEfemYZORPI14/uZSs86SLkPvNGUpWb4mN6olC0
2h/U4SCpq8Oy390oNM0VNpoa+nsTu5yOFc34pMIvjwCJyIOYPaDnvw9FYgr2oOp7
cdOgFcSJ8q7I+Tx2yg60VW8tAT7UBkifc37UUuVbnOsqeVB3

--052--

B.1.4. S/MIME Encrypted and Signed Over a Simple Message, No Header Protection

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses no header protection.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 6720 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 3960 bytes
  ⇩ (unwraps to)
  └─╴text/plain 239 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: smime-enc-signed
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:03:02 -0500
User-Agent: Sample MUA Version 1.0

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iIpa80hPVJyC4gDtKLqF8Jb8VVKb4kdbTph6+pcRwnqIj6pEZq4G8FvquntzNn0o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DpixiwTJroJ6GeKOtBn/K5eCqxKoF3gKiH98DnH9NV1otBej74998NG6ATN5jpaZ
C46LiTJpMZpTx91EyasuT6eDW+lEGa6EWylC7x7zjjjwaNlqD2mMlNpnSm8L1oB3
vvcwP60GoLgyu50+M0C+hYxrNuyCG2aoX6bvzdFrh9DyLl8LEErVdOPj9r/hOMtB
PJzmiDqHIYaZv6+uyarrjfRG6dO+kCZDtzuAy/HEU+UXCuv27i99gkEyeMcasQSp
DkRjvnVJQlO1fMx/ttIGyyUbTH/jlBmLQ0cc+hrBeGGTYyKM5N6eB5WCukYSkfva
6p7zGiKUER1py0ZmcO4BN3UqPR6P9pJbJ0cNhpCTx7/pKa9OgDpT8+Ma1RxanOLK
mskKwQpnkJf+2ays9Rv0oYtbNfVzJJPrT8iVglD3aFwmCop0Ml/kW5sYFdPpFGsH
byzTzq3Fjw0AQ5UOG5Qq8EpsAlAJ3hy/5Vv4OaVizAoJz2fZXnQ9Bw00lud/outL
ZbRUEC72vJewbIAS1lzdJ7RLlpSMvB48/cA2dgeXqqfnvnAsMzgOIlaFlVID9H4m
/KtMJfKPkagrka91wFwLECu207zihtHmRbkkWlrswqA4SyumWfR5AEGW/sZ8g9LA
rugrt/sE6SpyYi5zzYL9/vNT61kQVy7UhUqcasQU+1CLVuaplAk4uvRso88wXYKn
SSQXesmy5m6eYOIevOmyUMQzzfwKswT49j/7hrHsECtzpyCOP0/8zBgGH8f/wg1r
/sZ/O+sZNu819qUaJhHSFIEx/CQKuHYv5ez6aT3BAtmPn0iWrFVzna3Ogo8XAL68
eDwN69Qm82ikDO2LFkKZrBzn/1dyZs/dT6lQYpsmhxJzoluZzW/sYFeOCX6fWs7n
fcrz9yMIDKvj70JrZp5jPRghFKHmqo5xh39TmeTsQFp2B8UlGD9YK6YfgSEaGbyL
3BpUjZN/713jmWYHzGvEQfx7vP3SaZBMZ4GSCoeBT2grQoUDe575H7UDJsmRVJ04
bO7iTWPZ1LdIC+oifedAhGhCoum+tApUYj+3BHz1xIAZJMCGARqgyKcnvjw5WVu3
fDna+4xJdNs0YK1uBkr6N9FBDfmQIuneIsQHAM7lZfucd1FenZhy1zNreqgls9QO
NncRNlltqmT2qmERXw8/HwcwNjR8FWrwbCCApsMgAZ0xWaRxpEct5lnGNbBpplEn
BrMafVecUlQgwa1jchA5ZiOuaZxizi1Pr9/eoaX93aa2u+6OpsyPqdadxwDeV1Do
4dg2NrDqQMFo3I1IcADeZEcEqPx8PV0tYjEeFZYsE0k3Qmcti+RuRj/rNTaXQ2Xw
VkgL1BG8POkxw0pVIKVyevcPtUD5tSlTxfp4qBFlEY/yrGCHy36q2mboBcRyYQry
oBnsvoEfrIE8FEz1rOJVM+HN2udrKVJZzEPySflZvbDzxINcqDu09r3UO+L+ymW5
9/ncHCMyoa0KbQ08q9i8VsGchL2FF5Q66g7I8U9u7R7V4Fz8RvLOzs6bB/Oh7+Z9
0dTWreRYp9/82pQ0VSuvkWYiSPwiy37spaE8uALD5MvZOS3CqOwGI+o45uLBP/a6
dgalPv1kThe8/a25+FqiQP6boCsN9wgA+T3v3kRFibzFEtyqX8C6Vu795PpycZ14
/RGFTm2Df/U38DN/mlNhGgM6gMQr1YuSPieFJ+0/ctzGpSaS835d+DkQVvS3zT3/
5EpybkOZrqf6erhNTVa8Onr3ZNdt9QyNUCmwxpYVvV2exwoVfcIjQgCxwehySLW5
UprvrRNgHo0OBMH+UmSggBfT7/omejxHgAJz5WCl/P+DiQ/dZcBK1OCRh1ZkocLB
WVpunKTMuLyqSqNG87nzXAgFCLYQRWeCQNcItSbJ4aed+sJIYxmEm2UzyKAk9eXI
dCZ/5fHOtmMDl645r/v9eSjeZd7Ed6MhGladuVlNm9Dl29sIzKcUu3zfZAqBlzFK
1RzPS3IUeM2VEJbK9AowEQ==

B.1.5. No Cryptographic Protections Over a Complex Message

This message uses no cryptographic protection at all. Its body is a multipart/alternative message with an inline image/png attachment.

It has the following structure:

└┬╴multipart/mixed 1406 bytes
 ├┬╴multipart/alternative 794 bytes
 │├─╴text/plain 206 bytes
 │└─╴text/html 304 bytes
 └─╴image/png inline 232 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="c39"
Subject: no-crypto-complex
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:00:02 -0500
User-Agent: Sample MUA Version 1.0

--c39
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="05a"

--05a
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

This is the no-crypto-complex message.

This message uses no cryptographic protection at all.  Its body
is a multipart/alternative message with an inline image/png
attachment.

--
Alice
[email protected]
--05a
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

<html><head><title></title></head><body>
<p>This is the <b>no-crypto-complex</b> message.</p>
<p>This message uses no cryptographic protection at all.  Its body
is a multipart/alternative message with an inline image/png
attachment.</p>
<p><tt>-- <br/>Alice<br/>[email protected]</tt></p></body></html>
--05a--

--c39
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

--c39--

B.1.6. S/MIME Signed-only signedData Over a Complex Message, No Header Protection

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses no header protection.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 5249 bytes
 ⇩ (unwraps to)
 └┬╴multipart/mixed 1288 bytes
  ├┬╴multipart/alternative 882 bytes
  │├─╴text/plain 258 bytes
  │└─╴text/html 353 bytes
  └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part-complex
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:01:02 -0500
User-Agent: Sample MUA Version 1.0

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IG11bHRpcGFydC9taXhlZDsgYm91bmRhcnk9IjMzZSINCg0KLS0zM2UNCk1JTUUt
VmVyc2lvbjogMS4wDQpDb250ZW50LVR5cGU6IG11bHRpcGFydC9hbHRlcm5hdGl2
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dGVudC1UcmFuc2Zlci1FbmNvZGluZzogN2JpdA0KDQpUaGlzIGlzIHRoZSBzbWlt
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ZQ0KQ29udGVudC1UeXBlOiBpbWFnZS9wbmcNCkNvbnRlbnQtVHJhbnNmZXItRW5j
b2Rpbmc6IGJhc2U2NA0KQ29udGVudC1EaXNwb3NpdGlvbjogaW5saW5lDQoNCmlW
Qk9SdzBLR2dvQUFBQU5TVWhFVWdBQUFCUUFBQUFVQ0FZQUFBQ05pUjBOQUFBQWNF
bEVRVlI0MnVWVE94YkENCk1BZ1M3MzluTzNUcFJ3MjBkcXBiZkFSUUVqT3l3aXdZ
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MDQ0N2dZRHBlQXJrK09uSkhrSWhBZlRQUmljaWhBZjVZSnJ3N3ZqdjBaV1JXTS91
bGkNCnZkUGYxUVoya0REOXhwcGQ4d0FBQUFCSlJVNUVya0pnZ2c9PQ0KDQotLTMz
ZS0tDQqgggemMIIDzzCCAregAwIBAgITDy0lvRE5l0rOQlSHoe49NAaKtDANBgkq
hkiG9w0BAQ0FADBVMQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBXRzEx
MC8GA1UEAxMoU2FtcGxlIExBTVBTIFJTQSBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0
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J2Zu5mY6kHm+hVB+IthjLeI7Htg6rNeuXq50/TuTSxX5R1I1EXGt8p6hAQVeA5oZ
2afHg4b97enV8gozR0/Nkug4AkXmbk7THNc8vvjMUJanZ/VmS4TgDqXjWShplcI3
lcvvBZMswt41/0HJvmSwqpS6oQcAx3Weag0yCNj1V9V9yu/3DjcYbwW2lJf5NbMH
bM1LY4X5chWfNEbkN6hQury/zxnlsukgn+fHbqvwDhJLAgFpW/jA/EB/WI+whUpq
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ATABMB4GA1UdEQQXMBWBE2FsaWNlQHNtaW1lLmV4YW1wbGUwEwYDVR0lBAwwCgYI
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546vzfN3DzAfBgNVHSMEGDAWgBSRMI58BxcMp/EJKGU2GmccaHb0WTANBgkqhkiG
9w0BAQ0FAAOCAQEAgUl4oJyxMpwWpAylOvK6NEbMl1gD5H14EC4Muxq1u0q2XgXO
SBHI6DfX/4LDsfx7fSIus8gWVY3WqMeuOA7IizkBD+GDEu8uKveERRXZncxGwy2M
fbH1Ib3U8QzTjqB8+dz2AwYeMxODWq9opwtA/lTOkRg8uuivZfg/m5fFo/QshlHN
aaTDVEXsU4Ps98Hm/3gznbvhdjFbZbi4oZ3tAadRlE5K9JiQaJYOnUmGpfB8PPwD
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DMR/sYeqAH83KM5p3el2lVh4OHhyI0qNuz5oShNaACSioQ23WxHGVy9vsdVfnbhs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fdkZwTVUoDHWjSVdIREobdPI2wdpnGxS/AB1VuiYpcebi4o=

B.1.7. S/MIME Signed-only multipart/signed Over a Complex Message, No Header Protection

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a multipart/alternative message with an inline image/png attachment. It uses no header protection.

It has the following structure:

└┬╴multipart/signed 5234 bytes
 ├┬╴multipart/mixed 1344 bytes
 │├┬╴multipart/alternative 938 bytes
 ││├─╴text/plain 278 bytes
 ││└─╴text/html 376 bytes
 │└─╴image/png inline 232 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="452";
 micalg="sha-256"
Subject: smime-multipart-complex
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:02:02 -0500
User-Agent: Sample MUA Version 1.0

--452
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="ac5"

--ac5
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="813"

--813
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

This is the smime-multipart-complex message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses no header protection.

--
Alice
[email protected]
--813
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

<html><head><title></title></head><body>
<p>This is the <b>smime-multipart-complex</b> message.</p>
<p>This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses no header protection.</p>
<p><tt>-- <br/>Alice<br/>[email protected]</tt></p></body></html>
--813--

--ac5
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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--ac5--

--452
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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--452--

B.1.8. S/MIME Encrypted and Signed Over a Complex Message, No Header Protection

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses no header protection.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 8690 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 5426 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1356 bytes
   ├┬╴multipart/alternative 950 bytes
   │├─╴text/plain 293 bytes
   │└─╴text/html 388 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: smime-enc-signed-complex
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:03:02 -0500
User-Agent: Sample MUA Version 1.0

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B.2. Signed-only Messages

These messages are signed-only, using different schemes of header protection and different S/MIME structure. The use no Header Confidentiality Policy because the hcp is only relevant when a message is encrypted.

B.2.1. S/MIME Signed-only signedData Over a Simple Message, Wrapped Message

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a text/plain message. It uses the Wrapped Message header protection scheme.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 4319 bytes
 ⇩ (unwraps to)
 └┬╴message/rfc822 inline 642 bytes
  └─╴text/plain 228 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:04:02 -0500
User-Agent: Sample MUA Version 1.0

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B.2.2. S/MIME Signed-only multipart/signed Over a Simple Message, Wrapped Message

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a text/plain message. It uses the Wrapped Message header protection scheme.

It has the following structure:

└┬╴multipart/signed 4562 bytes
 ├┬╴message/rfc822 inline 672 bytes
 │└─╴text/plain 256 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="8a8";
 micalg="sha-256"
Subject: smime-multipart-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:05:02 -0500
User-Agent: Sample MUA Version 1.0

--8a8
MIME-Version: 1.0
Content-Type: message/rfc822; protected-headers="wrapped"
Content-Disposition: inline

MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Subject: smime-multipart-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:05:02 -0500
User-Agent: Sample MUA Version 1.0

This is the smime-multipart-wrapped message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a text/plain
message. It uses the Wrapped Message header protection scheme.

--
Alice
[email protected]

--8a8
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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VtJqhT5xHHvMfDLUVW4sXwRugWKohiW+cjZ16SQ5zP14KJBpriMWv8A/4sJv5aC2
ImraEATJ1gIse53X6XPDt/+9BsXOrvbIvXRIbgMJBK8gIz6aO72n/dvm1fHjdBXv
9t75zqN+O821RiUiSbBoaB3FP0sl3prsZ4QRr3Yv7vpv/HoR

--8a8--

B.2.3. S/MIME Signed-only signedData Over a Simple Message, Injected Headers

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 4234 bytes
 ⇩ (unwraps to)
 └─╴text/plain 239 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:06:02 -0500
User-Agent: Sample MUA Version 1.0

MIIMMgYJKoZIhvcNAQcCoIIMIzCCDB8CAQExDTALBglghkgBZQMEAgEwggJbBgkq
hkiG9w0BBwGgggJMBIICSE1JTUUtVmVyc2lvbjogMS4wDQpDb250ZW50LVRyYW5z
ZmVyLUVuY29kaW5nOiA3Yml0DQpTdWJqZWN0OiBzbWltZS1vbmUtcGFydC1pbmpl
Y3RlZA0KTWVzc2FnZS1JRDogPHNtaW1lLW9uZS1wYXJ0LWluamVjdGVkQGxocC5l
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LjANCkNvbnRlbnQtVHlwZTogdGV4dC9wbGFpbjsgY2hhcnNldD0idXRmLTgiOyBw
cm90ZWN0ZWQtaGVhZGVycz0idjEiDQoNClRoaXMgaXMgdGhlIHNtaW1lLW9uZS1w
YXJ0LWluamVjdGVkIG1lc3NhZ2UuDQoNClRoaXMgaXMgYSBzaWduZWQtb25seSBT
L01JTUUgbWVzc2FnZSB2aWEgUEtDUyM3IHNpZ25lZERhdGEuICBUaGUNCnBheWxv
YWQgaXMgYSB0ZXh0L3BsYWluIG1lc3NhZ2UuIEl0IHVzZXMgdGhlIEluamVjdGVk
IEhlYWRlcnMNCmhlYWRlciBwcm90ZWN0aW9uIHNjaGVtZS4NCg0KLS0gDQpBbGlj
ZQ0KYWxpY2VAc21pbWUuZXhhbXBsZQ0KoIIHpjCCA88wggK3oAMCAQICEw8tJb0R
OZdKzkJUh6HuPTQGirQwDQYJKoZIhvcNAQENBQAwVTENMAsGA1UEChMESUVURjER
MA8GA1UECxMITEFNUFMgV0cxMTAvBgNVBAMTKFNhbXBsZSBMQU1QUyBSU0EgQ2Vy
dGlmaWNhdGlvbiBBdXRob3JpdHkwIBcNMTkxMTIwMDY1NDE4WhgPMjA1MjA5Mjcw
NjU0MThaMDsxDTALBgNVBAoTBElFVEYxETAPBgNVBAsTCExBTVBTIFdHMRcwFQYD
VQQDEw5BbGljZSBMb3ZlbGFjZTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoC
ggEBAJqVKfqLwaLjj+gBUCfkacKTg8cc2OtJ9ZSed6U3jUoiZVpMLcP3MUKtLeLg
9r1mAfIDlB/wlbdmadXPmrszyidmbuZmOpB5voVQfiLYYy3iOx7YOqzXrl6udP07
k0sV+UdSNRFxrfKeoQEFXgOaGdmnx4OG/e3p1fIKM0dPzZLoOAJF5m5O0xzXPL74
zFCWp2f1ZkuE4A6l41koaZXCN5XL7wWTLMLeNf9Byb5ksKqUuqEHAMd1nmoNMgjY
9VfVfcrv9w43GG8FtpSX+TWzB2zNS2OF+XIVnzRG5DeoULq8v88Z5bLpIJ/nx26r
8A4SSwIBaVv4wPxAf1iPsIVKarUCAwEAAaOBrzCBrDAMBgNVHRMBAf8EAjAAMBcG
A1UdIAQQMA4wDAYKYIZIAWUDAgEwATAeBgNVHREEFzAVgRNhbGljZUBzbWltZS5l
eGFtcGxlMBMGA1UdJQQMMAoGCCsGAQUFBwMEMA4GA1UdDwEB/wQEAwIFIDAdBgNV
HQ4EFgQUolNB1UQ8gCkVfAEj8OeOr83zdw8wHwYDVR0jBBgwFoAUkTCOfAcXDKfx
CShlNhpnHGh29FkwDQYJKoZIhvcNAQENBQADggEBAIFJeKCcsTKcFqQMpTryujRG
zJdYA+R9eBAuDLsatbtKtl4FzkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5
AQ/hgxLvLir3hEUV2Z3MRsMtjH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5U
zpEYPLror2X4P5uXxaP0LIZRzWmkw1RF7FOD7PfB5v94M5274XYxW2W4uKGd7QGn
UZROSvSYkGiWDp1JhqXwfDz8A0enITGXnoEkAFvvjiCqh64P1hIeMorj36pgL19o
WZD6YrzSWHUz1F00juyuOfQsqm6hvrDTqNpHNZ015fOURza1SkCvi9GFmNUPoVgw
ggPPMIICt6ADAgECAhM3QQV57XV/QqmiXDr0+GrOmqnXMA0GCSqGSIb3DQEBDQUA
MFUxDTALBgNVBAoTBElFVEYxETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhT
YW1wbGUgTEFNUFMgUlNBIENlcnRpZmljYXRpb24gQXV0aG9yaXR5MCAXDTE5MTEy
MDA2NTQxOFoYDzIwNTIwOTI3MDY1NDE4WjA7MQ0wCwYDVQQKEwRJRVRGMREwDwYD
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078oullsk4ASvSwjsCNo7sHUa4xQUl5JO6VqY18LANwORjrc9BaX4MguzsbFXBe6
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ls/gkUP2GxzymsO2kaYWTut3SryCqeHEFbZFkB4urMk4xrIJC3CzWruS2Q0FHbBl
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BgNVHQ8BAf8EBAMCBsAwHQYDVR0OBBYEFLv2zLItHQYSHJeuKWqQENMgZmZzMB8G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B.2.4. S/MIME Signed-only multipart/signed Over a Simple Message, Injected Headers

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a text/plain message. It uses the Injected Headers header protection scheme.

It has the following structure:

└┬╴multipart/signed 4487 bytes
 ├─╴text/plain 258 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="f1e";
 micalg="sha-256"
Subject: smime-multipart-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:07:02 -0500
User-Agent: Sample MUA Version 1.0

--f1e
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: smime-multipart-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:07:02 -0500
User-Agent: Sample MUA Version 1.0
Content-Type: text/plain; charset="utf-8"; protected-headers="v1"

This is the smime-multipart-injected message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a text/plain
message. It uses the Injected Headers header protection scheme.

--
Alice
[email protected]

--f1e
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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hvcNAQcBoIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJ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zkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLvLir3hEUV2Z3MRsMt
jH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5UzpEYPLror2X4P5uXxaP0LIZR
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A0enITGXnoEkAFvvjiCqh64P1hIeMorj36pgL19oWZD6YrzSWHUz1F00juyuOfQs
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saqpo1d3f9jSkbtAV5w3vzfog8919MxKI9H6l4KuElnAtJ7BtZcsl7dUy9u9COgE
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86vWp79p+KwM/+SyDdfgudIRGjbs/tmKaBvaH89a8SvuxhNxq/pxgDzpy/JWC8Er
AUDTbKrNVsYD+MfzMy9B0TlK2YLKoQ6rV0N1n2nXbW0e+Ztv0a/getNKAEAP+5hE
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yRXQPj+5oyduWiHRMLXG1+fs4tqxHZXN+WaUHvSIDqNXK3rj

--f1e--

B.2.5. S/MIME Signed-only signedData Over a Complex Message, Wrapped Message

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 5737 bytes
 ⇩ (unwraps to)
 └┬╴message/rfc822 inline 1689 bytes
  └┬╴multipart/mixed 1584 bytes
   ├┬╴multipart/alternative 946 bytes
   │├─╴text/plain 282 bytes
   │└─╴text/html 380 bytes
   └─╴image/png inline 232 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part-complex-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:04:02 -0500
User-Agent: Sample MUA Version 1.0

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biBpbmxpbmUKaW1hZ2UvcG5nIGF0dGFjaG1lbnQuIEl0IHVzZXMgdGhlIFdyYXBw
ZWQgTWVzc2FnZSBoZWFkZXIKcHJvdGVjdGlvbiBzY2hlbWUuCgotLSAKQWxpY2UK
YWxpY2VAc21pbWUuZXhhbXBsZQotLTQxMwpDb250ZW50LVR5cGU6IHRleHQvaHRt
bDsgY2hhcnNldD0idXMtYXNjaWkiCk1JTUUtVmVyc2lvbjogMS4wCkNvbnRlbnQt
VHJhbnNmZXItRW5jb2Rpbmc6IDdiaXQKCjxodG1sPjxoZWFkPjx0aXRsZT48L3Rp
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QWZUUFJpY2loQWY1WUpydzd2anYwWldSV00vdWxpCnZkUGYxUVoya0REOXhwcGQ4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B.2.6. S/MIME Signed-only multipart/signed Over a Complex Message, Wrapped Message

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme.

It has the following structure:

└┬╴multipart/signed 5653 bytes
 ├┬╴message/rfc822 inline 1747 bytes
 │└┬╴multipart/mixed 1642 bytes
 │ ├┬╴multipart/alternative 1002 bytes
 │ │├─╴text/plain 310 bytes
 │ │└─╴text/html 408 bytes
 │ └─╴image/png inline 232 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="aa9";
 micalg="sha-256"
Subject: smime-multipart-complex-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:05:02 -0500
User-Agent: Sample MUA Version 1.0

--aa9
MIME-Version: 1.0
Content-Type: message/rfc822; protected-headers="wrapped"
Content-Disposition: inline

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="a30"
Subject: smime-multipart-complex-wrapped
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:05:02 -0500
User-Agent: Sample MUA Version 1.0

--a30
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="844"

--844
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

This is the smime-multipart-complex-wrapped message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses the Wrapped Message header protection
scheme.

--
Alice
[email protected]
--844
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

<html><head><title></title></head><body>
<p>This is the <b>smime-multipart-complex-wrapped</b> message.</p>
<p>This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses the Wrapped Message header protection
scheme.</p>
<p><tt>-- <br/>Alice<br/>[email protected]</tt></p></body></html>
--844--

--a30
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

--a30--

--aa9
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"
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--aa9--

B.2.7. S/MIME Signed-only signedData Over a Complex Message, Injected Headers

This is a signed-only S/MIME message via PKCS#7 signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 5700 bytes
 ⇩ (unwraps to)
 └┬╴multipart/mixed 1614 bytes
  ├┬╴multipart/alternative 950 bytes
  │├─╴text/plain 293 bytes
  │└─╴text/html 388 bytes
  └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="signed-data"
Subject: smime-one-part-complex-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:06:02 -0500
User-Agent: Sample MUA Version 1.0

MIIQbQYJKoZIhvcNAQcCoIIQXjCCEFoCAQExDTALBglghkgBZQMEAgEwggaWBgkq
hkiG9w0BBwGgggaHBIIGg01JTUUtVmVyc2lvbjogMS4wDQpTdWJqZWN0OiBzbWlt
ZS1vbmUtcGFydC1jb21wbGV4LWluamVjdGVkDQpNZXNzYWdlLUlEOiA8c21pbWUt
b25lLXBhcnQtY29tcGxleC1pbmplY3RlZEBsaHAuZXhhbXBsZT4NCkZyb206IEFs
aWNlIDxhbGljZUBzbWltZS5leGFtcGxlPg0KVG86IEJvYiA8Ym9iQHNtaW1lLmV4
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c2VyLUFnZW50OiBTYW1wbGUgTVVBIFZlcnNpb24gMS4wDQpDb250ZW50LVR5cGU6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cyBhIHNpZ25lZC1vbmx5IFMvTUlNRSBtZXNzYWdlIHZpYSBQS0NTIzcgc2lnbmVk
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dXNlcyB0aGUgSW5qZWN0ZWQgSGVhZGVycyBoZWFkZXINCnByb3RlY3Rpb24gc2No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VokFQgqQ7XNDU+r3SeOWwks7AgMBAAGjga8wgawwDAYDVR0TAQH/BAIwADAXBgNV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B.2.8. S/MIME Signed-only multipart/signed Over a Complex Message, Injected Headers

This is a signed-only S/MIME message via PKCS#7 detached signature (multipart/signed). The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme.

It has the following structure:

└┬╴multipart/signed 5580 bytes
 ├┬╴multipart/mixed 1672 bytes
 │├┬╴multipart/alternative 1006 bytes
 ││├─╴text/plain 312 bytes
 ││└─╴text/html 410 bytes
 │└─╴image/png inline 232 bytes
 └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

Its contents are:

MIME-Version: 1.0
Content-Type: multipart/signed;
 protocol="application/pkcs7-signature"; boundary="f91";
 micalg="sha-256"
Subject: smime-multipart-complex-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:07:02 -0500
User-Agent: Sample MUA Version 1.0

--f91
MIME-Version: 1.0
Subject: smime-multipart-complex-injected
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:07:02 -0500
User-Agent: Sample MUA Version 1.0
Content-Type: multipart/mixed; boundary="099"; protected-headers="v1"

--099
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="9a5"

--9a5
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

This is the smime-multipart-complex-injected message.

This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses the Injected Headers header protection
scheme.

--
Alice
[email protected]
--9a5
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

<html><head><title></title></head><body>
<p>This is the <b>smime-multipart-complex-injected</b> message.</p>
<p>This is a signed-only S/MIME message via PKCS#7 detached
signature (multipart/signed).  The payload is a
multipart/alternative message with an inline image/png
attachment. It uses the Injected Headers header protection
scheme.</p>
<p><tt>-- <br/>Alice<br/>[email protected]</tt></p></body></html>
--9a5--

--099
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

--099--

--f91
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"
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--f91--

B.3. Encrypted-and-signed Messages

These messages are encrypted and signed. They use PKCS#7 signedData inside envelopedData, with different header protection schemes and different Header Confidentiality Policies.

B.3.1. S/MIME Encrypted and Signed Over a Simple Message, Wrapped Message With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Wrapped Message header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7540 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4580 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 783 bytes
   └─╴text/plain 321 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:08:02 -0500
User-Agent: Sample MUA Version 1.0
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B.3.2. S/MIME Encrypted and Signed Over a Simple Message, Injected Headers With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7435 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4498 bytes
  ⇩ (unwraps to)
  └─╴text/plain 333 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:09:02 -0500
User-Agent: Sample MUA Version 1.0

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B.3.3. S/MIME Encrypted and Signed Over a Simple Message, Injected Headers With hcp_minimal (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7670 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4674 bytes
  ⇩ (unwraps to)
  └─╴text/plain 423 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:10:02 -0500
User-Agent: Sample MUA Version 1.0

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B.3.4. S/MIME Encrypted and Signed Over a Simple Message, Wrapped Message With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Wrapped Message header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7735 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4712 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 878 bytes
   └─╴text/plain 319 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:11:02 -0500

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B.3.5. S/MIME Encrypted and Signed Over a Simple Message, Injected Headers With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7605 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4630 bytes
  ⇩ (unwraps to)
  └─╴text/plain 331 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:12:02 -0500

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B.3.6. S/MIME Encrypted and Signed Over a Simple Message, Injected Headers With hcp_strong (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7845 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4806 bytes
  ⇩ (unwraps to)
  └─╴text/plain 420 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:13:02 -0500

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B.3.7. S/MIME Encrypted and Signed Reply Over a Simple Message, Wrapped Message With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Wrapped Message header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7800 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4770 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 920 bytes
   └─╴text/plain 327 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:14:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To: <[email protected]>
References: <[email protected]>

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TCet+3Ol0Otq5uCRkncOGVDbrJO2yqONU4Sq0oksMt6ZQIEZM8150kh+bVxu/ixt
86+BxkTFfKo+yOL5/K0Qo0J2WK1ADN16IKZbrr3kQFuVOnHmKcZrt/kwttlO8iFj

B.3.8. S/MIME Encrypted and Signed Reply Over a Simple Message, Injected Headers With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7695 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4692 bytes
  ⇩ (unwraps to)
  └─╴text/plain 339 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:15:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To: <[email protected]>
References: <[email protected]>

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B.3.9. S/MIME Encrypted and Signed Reply Over a Simple Message, Injected Headers With hcp_minimal (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7975 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4898 bytes
  ⇩ (unwraps to)
  └─╴text/plain 435 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:16:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To:
 <[email protected]>
References:
 <[email protected]>

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B.3.10. S/MIME Encrypted and Signed Reply Over a Simple Message, Wrapped Message With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Wrapped Message header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 8020 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4930 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 1038 bytes
   └─╴text/plain 325 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:17:02 -0500

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C0nuJO5Lu+dbA+wkaMCEBw==

B.3.11. S/MIME Encrypted and Signed Reply Over a Simple Message, Injected Headers With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 7930 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4856 bytes
  ⇩ (unwraps to)
  └─╴text/plain 337 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:18:02 -0500

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B.3.12. S/MIME Encrypted and Signed Reply Over a Simple Message, Injected Headers With hcp_strong (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a text/plain message. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 8190 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 5058 bytes
  ⇩ (unwraps to)
  └─╴text/plain 432 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 10:19:02 -0500

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B.3.13. S/MIME Encrypted and Signed Over a Complex Message, Wrapped Message With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9665 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6148 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 1923 bytes
   └┬╴multipart/mixed 1818 bytes
    ├┬╴multipart/alternative 1132 bytes
    │├─╴text/plain 375 bytes
    │└─╴text/html 473 bytes
    └─╴image/png inline 232 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:08:02 -0500
User-Agent: Sample MUA Version 1.0

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B.3.14. S/MIME Encrypted and Signed Over a Complex Message, Injected Headers With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9620 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6114 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1848 bytes
   ├┬╴multipart/alternative 1136 bytes
   │├─╴text/plain 387 bytes
   │└─╴text/html 482 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:09:02 -0500
User-Agent: Sample MUA Version 1.0

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B.3.15. S/MIME Encrypted and Signed Over a Complex Message, Injected Headers With hcp_minimal (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10205 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6548 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2157 bytes
   ├┬╴multipart/alternative 1431 bytes
   │├─╴text/plain 485 bytes
   │└─╴text/html 637 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:10:02 -0500
User-Agent: Sample MUA Version 1.0

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B.3.16. S/MIME Encrypted and Signed Over a Complex Message, Wrapped Message With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9840 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6276 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 2016 bytes
   └┬╴multipart/mixed 1911 bytes
    ├┬╴multipart/alternative 1128 bytes
    │├─╴text/plain 373 bytes
    │└─╴text/html 471 bytes
    └─╴image/png inline 232 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:11:02 -0500

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3EUkKKEGHIMQy1GrVMcAiQ==

B.3.17. S/MIME Encrypted and Signed Over a Complex Message, Injected Headers With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9795 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6246 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1941 bytes
   ├┬╴multipart/alternative 1132 bytes
   │├─╴text/plain 385 bytes
   │└─╴text/html 480 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:12:02 -0500

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B.3.18. S/MIME Encrypted and Signed Over a Complex Message, Injected Headers With hcp_strong (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10380 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6676 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2248 bytes
   ├┬╴multipart/alternative 1425 bytes
   │├─╴text/plain 482 bytes
   │└─╴text/html 634 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:13:02 -0500

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B.3.19. S/MIME Encrypted and Signed Reply Over a Complex Message, Wrapped Message With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9970 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6366 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 2082 bytes
   └┬╴multipart/mixed 1977 bytes
    ├┬╴multipart/alternative 1144 bytes
    │├─╴text/plain 381 bytes
    │└─╴text/html 479 bytes
    └─╴image/png inline 232 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:14:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To:
 <[email protected]>
References:
 <[email protected]>

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e/YMYKXibjeYXgEA8ln+Pw==

B.3.20. S/MIME Encrypted and Signed Reply Over a Complex Message, Injected Headers With hcp_minimal

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 9925 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6342 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2009 bytes
   ├┬╴multipart/alternative 1148 bytes
   │├─╴text/plain 393 bytes
   │└─╴text/html 488 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:15:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To:
 <[email protected]>
References:
 <[email protected]>

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B.3.21. S/MIME Encrypted and Signed Reply Over a Complex Message, Injected Headers With hcp_minimal (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_minimal Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10510 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6766 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2314 bytes
   ├┬╴multipart/alternative 1435 bytes
   │├─╴text/plain 487 bytes
   │└─╴text/html 639 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID:
 <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:16:02 -0500
User-Agent: Sample MUA Version 1.0
In-Reply-To:
 <[email protected]>
References:
 <[email protected]>

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B.3.22. S/MIME Encrypted and Signed Reply Over a Complex Message, Wrapped Message With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Wrapped Message header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10185 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6526 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 inline 2198 bytes
   └┬╴multipart/mixed 2093 bytes
    ├┬╴multipart/alternative 1140 bytes
    │├─╴text/plain 379 bytes
    │└─╴text/html 477 bytes
    └─╴image/png inline 232 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:17:02 -0500

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B.3.23. S/MIME Encrypted and Signed Reply Over a Complex Message, Injected Headers With hcp_strong

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10140 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6502 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2125 bytes
   ├┬╴multipart/alternative 1144 bytes
   │├─╴text/plain 391 bytes
   │└─╴text/html 486 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:18:02 -0500

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B.3.24. S/MIME Encrypted and Signed Reply Over a Complex Message, Injected Headers With hcp_strong (+ Legacy Display)

This is a encrypted and signed S/MIME message using PKCS#7 envelopedData around signedData. The payload is a multipart/alternative message with an inline image/png attachment. It uses the Injected Headers header protection scheme with the hcp_strong Header Confidentiality Policy with a "Legacy Display" part.

It has the following structure:

└─╴application/pkcs7-mime [smime.p7m] 10790 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6968 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2460 bytes
   ├┬╴multipart/alternative 1449 bytes
   │├─╴text/plain 494 bytes
   │└─╴text/html 646 bytes
   └─╴image/png inline 236 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <[email protected]>
From: Alice <[email protected]>
To: Bob <[email protected]>
Date: Sat, 20 Feb 2021 12:19:02 -0500

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Appendix C. Composition Examples

This section offers step-by-step examples of message composition.

C.1. New message composition

A typical MUA composition interface offers the user a place to indicate the message recipients, the subject, and the body. Consider a composition window filled out by the user like so:

Composing New Message Send To: Alice <[email protected]> Subject: Handling the Jones contract Please review and approve or decline by Thursday, it's critical! Thanks, Bob -- Bob Gonzalez ACME, Inc.
Figure 1: Example Message Composition Interface

When Bob clicks "Send", his MUA generates values for Message-ID, From, and Date header fields, and converts the message body into the appropriate format.

C.1.1. Unprotected message

The resulting message would look something like this if it was sent without cryptographic protections:

Date: Wed, 11 Jan 2023 16:08:43 -0500
From: Bob <[email protected]>
To: Alice <[email protected]>
Subject: Handling the Jones contract
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0

Please review and approve or decline by Thursday, it's critical!

Thanks,
Bob

--
Bob Gonzalez
ACME, Inc.

C.1.2. Encrypted with hcp_minimal and Legacy Display

Now consider the message to be generated if it is to be cryptographically signed and encrypted, using HCP hcp_minimal, and the legacy variable is set.

For each header field, Bob's MUA passes its name and value through hcp_minimal. This returns the same value for every header field, except that:

hcp_minimal("Subject", "Handling the Jones contract") yields "[...]".

C.1.2.1. Cryptographic Payload

The Cryptographic Payload that will be signed and then encrypted is very similar to the unprotected message in Appendix C.1.1. Note the addition of:

  • the protected-headers="v1" parameter for the Content-Type
  • the appropriate HP-Obscured header for Subject,
  • the hp-legacy-display="1" parameter for the Content-Type
  • the Legacy Display Element (the simple pseudo-header and its trailing newline) in the main body part. 
Date: Wed, 11 Jan 2023 16:08:43 -0500
From: Bob <[email protected]>
To: Alice <[email protected]>
Subject: Handling the Jones contract
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
 protected-headers="v1"
MIME-Version: 1.0
HP-Obscured: Subject: [...]

Subject: Handling the Jones contract

Please review and approve or decline by Thursday, it's critical!

Thanks,
Bob

--
Bob Gonzalez
ACME, Inc.
C.1.2.2. External header section

The Cryptographic Payload from Appendix C.1.2.1 is then wrapped in the appropriate cryptographic layers. For this example, using S/MIME, it is wrapped in an application/pkcs7-mime; smime-type="signed-data" layer, which is in turn wrapped in a application/pkcs7-mime; smime-type="enveloped-data" layer.

Then an external header section is applied to the outer MIME object, which looks like this:

Date: Wed, 11 Jan 2023 16:08:43 -0500
From: Bob <[email protected]>
To: Alice <[email protected]>
Subject: [...]
Message-ID: <[email protected]>
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
MIME-Version: 1.0

Note that the Subject header field has been obscured appropriately by hcp_minimal. The output of the CMS enveloping operation is base64-encoded and forms the body of the message.

C.2. Composing a Reply

Next we consider a typical MUA reply interface, where we see Alice replying to Bob's message from Appendix C.1.

When Alice clicks "Reply" to Bob's signed-and-encrypted message with header protection, she might see something like this:

Replying to Bob ("Handling the Jones Contract") Send To: Bob <[email protected]> Subject: Re: Handling the Jones contract On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote: > Please review and approve or decline by Thursday, > it's critical! > > Thanks, > Bob > > -- > Bob Gonzalez > ACME, Inc. -- Alice Jenkins ACME, Inc.
Figure 2: Example Message Reply Interface (unedited)

Note that because Alice's MUA is aware of header protection, it knows what the correct Subject header is, even though it was obscured. It also knows to avoid including the Legacy Display Element in the quoted/attributed text that it includes in the draft reply.

Once Alice has edited the reply message, it might look something like this:

Replying to Bob ("Handling the Jones Contract") Send To: Bob <[email protected]> Subject: Re: Handling the Jones contract On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote: > Please review and approve or decline by Thursday, > it's critical! I'll get right on it, Bob! Regards, Alice -- Alice Jenkins ACME, Inc.
Figure 3: Example Message Reply Interface (edited)

When Alice clicks "Send", the MUA generates values for Message-ID, From, and Date header fields, populates the In-Reply-To, and References header fields, and also converts the reply body into the appropriate format.

C.2.1. Unprotected message

The resulting message would look something like this if it were to be sent without any cryptographic protections:

Date: Wed, 11 Jan 2023 16:48:22 -0500
From: Alice <[email protected]>
To: Bob <[email protected]>
Subject: Re: Handling the Jones contract
Message-ID: <[email protected]>
In-Reply-To: <[email protected]>
References: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0

On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

> Please review and approve or decline by Thursday,
> it's critical!

I'll get right on it, Bob!

Regards,
Alice

--
Alice Jenkins
ACME, Inc.

Of course, this would leak not only the contents of Alice's message, but also the contents of Bob's initial message, as well as the Subject header field! So Alice's MUA won't do that; it is going to create a signed-and-encrypted message to submit to the network.

C.2.2. Encrypted with hcp_null and Legacy Display

This example assumes that Alice's MUA uses hcp_null, not hcp_minimal. That is, by default, it does not obscure or remove any header fields, even when encrypting.

However, it follows the guidance in Section 2.5.8.1, and will make use of the HP-Obscured field in the Cryptographic Payload of Bob's original message (Appendix C.1.2.1) to determine what to obscure.

When crafting the Cryptographic Payload, its baseline HCP (hcp_null) leaves each field untouched. But it also knows that In-Reply-To, References, To, and Subject are all derived from headers in Bob's original message.

For each of these header fields, it observes whether the origin header field was signed-and-encrypted or merely signed in Bob's original message.

In-Reply-To and References derive from Bob's original message's Message-ID field, which was merely signed. The To header field is derived from Bob's original message's From field, which was also merely signed. So these three header fields are passed through untouched.

But the Subject header field is derived from Bob's original message's Subject field (by prefixing Re: to it), and that header field is signed-and-encrypted, which the MUA can tell because the HP-Obscured: Subject entry in the Cryptographic Payload of Bob's message.

So Alice's MUA generates a new external Subject header by applying its derivation rules to the HP-Obscured: Subject value from Bob's message, yielding the value Re: [...].

C.2.2.1. Cryptographic Payload

Consesquently, the Cryptographic Payload for Alice's reply looks like this:

Date: Wed, 11 Jan 2023 16:48:22 -0500
From: Alice <[email protected]>
To: Bob <[email protected]>
Subject: Re: Handling the Jones contract
Message-ID: <[email protected]>
In-Reply-To: <[email protected]>
References: <[email protected]>
Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
 protected-headers="v1"
MIME-Version: 1.0
HP-Obscured: Subject: Re: [...]

Subject: Re: Handling the Jones contract

On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

> Please review and approve or decline by Thursday,
> it's critical!

I'll get right on it, Bob!

Regards,
Alice

--
Alice Jenkins
ACME, Inc.

Note the following features:

  • the protected-header="v1" parameter to Content-Type
  • the appropriate HP-Obscured header for Subject,
  • the hp-legacy-display="1" parameter for the Content-Type
  • the Legacy Display Element (the simple pseudo-header and its trailing newline) in the main body part.
C.2.2.2. External header section

The Cryptographic Payload from Appendix C.2.2.1 is then wrapped in the appropriate cryptographic layers. For this example, using S/MIME, it is wrapped in an application/pkcs7-mime; smime-type="signed-data" layer, which is in turn wrapped in a application/pkcs7-mime; smime-type="enveloped-data" layer.

Then an external header section is applied to the outer MIME object, which looks like this:

Date: Wed, 11 Jan 2023 16:48:22 -0500
From: Alice <[email protected]>
To: Bob <[email protected]>
Subject: Re: [...]
Message-ID: <[email protected]>
In-Reply-To: <[email protected]>
References: <[email protected]>
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
MIME-Version: 1.0

Note that the Subject header field has been obscured appropriately even though hcp_null would not have touched it by default. The output of the CMS enveloping operation is base64-encoded and forms the body of the message.

Appendix D. Rendering Examples

This section offers example Cryptographic Payloads (the content within the Cryptographic Envelope) that contain Legacy Display elements.

D.1. Example text/plain Cryptographic Payload with Legacy Display Elements

Here is a simple one-part Cryptographic Payload (headers and body) of a message that includes Legacy Display elements:

Date: Fri, 21 Jan 2022 20:40:48 -0500
From: Alice <[email protected]>
To: Bob <[email protected]>
Subject: Dinner plans
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
 protected-headers="v1"

Subject: Dinner plans

Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.

A compatible MUA will recognize the hp-legacy-display="1" parameter and render the body of the message as:

Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.

A legacy decryption-capable MUA that is unaware of this mechanism will ignore the hp-legacy-display="1" parameter and instead render the body including the Legacy Display elements:

Subject: Dinner plans

Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.

D.2. Example text/html Cryptographic Payload with Legacy Display Elements

Here is a modern one-part Cryptographic Payload (headers and body) of a message that includes Legacy Display elements:

Date: Fri, 21 Jan 2022 20:40:48 -0500
From: Alice <[email protected]>
To: Bob <[email protected]>
Subject: Dinner plans
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: text/html; charset="us-ascii"; hp-legacy-display="1";
 protected-headers="v1"

<html><head><title></title></head><body>
<div class="header-protection-legacy-display">
<pre>Subject: Dinner plans</pre>
</div>
<p>
Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.
</p>
</body>
</html>

A compatible MUA will recognize the hp-legacy-display="1" parameter and mask out the Legacy Display div, rendering the body of the message as a simple paragraph:

Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.

A legacy decryption-capable MUA that is unaware of this mechanism will ignore the hp-legacy-display="1" parameter and instead render the body including the Legacy Display elements:

Subject: Dinner plans

Let's meet at Rama's Roti Shop at 8pm and go to the park
from there.

Appendix E. Document Changelog

[[ RFC Editor: This section is to be removed before publication ]]

Authors' Addresses

Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Bernie Hoeneisen
pEp Foundation
Oberer Graben 4
CH- CH-8400 Winterthur
Switzerland
URI: https://pep.foundation/
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom