Header Protection for S/MIME

Internet-Draft	Header Protection S/MIME	November 2020
Hoeneisen, et al.	Expires 6 May 2021	[Page]

Abstract

S/MIME version 3.1 has introduced a feasible standardized option to accomplish Header Protection. However, implementations of Header Protection can cause rendering issues on the receiving side. Clearer specifications regarding message processing, particularly with respect to header sections, are needed in order to resolve these rendering issues.¶

In order to help implementers to correctly compose and render email messages with Header Protection, this document updates S/MIME Header Protection specifications with additional guidance on MIME format, sender and receiver processing.¶

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.¶

A way to provide end-to-end protection for the Header Section of an email message has been standardized for S/MIME version 3.1 and later (cf. [RFC8551]):¶

In order to protect outer, non-content-related message header fields (for instance, the "Subject", "To", "From", and "Cc" fields), the sending client MAY wrap a full MIME message in a message/RFC822 wrapper in order to apply S/MIME security services to these header fields.¶

Unfortunately, implementations of Header Protection can cause rendering issues on the receiving side. In some cases, the user sees an attachment suggesting a forwarded email message, which - in fact - contains the protected email message that should be rendered directly. For these cases, the user can click on the attachment to view the protected message. However, there have also been reports of email clients displaying garbled text, or sometimes nothing at all. In those cases the email clients on the receiving side are (most likely) not fully MIME-capable.¶

The following shortcomings have been identified to cause these issues:¶

Broken or incomplete implementations¶
Lack of a simple means to distinguish "forwarded message" and "wrapped message" (for the sake of Header Protection)¶
Not enough guidance with respect to handling of Header Fields on both the sending and the receiving side¶

Furthermore, the need (technical) Data Minimization, which includes data sparseness and hiding all technically concealable information, has grown in importance over the past several years. In addition, backwards compatibility must be considered when it is possible to do so without compromising privacy and security.¶

No mechanism for Header Protection has been standardized for PGP/MIME (Pretty Good Privacy) [RFC3156] yet. PGP/MIME developers have implemented ad-hoc header-protection, and would like to see a specification that is applicable to both S/MIME and PGP/MIME.¶

This document describes the problem statement (Section 2), generic use cases (Section 3) and the specification for Header Protection (Section 4) with guidance on MIME format, sender and receiver processing .¶

[I-D.ietf-lamps-header-protection-requirements] defines the requirements that this specification is based on.¶

This document is in an early draft state and contains a proposal on which to base future discussions of this topic. In any case, the final mechanism is to be determined by the IETF LAMPS WG.¶

1.1. Other Protocols to Protect Email Headers

A range of protocols for the protection of electronic mail (email) exists, which allows one to assess the authenticity and integrity of the email headers section or selected Header Fields from the domain-level perspective, specifically DomainKeys Identified Mail (DKIM) However, integrity protection and proof of authenticity are both tied to the domain name of the sending e-mail address, not the sending address itself, so these protocols do not provide end-to-end protection, and are incapable of providing any form of confidentiality.[RFC6376], as used by Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489]. These protocols provide a domain-based reputation mechanism that can be used to mitigate some forms of unsolicited email (spam). At the same time, these protocols can provide a level of cryptographic integrity and authenticity for some headers, depending on how they are used.¶

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶

1.3. Terms

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

Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A form of active wiretapping attack in which the attacker intercepts and selectively modifies communicated data to masquerade as one or more of the entities involved in a communication association."¶

Note: Historically, MITM has stood for 'Man-in-the-middle'. However, to indicate that the entity in the middle is not always a human attacker, MITM can also stand for 'Machine-in-the-middle' or 'Meddler-in-the-middle'.¶
S/MIME: Secure/Multipurpose Internet Mail Extensions (cf. [RFC8551])¶
PGP/MIME: MIME Security with OpenPGP (cf. [RFC3156])¶
Message: An Email Message consisting of Header Fields (collectively called "the Header Section of the message") followed, optionally, by a Body; cf. [RFC5322].¶

Note: To avoid ambiguity, this document does not use 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; cf. [RFC5322].¶
Header Field (HF): cf. [RFC5322] Header Fields are lines beginning with a field name, followed by a colon (":"), followed by a field body (value), and terminated by CRLF.¶
Header Section (HS): The Header Section is a sequence of lines of characters with special syntax as defined in [RFC5322]. It is the (top) section of a Message containing the Header Fields.¶
Body: The Body is simply a sequence of bytes 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); cf [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.¶
MIME Header Fields: Header Fields describing content of a MIME entity [RFC2045], in particular the MIME structure. Each MIME Header Field name starts with "Content-" prefix.¶
MIME Header Section (part): The collection of MIME Header Fields. "MIME Header Section" refers to a Header Sections that contains only MIME Header Fields, whereas "MIME Header Section part" refers to the MIME Header Fields of a Header Section that - in addition to MIME Header Fields - also contains non-MIME Header Fields.¶
Essential Header Fields (EHF): The minimum set of Header Fields an Outer Message Header Section SHOULD contain; cf. Section 4.1.2.4.¶
Header Protection (HP): cryptographic protection of email Header Sections (or parts of it) for signatures and/or encryption¶
Protection Levels (PL): The level of protection applied to a Message, e.g. 'signature and encryption' or 'signature only' (cf. Section 3.2).¶
Protected: Portions of a message that have had any Protection Levels applied.¶
Protected Message: A Message that has had any Protection Levels applied.¶
Unprotected: Portions of a Message that has had no Protection Levels applied.¶
Unprotected Message: A Message that has had no Protection Levels applied.¶
Submission Entity: The entity which executes further processing of the Message (incl. transport towards the receiver), after protection measures have been applied to the Message.¶

Note: The Submission Entity varies among implementations, mainly depending on the stage where protection measures are applied: E.g. a Message Submission Agent (MSA) [RFC6409] or another (proprietary) solution. The latter is particularly relevant, if protection is implemented as a plugin solution. Some implementations may determine the destination recipients by reading the To, Cc and Bcc Header Fields of the Outer Message.¶
Original Message (OrigM): The Message to be protected before any protection-related processing has been applied on the sending side. If the source is not a "message/rfc822" Message, OrigM is defined as the "virtual" Message that would be constructed for sending it as unprotected email.¶
Inner Message (InnerM): The Message to be protected which has had wrapping and protection measures aapplied on the sending side OR the resulting Message once decryption and unwrapping on the receiving side has been performed. Typically, the Inner Message is in clear text. The Inner Message is a subset of (or the same as) the Original Message (cf. Section 4.1.2.1). The Inner Message must be the same on the sending and the receiving side.¶
Outer Message (OuterM): The Message as provided to the Submission Entity or received from the last hop respectively. The Outer Message normally differs on the sending and the receiving side (e.g. new Header Fields are added by intermediary nodes).¶
Receiving User Facing Message (RUFM): The Message used for rendering at the receiving side. Typically this is the same as the Inner Message.¶
Data Minimization: Data sparseness and hiding of all technically concealable information whenever possible.¶
Cryptographic Layer, Cryptographic Payload, and Cryptographic Envelope are all used as defined in [I-D.dkg-lamps-e2e-mail-guidance]¶

3. Use Cases

In the following, the reader can find a list of the generic use cases that need to be addressed for Messages with Header Protection (HP). These use cases apply regardless of technology (S/MIME, PGP/MIME, etc.) used to achieve HP.¶

3.1. Interactions

The following use cases assume that at least the sending side supports Header Protection as specified in this document. Receiving sides that support this specification are expected to be able to distinguish between Messages that use Header Protection as specified in this document, and (legacy) Mail User Agents (MUAs) which do not implement this specification.¶

[[ TODO: Verify once solution is stable and update last sentence. ]]¶

3.1.1. Main Use Case

Both the sending and receiving side (fully) support Header Protection as specified in this document.¶

The main use case is specified in Section 4.1.¶

3.1.2. Backward Compatibility Use Cases

Regarding backward compatibility, the main distinction is based on whether or not the receiving side conforms to MIME according to [RFC2046], ff., which in particular also includes Section 2 of [RFC2049] on "MIME Conformance". The following excerpt is contextually relevant:¶

  A mail user agent that is MIME-conformant MUST:

  [...]

           -- Recognize and display at least the RFC822 message
           encapsulation (message/rfc822) in such a way as to
           preserve any recursive structure, that is, displaying
           or offering to display the encapsulated data in
           accordance with its media type.

           -- Treat any unrecognized subtypes as if they were
           "application/octet-stream".

  [...]

  An MUA that meets the above conditions is said to be MIME-
  conformant.  A MIME-conformant MUA is assumed to be "safe" to send
  virtually any kind of properly-marked data to users of such mail
  systems, because these systems are, at a minimum, capable of treating
  the data as undifferentiated binary, and will not simply
  splash it onto the screen of unsuspecting users.

[[ TODO: The compatibility of legacy HP systems with this new solution, and how to handle issues surrounding future maintenance for these legacy systems, will be decided by the LAMPS WG. ]]¶

3.1.2.1. Receiving Side MIME-Conformant

The sending side (fully) supports Header Protection as specified in this document, while the receiving side does not support this specification. However, the receiving side is MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2).¶

This use case is specified in Section 4.2.1.¶

Note: This case should perform as expected if the sending side applies this specification as outlined in Section 4.1.¶

[[ TODO: Verify once solution is stable and update last sentence. ]]¶

3.1.2.2. Receiving Side Not MIME-Conformant

The sending side (fully) supports Header Protection as specified in this document, while the receiving side does not support this specification. Furthermore, the receiving side is not MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2).¶

This use case is specified in Section 4.2.2.¶

3.2. Protection Levels

3.2.1. In-Scope

The following Protection Levels are in scope for this document:¶

a) Signature and encryption¶

Messages containing a cryptographic signature, which are also
encrypted.

b) Signature only¶

Messages containing a cryptographic signature, but which are not
encrypted.

3.2.2. Out-of-Scope

Legacy implementations, implementations not (fully) compliant with this document or corner-cases may lead to further Protection Levels to appear on the receiving side, such as (list not exhaustive):¶

Triple wrap¶
Encryption only¶
Encryption before signature¶
Signature and encryption, but:¶
- Signature fails to validate¶
- Signature validates but the signing certificate revoked¶
Signature only, but:¶
- with multiple valid signatures, layered atop each other¶

These Protection Levels, as well as any further Protection Levels not listed in Section 3.2.1 are beyond the scope of this document.¶

4. Specification

This section contains the specification for Header Protection in S/MIME to update and clarify Section 3.1 of [RFC8551] (S/MIME 4.0).¶

Note: It is likely that PGP/MIME [RFC3156] will also incorporate this specification or parts of it.¶

This specification applies to the Protection Levels "signature & encryption" and "signature only" (cf. Section 3.2):¶

Sending and receiving sides MUST implement the "signature and encryption" Protection Level, which SHOULD be used as default on the sending side.¶

Certain implementations may decide to send "signature only" Messages, depending on the circumstances and customer requirements. Sending sides MAY and receiving sides MUST implement "signature only" Protection Level.¶

It generally is NOT RECOMMENDED to send a Message with any other Protection Level. On the other hand, the receiving side must be prepared to receive Messages with other Protection Levels.¶

[[ TODO: Further study is necessary to determine whether - and if yes to what extent - additional guidance for handling messages with other Protection Levels, e.g. "encryption only" at the receiving side should be included in this document. ]]¶

4.1. Main Use Case

This section applies to the main use case, where the sending and receiving side (fully) support Header Protection as specified herein (cf. Section 3.1.1).¶

Note: The sending side specification of the main use case is also applicable to the cases where the sending side (fully) supports Header Protection as specified herein, while the receiving side does not, but is MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2 and Section 3.1.2.1).¶

Further backward compatibility cases are defined in Section 4.2.¶

4.1.1. MIME Format

4.1.1.1. Introduction

As per S/MIME version 3.1 and later (cf. [RFC8551]), the sending client MAY wrap a full MIME message in a message/RFC822 wrapper in order to apply S/MIME security services to these header fields.¶

To help the receiving side to distinguish between a forwarded and a wrapped message, the Content-Type header field parameter "forwarded" is added as defined in [I-D.melnikov-iana-reg-forwarded].¶

The simplified (cryptographic overhead not shown) MIME structure of such an Email Message looks as follows:¶

  <Outer Message Header Section (unprotected)>

  <Outer Message Body (protected)>

    <MIME Header Section (wrapper)>

      <Inner Message Header Section>

      <Inner Message Body>

The following example demonstrates how an Original Message might be protected, i.e., the Original Message is contained as Inner Message in the Protected Body of an Outer Message. It illustrates the first Body part (of the Outer Message) as a "multipart/signed" (application/pkcs7-signature) media type:¶

Lines are prepended as follows:¶

"O: " Outer Message Header Section¶
"I: " Message Header Section¶
"W: " Wrapper (MIME Header Section)¶


  O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
  O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
  O: Subject: Meeting at my place
  O: From: "Alexey Melnikov" <alexey.melnikov@example.net>
  O: To: somebody@example.net
  O: MIME-Version: 1.0
  O: Content-Type: multipart/signed; charset=us-ascii; micalg=sha1;
  O:  protocol="application/pkcs7-signature";
  O:  boundary=boundary-AM

     This is a multipart message in MIME format.
     --boundary-AM
  W: Content-Type: message/RFC822; forwarded=no
  W:
  I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
  I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
  I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
  I: MIME-Version: 1.0
  I: MMHS-Primary-Precedence: 3
  I: Subject: Meeting at my place
  I: To: somebody@example.net
  I: X-Mailer: Isode Harrier Web Server
  I: Content-Type: text/plain; charset=us-ascii

     This is an important message that I don't want to be modified.

     --boundary-AM
     Content-Transfer-Encoding: base64
     Content-Type: application/pkcs7-signature

     [[base-64 encoded signature]]

     --boundary-AM--

The Outer Message Header Section is unprotected, while the remainder (Outer Message Body) is protected. The Outer Message Body consists of the wrapper (MIME Header Section) and the Inner Message (Header Section and Body).¶

The wrapper is a simple MIME Header Section with media type "message/rfc822" containing a Content-Type header field parameter "forwarded=no" followed by an empty line.¶

If the source is an Original (message/rfc822) Message, the Inner Message Header Section is typically the same as (or a subset of) the Original Message Header Section (cf. Section 4.1.2.1), and the Inner Message Body is typically the same as the Original Message Body.¶

The Inner Message itself may contain any MIME structure.¶

Note: It is still to be decided by the LAMPS WG whether or not to recommend an alternative MIME format as described in Appendix B.1.1.1 (instead of the currently standardized and above defined format).¶

4.1.2. Sending Side

To ease explanation, the following describes the case where an Original (message/rfc822) Message to be protected is present. If this is not the case, Original Message means the (virtual) Message that would be constructed for sending it as unprotected email.¶

4.1.2.1. Inner Message Header Fields

It is RECOMMENDED that the Inner Message contains all Header Fields of the Original Message with the exception of the following Header Field, which MUST NOT be included within the Inner Message nor within any other protected part of the Message:¶

Bcc¶

[[ TODO: Bcc handling needs to be further specified (see also Appendix A.1). Certain MUAs cannot properly decrypt Messages with Bcc recipients. ]]¶

4.1.2.2. Wrapper

The wrapper is a simple MIME Header Section followed by an empty line preceding the Inner Message (inside the Outer Message Body). The media type of the wrapper MUST be "message/RFC822" and MUST contain the Content-Type header field parameter "forwarded=no" as defined in [I-D.melnikov-iana-reg-forwarded]. The wrapper unambiguously delimits the Inner Message from the rest of the Message.¶

4.1.2.3. Cryptographic Layers / Envelope

[[ TODO: Basically refer to S/MIME standards ]]¶

4.1.2.4. Outer Message Header Fields

4.1.2.4.1. Encrypted Messages

To maximize Privacy, it is strongly RECOMMENDED to follow the principle of Data Minimization (cf. Section 2.1).¶

However, the Outer Message Header Section SHOULD contain the Essential Header Fields and, in addition, MUST contain the Header Fields of the MIME Header Section part to describe Cryptographic Layer of the protected MIME subtree as per [RFC8551].¶

The following Header Fields are defined as the Essential Header Fields:¶

From¶
To (if present in the Original Message)¶
Cc (if present in the Original Message)¶
Bcc (if present in the Original Message, see also Section 4.1.2.1 and Appendix A.1)¶
Date¶
Message-ID¶
Subject¶

Further processing by the Submission Entity normally depends on part of these Header Fields, e.g. From and Date HFs are required by [RFC5322]. Furthermore, not including certain Header Fields may trigger spam detection to flag the Message, and/or lead to user experience (UX) issues.¶

For further Data Minimization, the value of the Subject Header Field SHOULD be obfuscated as follows:¶

* Subject: [...]

and it is RECOMMENDED to replace the Message-ID by a new randomly generated Message-ID.¶

In addition, the value of other Essential Header Fields MAY be obfuscated.¶

Non-Essential Header Fields SHOULD be omitted from the Outer Message Header Section where possible. If Non-essential Header Fields are included in the Outer Message Header Section, those MAY be obfuscated too.¶

Header Fields that are not obfuscated should contain the same values as in the Original Message.¶

If an implementation obfuscates the From, To, and/or Cc Header Fields, it may need to provide access to the clear text content of these Header Fields to the Submission Entity for processing purposes. This is particularly relevant, if proprietary Submission Entities are used. Obfuscation of Header Fields may adversely impact spam filtering.¶

(A use case for obfuscation of all Outer Message Header Fields is routing email through the use of onion routing or mix networks, e.g. [pEp.mixnet].)¶

The MIME Header Section part is the collection of MIME Header Fields describing the following MIME structure as defined in [RFC2045]. A MIME Header Section part typically includes the following Header Fields:¶

Content-Type¶
Content-Transfer-Encoding¶
Content-Disposition¶

The following example shows the MIME Header Section part of an S/MIME signed Message (using application/pkcs7-mime with SignedData):¶

   MIME-Version: 1.0
   Content-Type: application/pkcs7-mime; smime-type=signed-data;
      name=smime.p7m
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7m

Depending on the scenario, further Header Fields MAY be exposed in the Outer Message Header Section, which is NOT RECOMMENDED unless justified. Such Header Fields may include e.g.:¶

References¶
Reply-To¶
In-Reply-To¶

4.1.2.4.2. Unencrypted Messages

The Outer Message Header Section of unencrypted Messages SHOULD contain at least the Essential Header Fields and, in addition, MUST contain the Header Fields of the MIME Header Section part to describe Cryptographic Layer of the protected MIME subtree as per [RFC8551]. It may contain further Header Fields, in particular those also present in the Inner Message Header Section.¶

4.1.2.5. Sending Side Message Processing

For a protected Message the following steps are applied before a Message is handed over to the Submission Entity:¶

4.1.2.5.1. Step 1: Decide on Protection Level and Information Disclosure

The implementation which applies protection to a Message must decide:¶

Which Protection Level (signature and/or encryption) shall be applied to the Message? This depends on user request and/or local policy as well as availability of cryptographic keys.¶
Which Header Fields of the Original Message shall be part of the Outer Message Header Section? This typically depends on local policy. By default, the Essential Header Fields are part of the Outer Message Header Section; cf. Section 4.1.2.4.¶
Which of these Header Fields are to be obfuscated? This depends on local policy and/or specific Privacy requirements of the user. By default only the Subject Header Field is obfuscated; cf. Section 4.1.2.4.¶

4.1.2.5.2. Step 2: Compose the Outer Message Header Section

Depending on the decision in Section 4.1.2.5.1, the implementation shall compose the Outer Message Header Section. (Note that this also includes the necessary MIME Header Section part for the following protection layer.)¶

Outer Header Fields that are not obfuscated should contain the same values as in the Original Message (except for MIME Header Section part, which depends on the Protection Level selected in Section 4.1.2.5.1).¶

4.1.2.5.3. Step 3: Apply Protection to the Original Message

Depending on the Protection Level selected in Section 4.1.2.5.1, the implementation applies signature and/or encryption to the Original Message, including the wrapper (as per [RFC8551]), and sets the resulting package as the Outer Message Body.¶

The resulting (Outer) Message is then typically handed over to the Submission Entity.¶

[[ TODO: Example ]]¶

4.1.3. Receiving Side

4.1.3.1. Receiving User Facing Message Header Fields

The Receiving User Facing Message SHOULD be a verbatim copy of the Inner Message.¶

4.1.3.2. Receiving Side Message Processing

When a protected Message is received, the following steps are applied:¶

4.1.3.2.1. Step 1: Decrypt Message and/or check signature

Depending on the Protection Level, the received Message is decrypted and/or its signature is checked as per [RFC8551].¶

4.1.3.2.2. Step 2: Construct the Receiving User Facing Message

The Receiving User Facing Message is constructed according to Section 4.1.3.1.¶

The resulting Message is handed over for further processing, which typically involves rendering it for the user.¶

4.1.3.3. Step 3: Prepare Information Cyptographic Verification

[[ TODO: Signature valid, etc. ]]¶

4.2. Backward Compatibility Use Cases

4.2.1. Receiving Side MIME-Conformant

This section applies to the case where the sending side (fully) supports Header Protection as specified in this document, while the receiving side does not support this specification, but is MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2 and Section 3.1.2.1)¶

The sending side specification of the main use case (cf. Section 4.1) MUST ensure that receiving sides can still recognize and display or offer to display the encapsulated data in accordance with its media type (cf. [RFC2049], Section 2). In particular, receiving sides that do not support this specification, but are MIME-conformant according to [RFC2045], ff. can still recognize and display the Message intended for the user.¶

[[ TODO: Verify once solution is stable and update last sentence. ]]¶

4.2.2. Receiving Side Not MIME-Conformant

This section applies to cases where the sending side (fully) supports Header Protection as specified in this document, while the receiving side neither supports this specification nor is MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2 and Section 3.1.2.2).¶

[I-D.autocrypt-lamps-protected-headers] describes a possible way to achieve backward compatibility with existing S/MIME (and PGP/MIME) implementations that predate this specification and are not MIME-conformant (Legacy Display) either. It mainly focuses on email clients that do not render emails which utilize header protection in a user friendly manner, which may confuse the user. While this has been observed occasionally in PGP/MIME (cf. [RFC3156]), the extent of this problem with S/MIME implementations is still unclear. (Note: At this time, none of the samples in [I-D.autocrypt-lamps-protected-headers] apply header protection as specified in Section 3.1 of [RFC8551], which is wrapping as Media Type "message/RFC822".)¶

Should serious backward compatibility issues with rendering at the receiving side be discovered, the Legacy Display format described in [I-D.autocrypt-lamps-protected-headers] may serve as a basis to mitigate those issues (cf. Section 4.2).¶

Another variant of backward compatibility has been implemented by pEp [I-D.pep-email], i.e. pEp Email Format 1.0. At this time pEp has implemented this for PGP/MIME, but not yet S/MIME.¶

9. References

9.1. Normative References

[I-D.dkg-lamps-e2e-mail-guidance]: Gillmor, D., "Guidance on End-to-End E-mail Security", Work in Progress, Internet-Draft, draft-dkg-lamps-e2e-mail-guidance-00, 31 October 2020, <http://www.ietf.org/internet-drafts/draft-dkg-lamps-e2e-mail-guidance-00.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, 29 October 2019, <http://www.ietf.org/internet-drafts/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, November 1996, <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, November 1996, <https://www.rfc-editor.org/info/rfc2046>.
[RFC2049]: Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996, <https://www.rfc-editor.org/info/rfc2049>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC5322]: Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <https://www.rfc-editor.org/info/rfc5322>.
[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, April 2019, <https://www.rfc-editor.org/info/rfc8551>.

9.2. Informative References

[I-D.autocrypt-lamps-protected-headers]: Einarsson, B., juga, j., and D. Gillmor, "Protected Headers for Cryptographic E-mail", Work in Progress, Internet-Draft, draft-autocrypt-lamps-protected-headers-02, 20 December 2019, <http://www.ietf.org/internet-drafts/draft-autocrypt-lamps-protected-headers-02.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, 4 November 2019, <http://www.ietf.org/internet-drafts/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-00, 10 July 2020, <http://www.ietf.org/internet-drafts/draft-pep-email-00.txt>.
[pEp.mixnet]: pEp Foundation, "Mixnet", June 2020, <https://dev.pep.foundation/Mixnet>.
[RFC3156]: Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, August 2001, <https://www.rfc-editor.org/info/rfc3156>.
[RFC4949]: Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>.
[RFC6376]: Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, <https://www.rfc-editor.org/info/rfc6376>.
[RFC6409]: Gellens, R. and J. Klensin, "Message Submission for Mail", STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, <https://www.rfc-editor.org/info/rfc6409>.
[RFC7489]: Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, <https://www.rfc-editor.org/info/rfc7489>.

Appendix B. Text Moved from Above

Note: Per an explicit request by the chair of the LAMPS WG to only present one option for the specification, the following text has been stripped from the main body of the draft. It is preserved in an Appendix for the time being and may be moved back to the main body or deleted, depending on the decision of the LAMPS WG.¶

B.1. MIME Format

Currently there are two options in discussion:¶

The option according to the current S/MIME specification (cf. [RFC8551])¶
An alternative option that is based on the former "memory hole" approach (cf. [I-D.autocrypt-lamps-protected-headers])¶

B.1.1. S/MIME Specification

Note: This is currently described in the main part of this document.¶

B.1.1.1. Alternative Option Autocrypt "Protected Headers" (Ex-"Memory Hole")

An alternative option (based on the former autocrypt "Memory Hole" approach) to be considered, is described in [I-D.autocrypt-lamps-protected-headers].¶

Unlike the option described in Appendix B.1.1, this option does not use a "message/RFC822" wrapper to unambiguously delimit the Inner Message.¶

Before choosing this option, the following two issues must be assessed to ensure no interoperability issues result from it:¶

How current MIME parser implementations treat non-MIME Header Fields, which are not part of the outermost MIME entity and not part of a Message wrapped into a MIME entity of media type "message/rfc822", and how such Messages are rendered to the user.¶

[I-D.autocrypt-lamps-protected-headers] provides some examples for testing this.¶
MIME-conformance, i.e. whether or not this option is (fully) MIME-conformant [RFC2045] ff., in particular also Section 5.1. of [RFC2046] on "Multipart Media Type). In the following an excerpt of paragraphs that may be relevant in this context:¶

      The only header fields that have defined meaning for body parts
      are those the names of which begin with "Content-".  All other
      header fields may be ignored in body parts.  Although they
      should generally be retained if at all possible, they may be
      discarded by gateways if necessary.  Such other fields are
      permitted to appear in body parts but must not be depended on.
      "X-" fields may be created for experimental or private
      purposes, with the recognition that the information they
      contain may be lost at some gateways.

      NOTE:  The distinction between an RFC 822 Message and a body
      part is subtle, but important.  A gateway between Internet and
      X.400 mail, for example, must be able to tell the difference
      between a body part that contains an image and a body part
      that contains an encapsulated Message, the body of which is a
      JPEG image.  In order to represent the latter, the body part
      must have "Content-Type: message/rfc822", and its body (after
      the blank line) must be the encapsulated Message, with its own
      "Content-Type: image/jpeg" header field.  The use of similar
      syntax facilitates the conversion of Messages to body parts,
      and vice versa, but the distinction between the two must be
      understood by implementors.  (For the special case in which
      parts actually are Messages, a "digest" subtype is also
      defined.)

The MIME structure of an Email Message looks as follows:¶

  <Outer Message Header Section (unprotected)>

  <Outer Message Body (protected)>

    <Inner Message Header Section>

    <Inner Message Body>

Lines are prepended as follows:¶

"O: " Outer Message Header Section¶
"I: " Message Header Section¶

  O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
  O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
  O: Subject: Meeting at my place
  O: From: "Alexey Melnikov" <alexey.melnikov@example.net>
  O: MIME-Version: 1.0
  O: Content-Type: multipart/signed; charset=us-ascii; micalg=sha1;
  O:  protocol="application/pkcs7-signature";
  O:  boundary=boundary-AM

     This is a multipart message in MIME format.
     --boundary-AM
  I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
  I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
  I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
  I: MIME-Version: 1.0
  I: MMHS-Primary-Precedence: 3
  I: Subject: Meeting at my place
  I: To: somebody@example.net
  I: X-Mailer: Isode Harrier Web Server
  I: Content-Type: text/plain; charset=us-ascii

     This is an important message that I don't want to be modified.

     --boundary-AM
     Content-Transfer-Encoding: base64
     Content-Type: application/pkcs7-signature

     [[base-64 encoded signature]]

     --boundary-AM--

The Outer Message Header Section is unprotected, while the remainder (Outer Message Body) is protected. The Outer Message Body consists of the Inner Message (Header Section and Body).¶

The Inner Message Header Section is the same as (or a subset of) the Original Message Header Section (cf. Section 4.1.2.1).¶

The Inner Message Body is the same as the Original Message Body.¶

The Original Message itself may contain any MIME structure.¶