Network Working Group B. Hoeneisen
Internet-Draft pEp Foundation
Intended status: Informational A. Melnikov
Expires: January 10, 2021 Isode Ltd
July 09, 2020

Header Protection for S⁠/⁠MIME
draft-ietf-lamps-header-protection-00

Abstract

Privacy and security issues with email header protection in S⁠/⁠MIME have been identified for some time. However, the desire to fix these issues has only recently been expressed in the IETF LAMPS Working Group. The existing S⁠/⁠MIME specification is to be updated regarding header protection.

This document describes the problem statement, generic use cases, and the S⁠/⁠MIME specification for 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 January 10, 2021.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

A range of protocols for the protection of electronic mail (email) exists, which allows to assess the authenticity and integrity of the email headers section or selected header fields (HF) from the domain-level perspective, specifically DomainKeys Identified Mail (DKIM) [RFC6376] and Sender Policy Framework (SPF) [RFC7208], and Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489]. These protocols, while essential to responding to a range of attacks on email, do not offer (full) end-to-end protection to the header section and are not capable of providing privacy for the information contained therein.

The need for means of Data Minimization, which includes data sparseness and hiding all technically concealable information whenever possible, has grown in importance over the past several years.

A standard for end-to-end protection of the email header section exists for S⁠/⁠MIME version 3.1 and later. (cf. [RFC8551]):

No mechanism for header protection (HP) has been standardized for PGP/MIME (Pretty Good Privacy) [RFC3156] yet.

Several varying implementations of end-to-end protections for email header sections exist, though the total number of such implementations appears to be rather low.

Some LAMPS WG participants expressed the opinion that regardless of the mechanism chosen, it should not be limited to S⁠/⁠MIME, but also applicable to PGP/MIME.

This document describes the problem statement (Section 2), generic use cases (Section 3) and the specification for Header Protection (Section 4).

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

This document is in 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. 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].

1.2. Terms

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

2. Problem Statement

The LAMPS charter contains the following Work Item:

In the following a set of challenges to be addressed:

[[ TODO: enhance this section, add more items to the following ]]

2.1. Privacy

2.2. Security

2.3. Usability

2.4. Interoperability

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 Header Protection – as specified in this document – has been applied to and (legacy) Mail user Agents (MUAs) not implementing this specification.

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

3.1.1. Main Use Case

Both peers (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 backwards 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”. In the following an excerpt of paragraphs relevant in this context:

  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".
 
  [...]
 
  A user agent that meets the above conditions is said to be MIME-
  conformant.  The meaning of this phrase is that it is assumed to be
  "safe" to send virtually any kind of properly-marked data to users
  of such mail systems, because such systems will at least be able to
  treat the data as undifferentiated binary, and will not simply
  splash it onto the screen of unsuspecting users.

Note: 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. 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 is expected to just work fine, if the sending side applies specification for the main use case 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. The receiving side is not MIME-conformant according to [RFC2045], ff. (cf. Section 3.1.2) either.

This use case is specified in Section 4.2.2.

3.2. Protection Levels

The following protection levels need to be considered:

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.

c) Encryption only

Messages that are encrypted, but do not contain a cryptographic
signature.

4. Specification

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

Furthermore, 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 “signature and encryption”, which is the default to use on the sending side.

Certain implementations MAY decide to send “signature only” messages, depending on the circumstances and customer requirements. Sending side MAY and receiving sides MUST implement “signature only”.

It generally is NOT RECOMMENDED to send a message with protection level “encryption only”. On the other hand, messages with protection level “encryption only” might arrive at the receiving side. While not targeted to protection level “encryption only”, this specification is assumed to also function for “encryption only”. Receiving sides SHOULD implement “encryption only”.

Note: It is for further study whether or not more guidance for handling messages with protection level “encryption only” at the receiving side is needed.

4.1. Main Use Case

This section applies to the main use case, where both peers (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 support this specification, 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

Currently there are two options in discussion:

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

4.1.1.1. S/MIME Specification

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 forwarded and wrapped message, a Content-Type header field parameter “forwarded” is added as defined in [I-D.melnikov-iana-reg-forwarded]. Certain mailing applications might display the Inner Message as attachment otherwise.

The MIME structure of 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 header section and payload of a protected body part might look like. For example, this will be the first body part of a multipart/signed message or the signed and/or encrypted payload of the application/pkcs7-mime body part. Lines prepended by “O: “ are the Outer Message Header Section. Lines prepended by “I: “ are the Inner Message Header Section. Lines prepended by “W: “ are the 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=.cbe16d2a-e1a3-4220-b821-38348fc97237

     This is a multipart message in MIME format.
     --.cbe16d2a-e1a3-4220-b821-38348fc97237
  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.

     --.cbe16d2a-e1a3-4220-b821-38348fc97237
     Content-Transfer-Encoding: base64
     Content-Type: application/pkcs7-signature

     [[base-64 encoded signature]]

     --.cbe16d2a-e1a3-4220-b821-38348fc97237--

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.

The Inner Message Header Section is the same as (or a subset of) the Original Message Header Section (cf. Section 4.1.2).

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

The Original Message itself may contain any MIME structure.

4.1.1.2. 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 Section 4.1.1.1, this option does not use a “message/RFC822” wrapper to unambiguously delimit the Inner Message.

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

  1. 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.
  2. 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>

The following example demonstrates how the header section and payload of a protected body part might appear. For example, this will be the first body part of a multipart/signed message or the signed and/or encrypted payload of the application/pkcs7-mime body part. Lines prepended by “O: “ are the outer header section. Lines prepended by “I: “ are the inner 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=.cbe16d2a-e1a3-4220-b821-38348fc97237

     This is a multipart message in MIME format.
     --.cbe16d2a-e1a3-4220-b821-38348fc97237
  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.

     --.cbe16d2a-e1a3-4220-b821-38348fc97237
     Content-Transfer-Encoding: base64
     Content-Type: application/pkcs7-signature

     [[base-64 encoded signature]]

     --.cbe16d2a-e1a3-4220-b821-38348fc97237--

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).

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

The Original Message itself may contain any MIME structure.

4.1.2. Inner Message Header Fields

It is RECOMMENDED that the Inner Messages contains all the 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:

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

4.1.3. 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 delimits unambiguously the Inner Message from the rest of the message.

4.1.4. Outer Message Header Fields

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 the encryption or signature as per [RFC8551].

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

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. In addition, the value of other Essential Header Fields MAY be obfuscated. Further Header Fields MAY be obfuscated, though simply not adding those to the Outer Message Header Section SHOULD be preferred over obfuscation. Header Field obfuscation is further specified in Section 4.1.4.1. Header Fields not obfuscated should contain the same values as in the Original Message.

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:

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.:

4.1.4.1. Obfuscation of Outer Message Header Fields

If the values of the following Outer Message Header Fields are obfuscated, those SHOULD assume the following values:

* Subject: ...
* Message-ID: <new randomly generated Message-ID> 
* Date: Thu, 01 Jan 1970 00:00:00 +0000 (UTC)

[[ TODO: Consider alternatives for Date e.g. set to Monday 9am of the same week. ]]

In certain implementations also the From, To, and/or Cc Header Field MAY be obfuscated. Those may be replaced by e.g.

Such implementations may need to ensure that the Submission Entity has access to the content of these Header Fields in clear text and is capable of processing those. This is particularly relevant, if proprietary Submission Entities are used.

A use case for obfuscation of all Outer Message Header Fields is mixnet networks, i.e. “onion routing” for email (e.g. [pEp.mixnet]).

Note: It is for further study to what extent Header Field obfuscation adversely impacts spam filtering.

4.1.5. Receiving User Facing Message Header Fields

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

4.1.6. Header Field Flow

The Following figure depicts the different message representations (OrigM, InnerM, OuterM, RUFM) and which parts those are constructed from:

OrigM        InnerM       Outer(S)            OuterM(R)    RUFM

                                              <Trace-HF>
                          From (OrigM)      = From
                          To (OrigM)        = To
                          Cc (OrigM)        = Cc
                          Bcc (OrigM)       = Bcc*
                          Date (OrigM)      = Date
                          Message-ID (OrigM)= Message-ID
                          Subject (new)     = Subject
                          <MIME-HSp> (new)  = <MIME-HSp>

                          PROTECTED:          PROTECTED:
                          <Wrapper> (new)   = <Wrapper>
From       > From       > From              = From       > From
To         > To         > To                = To         > To
Cc*        > Cc         > Cc                = Cc         > Cc
Bcc*
Date       > Date       > Date              = Date       > Date
Message-ID > Message-ID > Message-ID        = Message-ID > Message-ID
Subject    > Subject    > Subject           = Subject    > Subject
<More HF>  > <More HF>  > <More HF>         = <More HF>  > <More-HF>
<MIME-HSp> > <MIME-HSp> > <MIME-HSp>        = <MIME-HSp> > <MIME-HSp>
<Body>     > <Body>     > <Body>            = <Body>     > <Body>
                          <Signature>* (new)= <Signature>

Legend:

4.1.7. Sending Side Message Processing

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

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

The entity applying protection to a message must decide:

4.1.7.2. Step 2: Compose the Outer Message Header Section

Depending on the decision in Section 4.1.7.1, 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 4.1.7.1).

4.1.7.3. Step 3: Apply Protection to the Original Message

Depending on the Protection Level selected in Section 4.1.7.1, apply signature and/or encryption to the Original Message, including the wrapper (as per [RFC8551]), and set the result to the message as Outer Message Body.

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

[[ TODO: Example ]]

4.1.8. Receiving Side Message Processing

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

4.1.8.1. 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].

4.1.8.2. Step 2: Construct the Receiving User Facing Message

The Receiving User Facing Message is constructed according to Section 4.1.5.

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

Note: Further study is needed to determine whether or not the Outer Message Header Section, as received from the last hop, is preserved for the user, and if so, how this is to be achieved.

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. {#main-use-case}}) MUST ensure that receiving sides, that do not support this specification, but are MIME-conformant according to [RFC2045], ff. can still recognize and display the Inner Message (or rather the RUFM) 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 (cf. [RFC2049], Section 2).

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

4.2.2. Receiving Side Not 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 neither supports this specification and nor is MIME-conformant according to [RFC2045], ff. (cf. {{uc-ia-backward-compatibility-use-cases}) 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 using header protection (in a user friendly manner) and 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 receiver 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.

5. Security Considerations

[[ TODO ]]

6. Privacy Considerations

[[ TODO ]]

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, Claudio Luck, Daniel Kahn Gillmor, David Wilson, Hernani Marques, Krista Bennett, Kelly Bristol, Robert Williams, Sofia Balicka, Steve Kille, Volker Birk, and Wei Chuang.

9. References

9.1. Normative References

[I-D.ietf-lamps-header-protection-requirements] Melnikov, A. and B. Hoeneisen, "Problem Statement and Requirements for Header Protection", Internet-Draft draft-ietf-lamps-header-protection-requirements-01, October 2019.
[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.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[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.

9.2. Informative References

[I-D.autocrypt-lamps-protected-headers] Einarsson, B., juga, j. and D. Gillmor, "Protected Headers for Cryptographic E-mail", Internet-Draft draft-autocrypt-lamps-protected-headers-02, December 2019.
[I-D.melnikov-iana-reg-forwarded] Melnikov, A. and B. Hoeneisen, "IANA Registration of Content-Type Header Field Parameter 'forwarded'", Internet-Draft draft-melnikov-iana-reg-forwarded-00, November 2019.
[I-D.pep-email] Marques, H., "pretty Easy privacy (pEp): Email Formats and Protocols", Internet-Draft draft-marques-pep-email-02, October 2018.
[pEp.mixnet] pEp Foundation, "Mixnet", June 2020.
[RFC3156] Elkins, M., Del Torto, D., Levien, R. and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, August 2001.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.

Appendix A. Additional information

A.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. / cf. 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 B. Document Changelog

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

Appendix C. Open Issues

[[ RFC Editor: This section should be empty and is to be removed before publication. ]]

Authors' Addresses

Bernie Hoeneisen pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: bernie.hoeneisen@pep.foundation URI: https://pep.foundation/
Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex, TW12 2NP UK EMail: alexey.melnikov@isode.com