Internet-Draft Header Protection S/MIME May 2022
Gillmor, et al. Expires 22 November 2022 [Page]
Workgroup:
LAMPS Working Group
Internet-Draft:
draft-ietf-lamps-header-protection-latest
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 S/MIME

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. Furthermore, it 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:spasm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/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 22 November 2022.

Table of Contents

1. Introduction

Privacy and security issues regarding email Header Protection in S/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 structures for how message headers can be cryptographically protected, and provides guidance for implementers of MUAs that generate and interpret such messages. It 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 email header sections or fields and provides guidance to implementers.

One scheme is the form specified in S/MIME 3.1 and later, which 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).

Consequently, another form of header protection is introduced, where 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 2.3.3 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 the worst 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 email message, which -- in fact -- contains the protected email 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 that cannot be delivered is less useful than a message with perfect cryptographic protections. 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 Email Header Fields

A separate pair of protocols also provides some cryptographic protection for the email 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 email (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 DKIM+DMARC not provide end-to-end protection. DKIM+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, DKIM+DMARC only provides cryptographic integrity and authentication, not encryption. So cryptographic confidentiality is not available from that suite.

DKIM+DMARC 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 Email 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: cryptographic protection of email Header Sections (or parts of it) for signatures and/or encryption
  • Cryptographic Layer, Cryptographic Payload, Cryptographic Envelope, 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.

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.

1.9.1. Out of Scope

While the generation guidance aims to provide minimal disruption for any legacy client, such a client by definition does not implement this document.

Therefore, the document does not attempt to provide guidance for legacy clients.

Furthermore, this document does not explicitly contemplate unusual (and tricky) 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.

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 2.3.3), and MAY generate the Wrapped Message scheme (Section 2.3.4).

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 Cryptograhic Payload itself. These decorative copies within the message are known as "legacy display elements".

Composing a message with the Injected Headers scheme is described in Section 2.3.3. 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 2.3.4. 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

[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 header field name name and 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.

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

The revised algorithm for applying 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 payload 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 2.3.3.1 for text/plain and Section 2.3.3.2 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 of MIME part payload with value v
  • Set the protected-headers parameter on the Content-Type of payload to v1
  • Apply crypto to payload, producing MIME tree output
  • If crypto contains encryption:

    • Create new empty list of header field names and values newh
    • For header field name and value (h,v) in origheaders:

      • Let newval be hcp(h,v)
      • If newval is not null:

        • Add newh[h] to newval
    • Set origheaders to newh
  • For each header field name and value (h,v) in origheaders:

    • Add header field h of 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.

2.3.3.1. 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", "alice@example.net"), ("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: alice@example.net

I think we should skip the meeting.
2.3.3.2. 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 2.3.3.1). 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 2.3.3.1, 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", "alice@example.net"), ("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: alice@example.net</pre></div>
<p>I think we should skip the meeting.</p>
</body></html>
2.3.3.3. 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 [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.

2.3.3.4. 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.

2.3.4. 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 forwarded parameter as described in [I-D.melnikov-iana-reg-forwarded].

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 of origbody with value v
  • 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; forwarded=no, and whose body is origbody.
  • Else:

    • Let payload be a new MIME part with one header field: Content-Type: message/rfc822; forwarded=no, and whose body is origbody.
  • Apply crypto to payload, yielding MIME tree output
  • If crypto contains encryption:

    • Create new empty list of header field names and values newh
    • For header field name and value (h,v) in origheaders:

      • Let newval be hcp(h,v)
      • If newval is not null:

        • Append (h,newval) to newh
    • Set origheaders to newh
  • For header field name and value (h,v) in origheaders:

    • Add header field h of 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.

2.3.5. 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 2.3.3 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. Minimalist 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.

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 [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 forwarded has a value of no. 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.

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.

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. As in Section 2.5.4.2, each rendered header field found in G should be compared against the header field of the same name from E. If the value found in E matches the value found in G, the header field should be marked as signed-only. If no matching header field was found in E, or the value found did not match the value from G, the header field should be marked as signed-and-encrypted.

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 or generating a draft reply.

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 with 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.

A straightforward way for an HTML-capable MUA to do this is 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 forwarded is set to no, 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. Each rendered header field found in Q should be compared against the header field of the same name from O. If the value found in O matches the value found in Q, the header field should be marked as signed-only. If no matching header field was found in O, or the value found did not match the value from Q, the header field should be marked as signed-and-encrypted.

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 mssages 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 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.

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 email 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 email messages that also provide header protection [I-D.pep-email]. Similar to the "Wrapped Messages" scheme described in Section 2.3.4 and Section 2.5.4, pEp email 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 [I-D.ietf-lamps-e2e-mail-guidance], an MUA in this position MUST NOT leak previously-encrypted content in the clear in a followup 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.

[[ TODO: formally describe how a replying MUA should generate a message-specific Header Protection policy based on the cryptographic status of the headers of the incoming message ]]

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 (e.g., to test whether they are covered by a valid DKIM-Signature, see [RFC6376]) before rendering or acting on them.

Specific examples 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.

FIXME: other examples of unprotected transit header fields?

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. [[ TODO: update to refer to whichever one we recommend ]]

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.

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 section in [I-D.ietf-lamps-e2e-mail-guidance].

5. Security Considerations

[[ TODO ]]

6. Privacy Considerations

[[ TODO: add more 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 where the protected encrypted value does not match the value outside the message 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.

6.1.1. 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 bob@example.org, 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.

6.1.2. 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.

6.1.3. An Intermediary Can Trick a Recipient into Overestimating 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.

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 recipient would believe that the field has a cryptographic status of signed+encrypted.

An attacker can thus induce a false sense of confidentiality in a recipient whose MUA actually did not provide that level of privacy.

6.1.4. 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.

7. IANA Considerations

This document requests no action from IANA.

[[ RFC Editor: This section may be removed before publication. ]]

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, Claudio Luck, David Wilson, Hernani Marques, juga, Krista Bennett, Kelly Bristol, Lars Rohwedder, 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., "Guidance on End-to-End E-mail Security", Work in Progress, Internet-Draft, draft-ietf-lamps-e2e-mail-guidance-02, , <https://www.ietf.org/archive/id/draft-ietf-lamps-e2e-mail-guidance-02.txt>.
[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://www.ietf.org/archive/id/draft-ietf-lamps-header-protection-requirements-01.txt>.
[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/info/rfc2045>.
[RFC2046]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, , <https://www.rfc-editor.org/info/rfc2046>.
[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/info/rfc2119>.
[RFC5322]
Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, , <https://www.rfc-editor.org/info/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/info/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/info/rfc8551>.

9.2. Informative References

[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/>.
[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://www.ietf.org/archive/id/draft-ietf-lamps-samples-08.txt>.
[I-D.melnikov-iana-reg-forwarded]
Melnikov, A. and B. Hoeneisen, "IANA Registration of Content-Type Header Field Parameter 'forwarded'", Work in Progress, Internet-Draft, draft-melnikov-iana-reg-forwarded-00, , <https://www.ietf.org/archive/id/draft-melnikov-iana-reg-forwarded-00.txt>.
[I-D.pep-email]
Marques, H., "pretty Easy privacy (pEp): Email Formats and Protocols", Work in Progress, Internet-Draft, draft-pep-email-01, , <https://www.ietf.org/archive/id/draft-pep-email-01.txt>.
[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-00, , <https://www.ietf.org/archive/id/draft-pep-general-00.txt>.
[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>.
[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/info/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/info/rfc3156>.
[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/info/rfc6376>.
[RFC6532]
Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, , <https://www.rfc-editor.org/info/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/info/rfc7489>.

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+encrypted Messages in List View

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

A.2. Problems when Rendering a signed+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+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://bob@header-protection.cmrg.net/ (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: <no-crypto@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:00:02 -0500

This is the no-crypto message.

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

--
Alice
alice@smime.example

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: <smime-one-part@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:01:02 -0500
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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 4156 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="76c";
 micalg="sha-256"
Subject: smime-multipart
Message-ID: <smime-multipart@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:02:02 -0500

--76c
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
alice@smime.example

--76c
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"
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--76c--

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: <smime-enc-signed@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:03:02 -0500
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sAa1LzD78Dg4FKO8t3d13Q==

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 1371 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="428"
Subject: no-crypto-complex
Message-ID: <no-crypto-complex@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:00:02 -0500

--428
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="db9"

--db9
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
alice@smime.example
--db9
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/>alice@smime.example</tt></p></body></html>
--db9--

--428
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

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

--428--

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: <smime-one-part-complex@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:01:02 -0500

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dX9CqaJcOvT4as6aqdcwDQYJKoZIhvcNAQENBQAwVTENMAsGA1UEChMESUVURjER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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 5199 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="e18";
 micalg="sha-256"
Subject: smime-multipart-complex
Message-ID: <smime-multipart-complex@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:02:02 -0500

--e18
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="831"

--831
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="a1e"

--a1e
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
alice@smime.example
--a1e
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/>alice@smime.example</tt></p></body></html>
--a1e--

--831
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

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

--831--

--e18
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"
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saqpo1d3f9jSkbtAV5w3vzfog8919MxKI9H6l4KuElnAtJ7BtZcsl7dUy9u9COgE
ykRiVokFQgqQ7XNDU+r3SeOWwks7AgMBAAGjga8wgawwDAYDVR0TAQH/BAIwADAX
BgNVHSAEEDAOMAwGCmCGSAFlAwIBMAEwHgYDVR0RBBcwFYETYWxpY2VAc21pbWUu
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VR0OBBYEFLv2zLItHQYSHJeuKWqQENMgZmZzMB8GA1UdIwQYMBaAFJEwjnwHFwyn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--e18--

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: <smime-enc-signed-complex@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:03:02 -0500

MIIZDAYJKoZIhvcNAQcDoIIY/TCCGPkCAQAxggMQMIIBhAIBADBsMFUxDTALBgNV
BAoTBElFVEYxETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhTYW1wbGUgTEFN
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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] 4213 bytes
 ⇩ (unwraps to)
 └┬╴message/rfc822 566 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: <smime-one-part-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:04:02 -0500

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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 4451 bytes
 ├┬╴message/rfc822 596 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="20c";
 micalg="sha-256"
Subject: smime-multipart-wrapped
Message-ID: <smime-multipart-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:05:02 -0500

--20c
MIME-Version: 1.0
Content-Type: message/rfc822; forwarded="no"

MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Subject: smime-multipart-wrapped
Message-ID: <smime-multipart-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:05:02 -0500

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
alice@smime.example

--20c
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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arUCAwEAAaOBrzCBrDAMBgNVHRMBAf8EAjAAMBcGA1UdIAQQMA4wDAYKYIZIAWUD
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8OeOr83zdw8wHwYDVR0jBBgwFoAUkTCOfAcXDKfxCShlNhpnHGh29FkwDQYJKoZI
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zkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLvLir3hEUV2Z3MRsMt
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bKZa1oPTUNlm6QpivtqDIdqGJdGf8L1zLfXBuo2zL3HR+M9CDr4Opq2JCkzP0Qhp
7poIccGE6I9Tsg+RrOA9iCQsPn1+Tg8YedjGzUWF07rNmT0TzPCVzUAuBlr+JJtz
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MC8GCSqGSIb3DQEJBDEiBCCcDIxr7wd3VCCz1VBG9nySvUJ/Fhzo26f78El/UUbj
jTANBgkqhkiG9w0BAQEFAASCAQBUmMGL40IZQmt3Nad/ymEUOLu3Dgfd/nYKuj6P
fjKYJFb9UhwtufZK9/WyVtytLsFJMYHZgUSWU3VbHk1L/cO0469Rbqo6CqlLRJPK
uN2Eul2UCa+3ovMIQ8g0NBflXrdfR0OVRqvfO91hLFkTxLfCDUG8ziRWOLWucgZg
zkVXqEzvFyOtsSbr3GAY817wWgl1+PTFchO4XF+rg7cNysKqGLtjxP9lN3PcURYv
TmooTPY46kheab7ZAzKqQI6go7somKmMqD7UsctMLSVZo+EX5/N9vq5znv7bfpoE
Rgd+NZNQD+VYDIOU1FI5ZjyjHpRmcFpywjvHNbTBGlYhv3q4

--20c--

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] 4185 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: <smime-one-part-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:06:02 -0500

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hkiG9w0BBwGgggIoBIICJE1JTUUtVmVyc2lvbjogMS4wDQpDb250ZW50LVRyYW5z
ZmVyLUVuY29kaW5nOiA3Yml0DQpTdWJqZWN0OiBzbWltZS1vbmUtcGFydC1pbmpl
Y3RlZA0KTWVzc2FnZS1JRDogPHNtaW1lLW9uZS1wYXJ0LWluamVjdGVkQGxocC5l
eGFtcGxlPg0KRnJvbTogQWxpY2UgPGFsaWNlQHNtaW1lLmV4YW1wbGU+DQpUbzog
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dD0idXRmLTgiOyBwcm90ZWN0ZWQtaGVhZGVycz0idjEiDQoNClRoaXMgaXMgdGhl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BQADggEPADCCAQoCggEBAJqVKfqLwaLjj+gBUCfkacKTg8cc2OtJ9ZSed6U3jUoi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doizcGMk53riugAocCn5zbzhW/JVdYn30UxfyrZlRAzEf7GHqgB/NyjOad3pdpVY
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MBwGCSqGSIb3DQEJBTEPFw0yMTAyMjAxNTA2MDJaMC8GCSqGSIb3DQEJBDEiBCA7
4grfze+Y7DQEGFAYHyyvRpNkuuZFR0V+RvSTvu4FGDANBgkqhkiG9w0BAQEFAASC
AQB1KYVvQNZpe3EKeM0XhJrlJNxneVmZWFCEl5YFeRsO8FeIwJkV65YtFJKjOVVy
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+6dw3+ORgfc67VTHVnsVVfb0UPrWvdxFdL5xYdqXxlhDsLMEms2ttHHzvjC003Kq
As0xMHEmMpfdL5M69MAjvroOUv0SXETfQaxca7IKd+9xUNNRretZ9xz2kn2uD+k7
unTEyVGeHrWmQMw/8MdvEac/

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 4417 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="12b";
 micalg="sha-256"
Subject: smime-multipart-injected
Message-ID: <smime-multipart-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:07:02 -0500

--12b
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: smime-multipart-injected
Message-ID: <smime-multipart-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:07:02 -0500
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
alice@smime.example

--12b
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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G25nY/0eNARD6/oF0/sYonX2doizcGMk53riugAocCn5zbzhW/JVdYn30UxfyrZl
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bKZa1oPTUNlm6QpivtqDIdqGJdGf8L1zLfXBuo2zL3HR+M9CDr4Opq2JCkzP0Qhp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--12b--

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] 5631 bytes
 ⇩ (unwraps to)
 └┬╴message/rfc822 1613 bytes
  └┬╴multipart/mixed 1549 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: <smime-one-part-complex-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:04:02 -0500

MIIQOgYJKoZIhvcNAQcCoIIQKzCCECcCAQExDTALBglghkgBZQMEAgEwggZjBgkq
hkiG9w0BBwGgggZUBIIGUE1JTUUtVmVyc2lvbjogMS4wDQpDb250ZW50LVR5cGU6
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IGlzIHRoZSBzbWltZS1vbmUtcGFydC1jb21wbGV4LXdyYXBwZWQgbWVzc2FnZS4K
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bD48aGVhZD48dGl0bGU+PC90aXRsZT48L2hlYWQ+PGJvZHk+CjxwPlRoaXMgaXMg
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aWEgUEtDUyM3IHNpZ25lZERhdGEuICBUaGUKcGF5bG9hZCBpcyBhIG11bHRpcGFy
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oIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJKoZIhvcN
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AaOBrzCBrDAMBgNVHRMBAf8EAjAAMBcGA1UdIAQQMA4wDAYKYIZIAWUDAgEwATAe
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dw8wHwYDVR0jBBgwFoAUkTCOfAcXDKfxCShlNhpnHGh29FkwDQYJKoZIhvcNAQEN
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1/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLvLir3hEUV2Z3MRsMtjH2x9SG9
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7FOD7PfB5v94M5274XYxW2W4uKGd7QGnUZROSvSYkGiWDp1JhqXwfDz8A0enITGX
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qNpHNZ015fOURza1SkCvi9GFmNUPoVgwggPPMIICt6ADAgECAhM3QQV57XV/Qqmi
XDr0+GrOmqnXMA0GCSqGSIb3DQEBDQUAMFUxDTALBgNVBAoTBElFVEYxETAPBgNV
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FLv2zLItHQYSHJeuKWqQENMgZmZzMB8GA1UdIwQYMBaAFJEwjnwHFwyn8QkoZTYa
ZxxodvRZMA0GCSqGSIb3DQEBDQUAA4IBAQBziaI2p86poGkjd/4KkkOHG25nY/0e
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UNlm6QpivtqDIdqGJdGf8L1zLfXBuo2zL3HR+M9CDr4Opq2JCkzP0Qhp7poIccGE
6I9Tsg+RrOA9iCQsPn1+Tg8YedjGzUWF07rNmT0TzPCVzUAuBlr+JJtzOKypyQ3e
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A1UEAxMoU2FtcGxlIExBTVBTIFJTQSBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eQIT
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CwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0yMTAyMjAxNzA0MDJaMC8GCSqG
SIb3DQEJBDEiBCDMOILEox46FkWxHI/3mD5yDe0N8CAfZ/xaQnI0alyyOTANBgkq
hkiG9w0BAQEFAASCAQBWzuGAP7C0InZ86JeaKimYKXpArooRzZnso+wJtXhZlmTX
csHp783QCEKYE0F+rv1IrD+fcFULz8Lo7Mm+PWQbtkbx5uZR7IFLGlK+8i8wVCZj
1Bs2lgpZ/qg1qP+ddCPwZuywITEGnjjqg76OHJOgxJniG3/teIy6dHMI2OBogZjN
kdVSbBhOa9GnTtnWJd2zH7t0tV16NyH3+pNn4DTUWR2IvRgxHky/KT7cIOTfQj9C
HEizTljQMDvHhoHslWdwjAGjH3foH4CXP1/1bN+qBH2QAuRZ8+LueDcllQsPJXtc
fUseHVMstoHac0rajLjDZ8FXSLCkmto6RRSQVsT0

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 5542 bytes
 ├┬╴message/rfc822 1671 bytes
 │└┬╴multipart/mixed 1607 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="ce9";
 micalg="sha-256"
Subject: smime-multipart-complex-wrapped
Message-ID: <smime-multipart-complex-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:05:02 -0500

--ce9
MIME-Version: 1.0
Content-Type: message/rfc822; forwarded="no"

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="c33"
Subject: smime-multipart-complex-wrapped
Message-ID: <smime-multipart-complex-wrapped@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:05:02 -0500

--c33
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="bb6"

--bb6
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
alice@smime.example
--bb6
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/>alice@smime.example</tt></p></body></html>
--bb6--

--c33
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

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

--c33--

--ce9
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

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8QkoZTYaZxxodvRZMA0GCSqGSIb3DQEBDQUAA4IBAQBziaI2p86poGkjd/4KkkOH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--ce9--

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] 5651 bytes
 ⇩ (unwraps to)
 └┬╴multipart/mixed 1579 bytes
  ├┬╴multipart/alternative 950 bytes
  │├─╴text/plain 292 bytes
  │└─╴text/html 387 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: <smime-one-part-complex-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:06:02 -0500
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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 5510 bytes
 ├┬╴multipart/mixed 1637 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="34f";
 micalg="sha-256"
Subject: smime-multipart-complex-injected
Message-ID: <smime-multipart-complex-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:07:02 -0500

--34f
MIME-Version: 1.0
Subject: smime-multipart-complex-injected
Message-ID: <smime-multipart-complex-injected@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:07:02 -0500
Content-Type: multipart/mixed; boundary="193"; protected-headers="v1"

--193
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="db5"

--db5
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
alice@smime.example
--db5
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/>alice@smime.example</tt></p></body></html>
--db5--

--193
Content-Type: image/png
Content-Transfer-Encoding: base64
Content-Disposition: inline

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

--193--

--34f
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature; name="smime.p7s"

MIIJ4AYJKoZIhvcNAQcCoIIJ0TCCCc0CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
hvcNAQcBoIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJ
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acKTg8cc2OtJ9ZSed6U3jUoiZVpMLcP3MUKtLeLg9r1mAfIDlB/wlbdmadXPmrsz
yidmbuZmOpB5voVQfiLYYy3iOx7YOqzXrl6udP07k0sV+UdSNRFxrfKeoQEFXgOa
Gdmnx4OG/e3p1fIKM0dPzZLoOAJF5m5O0xzXPL74zFCWp2f1ZkuE4A6l41koaZXC
N5XL7wWTLMLeNf9Byb5ksKqUuqEHAMd1nmoNMgjY9VfVfcrv9w43GG8FtpSX+TWz
B2zNS2OF+XIVnzRG5DeoULq8v88Z5bLpIJ/nx26r8A4SSwIBaVv4wPxAf1iPsIVK
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AgEwATAeBgNVHREEFzAVgRNhbGljZUBzbWltZS5leGFtcGxlMBMGA1UdJQQMMAoG
CCsGAQUFBwMEMA4GA1UdDwEB/wQEAwIFIDAdBgNVHQ4EFgQUolNB1UQ8gCkVfAEj
8OeOr83zdw8wHwYDVR0jBBgwFoAUkTCOfAcXDKfxCShlNhpnHGh29FkwDQYJKoZI
hvcNAQENBQADggEBAIFJeKCcsTKcFqQMpTryujRGzJdYA+R9eBAuDLsatbtKtl4F
zkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLvLir3hEUV2Z3MRsMt
jH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5UzpEYPLror2X4P5uXxaP0LIZR
zWmkw1RF7FOD7PfB5v94M5274XYxW2W4uKGd7QGnUZROSvSYkGiWDp1JhqXwfDz8
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qm6hvrDTqNpHNZ015fOURza1SkCvi9GFmNUPoVgwggPPMIICt6ADAgECAhM3QQV5
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ykRiVokFQgqQ7XNDU+r3SeOWwks7AgMBAAGjga8wgawwDAYDVR0TAQH/BAIwADAX
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cml0eQITN0EFee11f0Kpolw69Phqzpqp1zALBglghkgBZQMEAgGgaTAYBgkqhkiG
9w0BCQMxCwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0yMTAyMjAxNzA3MDJa
MC8GCSqGSIb3DQEJBDEiBCBpheScfJ+ESh8/z2r5jHx3Lw+5VkH8zTicO3HRGxfm
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q5893RxO3g5zzxaYJP03zyO/WW7FmCJNNQbyZbQD8R4rvR0hVna0r7XoW4Q+WZfU
Dz29oLszzmumpedAaP7q/M0jySdSjWfQn1W5hHHhAMIlwcqt

--34f--

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] 7345 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4436 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 679 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: <smime-enc-signed-wrapped-minimal@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:08:02 -0500

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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] 7305 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4406 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: <smime-enc-signed-injected-minimal@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:09:02 -0500

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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] 7565 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4584 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:
 <smime-enc-signed-injected-minimal-legacy@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:10:02 -0500

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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] 7345 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4432 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 675 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: <73a42f8e-8f5a-5c62-b982-82ace766fd32@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:11:02 -0500

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mtSbNmSuhSGqYP3fKWV4inAgRQiDDw3bnazMh/mI17qMLa25lzP9IJ5RNDRRWCjf
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Rbb04HvCvu3bBNjy8D6yRlFVIVxH3Zy7+iz3fJ70VwlqqpmlnMsidx3v1ykAeK1t
uo42n/3t82Dx/5s3p9rZnhWXUdO0etjL88GpyzvdwtkYy3Nj/8afvB62iUwZ1fR5
rcnklWkphSq9HL6brXQsS3lODDHsy8xIJlu5RrGD2MOIOy/rbMxNT5WnGoZ6j/RJ
Spn1f944h2LkyVFFNgIlq1W6MLfTNBrZZ6kMpJ8X39iL5KmkrQ1me1rgJTtM4heK

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] 7305 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4402 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: <27139e00-e05f-581d-a339-d2bd43bd0f42@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 7540 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4576 bytes
  ⇩ (unwraps to)
  └─╴text/plain 419 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <fdccb76a-49ed-50c5-9030-e4aeb83d7f04@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 7605 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4626 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 816 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: <smime-enc-signed-wrapped-minimal-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:14:02 -0500
In-Reply-To: <smime-enc-signed-wrapped-minimal@lhp.example>
References: <smime-enc-signed-wrapped-minimal@lhp.example>

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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] 7585 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4600 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:
 <smime-enc-signed-injected-minimal-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:15:02 -0500
In-Reply-To: <smime-enc-signed-injected-minimal@lhp.example>
References: <smime-enc-signed-injected-minimal@lhp.example>

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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] 7845 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4806 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:
 <smime-enc-signed-injected-minimal-legacy-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:16:02 -0500
In-Reply-To:
 <smime-enc-signed-injected-minimal-legacy@lhp.example>
References:
 <smime-enc-signed-injected-minimal-legacy@lhp.example>

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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] 7605 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4616 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 810 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: <0e210732-9184-5855-9a95-2a635560d3a6@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:17:02 -0500

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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] 7565 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4592 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: <0b3ea6dd-0e91-5a91-9bc0-3d553f892983@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 10:18:02 -0500

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w7LgVMRJKMMDllquSaKDrQ==

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] 7845 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 4794 bytes
  ⇩ (unwraps to)
  └─╴text/plain 431 bytes

Its contents are:

Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-mime; name="smime.p7m";
 smime-type="enveloped-data"
Subject: [...]
Message-ID: <b10dcc75-cf43-5fd7-9e48-f932a9d68fb5@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 9470 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6002 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 1819 bytes
   └┬╴multipart/mixed 1755 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:
 <smime-enc-signed-complex-wrapped-minimal@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:08:02 -0500

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82mWq9DRGbLCrhxbp8iAu4omBxQe1mGGRRT2WtBwkAvQr2O6sX/RU3nBVt4NvHwN
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F15GHyRMedBfg/3T8VnZSj/39dJ//+xSogpITDQc4yW7u5WKDvS47xQJ142yh4k7
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wmSyyQOnKRpzyLDGZUuUehGyY4C4AZ7utFzxG8SBOdg=

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] 9515 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6028 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1785 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:
 <smime-enc-signed-complex-injected-minimal@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:09:02 -0500

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iskAuLrYCHbASEVkcHYOBw==

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] 10100 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6456 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2094 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:
 <smime-enc-signed-complex-injected-minimal-legacy@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:10:02 -0500

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cov6b9CP2WhxfoCWqatcsg==

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] 9470 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 5994 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 1813 bytes
   └┬╴multipart/mixed 1749 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: <95b9bb39-c028-5ff4-99b1-f179cb5d7585@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:11:02 -0500

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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] 9490 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6020 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1779 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: <23abef5f-8781-5c95-a46c-61e3a4464d58@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 10075 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6444 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2086 bytes
   ├┬╴multipart/alternative 1425 bytes
   │├─╴text/plain 481 bytes
   │└─╴text/html 633 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: <9cfcaae2-9fec-5aca-9a29-c98da35b262d@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 9775 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6222 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 1978 bytes
   └┬╴multipart/mixed 1914 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:
 <smime-enc-signed-complex-wrapped-minimal-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:14:02 -0500
In-Reply-To:
 <smime-enc-signed-complex-wrapped-minimal@lhp.example>
References:
 <smime-enc-signed-complex-wrapped-minimal@lhp.example>

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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] 9815 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6250 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1946 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:
 <smime-enc-signed-complex-injected-minimal-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:15:02 -0500
In-Reply-To:
 <smime-enc-signed-complex-injected-minimal@lhp.example>
References:
 <smime-enc-signed-complex-injected-minimal@lhp.example>

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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] 10445 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6720 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2283 bytes
   ├┬╴multipart/alternative 1455 bytes
   │├─╴text/plain 497 bytes
   │└─╴text/html 649 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: <smime-enc-signed-complex-injected-minimal-legacy-reply@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:16:02 -0500
In-Reply-To:
 <smime-enc-signed-complex-injected-minimal-legacy@lhp.example>
References:
 <smime-enc-signed-complex-injected-minimal-legacy@lhp.example>

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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] 9750 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6210 bytes
  ⇩ (unwraps to)
  └┬╴message/rfc822 1970 bytes
   └┬╴multipart/mixed 1906 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: <38a0b7ba-76e0-5351-93e9-f44877e20e6e@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 9795 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6238 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 1938 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: <c6774fdb-3ef5-5293-ab2d-eca8b66b4bbf@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
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] 10425 bytes
 ↧ (decrypts to)
 └─╴application/pkcs7-mime [smime.p7m] 6704 bytes
  ⇩ (unwraps to)
  └┬╴multipart/mixed 2273 bytes
   ├┬╴multipart/alternative 1449 bytes
   │├─╴text/plain 493 bytes
   │└─╴text/html 645 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: <acced3c9-111b-5a4f-bd80-34558da32b4d@lhp.example>
From: Alice <alice@smime.example>
To: Bob <bob@smime.example>
Date: Sat, 20 Feb 2021 12:19:02 -0500

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Appendix C. Additional information

C.1. Stored Variants of Messages with Bcc

Messages containing at least one recipient address in the Bcc header field may appear in up to three different variants:

  1. The Message for the recipient addresses listed in To or Cc header fields, which must not include the Bcc header field neither for signature calculation nor for encryption.
  2. The Message(s) sent to the recipient addresses in the Bcc header field, which depends on the implementation:

    a) One Message for each recipient in the Bcc header field separately, with a Bcc header field containing only the address of the recipient it is sent to.

    b) The same Message for each recipient in the Bcc header field with a Bcc header field containing an indication such as "Undisclosed recipients", but no addresses.

    c) The same Message for each recipient in the Bcc header field which does not include a Bcc header field (this Message is identical to 1. / see above).

  3. The Message stored in the 'Sent'-Folder of the sender, which usually contains the Bcc unchanged from the original Message, i.e., with all recipient addresses.

The most privacy preserving method of the alternatives (2a, 2b, and 2c) is to standardize 2a, as in the other cases (2b and 2c), information about hidden recipients is revealed via keys. In any case, the Message has to be cloned and adjusted depending on the recipient.

Appendix D. 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 <alice@example.net>
To: Bob <bob@example.net>
Subject: Dinner plans
Message-ID: <text-plain-legacy-display@lhp.example>
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 <alice@example.net>
To: Bob <bob@example.net>
Subject: Dinner plans
Message-ID: <text-html-legacy-display@lhp.example>
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 ]]

Appendix F. Open Issues

[[ RFC Editor: This section should be empty and 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
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom