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

Document Type Active Internet-Draft (lamps WG)
Authors Bernie Hoeneisen  , Alexey Melnikov 
Last updated 2020-07-13
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Network Working Group                                       B. Hoeneisen
Internet-Draft                                            pEp Foundation
Intended status: Informational                               A. Melnikov
Expires: January 14, 2021                                      Isode Ltd
                                                           July 13, 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 14, 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

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Security  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Usability . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Interoperability  . . . . . . . . . . . . . . . . . . . .   7
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Interactions  . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Main Use Case . . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  Backward Compatibility Use Cases  . . . . . . . . . .   7
     3.2.  Protection Levels . . . . . . . . . . . . . . . . . . . .   9
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Main Use Case . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  MIME Format . . . . . . . . . . . . . . . . . . . . .  10
       4.1.2.  Inner Message Header Fields . . . . . . . . . . . . .  15
       4.1.3.  Wrapper . . . . . . . . . . . . . . . . . . . . . . .  16
       4.1.4.  Outer Message Header Fields . . . . . . . . . . . . .  16
       4.1.5.  Receiving User Facing Message Header Fields . . . . .  18
       4.1.6.  Header Field Flow . . . . . . . . . . . . . . . . . .  18
       4.1.7.  Sending Side Message Processing . . . . . . . . . . .  20
       4.1.8.  Receiving Side Message Processing . . . . . . . . . .  21
     4.2.  Backward Compatibility Use Cases  . . . . . . . . . . . .  21
       4.2.1.  Receiving Side MIME-Conformant  . . . . . . . . . . .  21
       4.2.2.  Receiving Side Not MIME-Conformant  . . . . . . . . .  22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  23
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Appendix A.  Additional information . . . . . . . . . . . . . . .  25
     A.1.  Stored Variants of Messages with Bcc  . . . . . . . . . .  25
   Appendix B.  Document Changelog . . . . . . . . . . . . . . . . .  26
   Appendix C.  Open Issues  . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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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]):

      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.

   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.

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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:

   o  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."

   o  S/MIME: Secure/Multipurpose Internet Mail Extensions (cf.
      [RFC8551])

   o  PGP/MIME: MIME Security with OpenPGP (cf.  [RFC3156])

   o  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].

   o  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; cf. [RFC5322].

   o  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.

   o  Body: The Body is simply a sequence of characters 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.

   o  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.

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   o  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.

   o  Essential Header Fields (EHF): The minimum set of Header Fields an
      Outer Message Header Section SHOULD contain; cf. Section 4.1.4.

   o  Header Protection (HP): cryptographic protection of email Header
      Sections (or parts of it) for signatures and/or encryption

   o  Protection Levels (PL): One of 'signature and encryption',
      'signature only' or 'encryption only' (cf.  Section 3.2)

   o  Protected: Protected refers to the parts of a Message where
      protection measures of any Protection Level have been applied to.

   o  Protected Message: A Message that protection measures of any
      Protection Levels have been applied to.

   o  Unprotected: Unprotected refers to the parts of a Message where no
      protection measures of any Protection Levels have been applied to.

   o  Unprotected Message: A Message that no protection measures of any
      Protection Levels have been applied to.

   o  Submission Entity: The entity taking care of further processing of
      the Message (incl. transport towards the receiver), after
      protection measures have been applied to.

      Note: The Submission Entity varies among implementations, mainly
      depending on the stage, where protection measures are applied to:
      It could be 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.

   o  Original Message (OrigM): The message to be protected before any
      protection related processing has been applied on the sending
      side.

   o  Inner Message (InnerM): The message to be protected, i.e. which
      wrapping and protection measures are applied to on the sending
      side or the result of decryption and unwrapping on the receiving
      side respectively.  Typically, the Inner Message is in clear text.
      The Inner Message is a subset of (or the same as) the Original

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      Message (cf.  Section 4.1.2).  The Inner Message must be the same
      on the sending and the receiving side.

   o  Outer Message (OuterM): The Message as handed over 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).

   o  Receiving User Facing Message (RUFM): The message used for
      rendering at the receiving side.  Typically this is the same as
      the Inner Message.

   o  Data Minimization: Data sparseness and hiding of all technically
      concealable information whenever possible.

2.  Problem Statement

   The LAMPS charter contains the following Work Item:

      Update the specification for the cryptographic protection of email
      headers - both for signatures and encryption - to improve the
      implementation situation with respect to privacy, security,
      usability and interoperability in cryptographically-protected
      electronic mail.  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.

   In the following a set of challenges to be addressed:

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

2.1.  Privacy

   o  Data Minimization, which includes data sparseness and hiding all
      technically concealable information whenever possible

2.2.  Security

   o  MITM attacks (cf.  [RFC4949])

2.3.  Usability

   o  User interaction / User experience

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2.4.  Interoperability

   o  Interoperability with [RFC8551] implementations

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 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".  In the following an excerpt of
   paragraphs relevant in this context:

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     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.

   [[ 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).

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   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.

   [[ TODO: There are further "Protection Levels" to describe for the
   receiving side, e.g. encrypted and signed (only after encryption),
   etc. ]]

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 Protection
   Level "encryption only".  On the other hand, messages with Protection
   Level "encryption only" might arrive at the receiving side.  While

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   not targeted to Protection Level "encryption only", this
   specification is assumed to also function for "encryption only".
   Receiving sides SHOULD implement "encryption only".

   [[ TODO: Further study is necessary to determine whether - and if yes
   to what extent - additional guidance for handling messages with
   "encryption only" protection (as well as other variations) 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

   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 a forwarded and a
   wrapped message, the 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 an attachment
   otherwise.

   The MIME structure of an Email message looks as follows:

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     <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  "O: " Outer Message Header Section

   o  "I: " Message Header Section

   o  "W: " Wrapper (MIME Header Section)

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     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.

   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.

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   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, the following 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.

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         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 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  "O: " Outer Message Header Section

   o  "I: " Message Header Section

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     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).

   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 Message contains all Header Fields
   of the Original Message with the exception of the following Header

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   Field, which MUST NOT be included within the Inner Message nor within
   any other protected part of the message:

   o  Bcc

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

4.1.3.  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 unambiguously
   delimits 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:

   o  From

   o  To (if present in the Original Message)

   o  Cc (if present in the Original Message)

   o  Bcc (if present in the Original Message, see also Section 4.1.2
      and Appendix A.1)

   o  Date

   o  Message-ID

   o  Subject

   Further processing by the Submission Entity normally depends on part
   of these Header Fields, e.g.  From and Date HFs are required by

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   [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:

   o  Content-Type

   o  Content-Transfer-Encoding

   o  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.:

   o  References

   o  Reply-To

   o  In-Reply-To

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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.  The Impact of obfuscated Date HF content to certificate
   validation is for further study, in particular regarding legacy
   clients. ]]

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

   o  To: Obfuscated <anonymous@anonymous.invalid>

   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
   routing email using onion routing or mix networks (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:

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   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:

   o  OuterM(S): Outer Message (OuterM) at sending side (before handing
      it over to the Submission Entity)

   o  OuterM(R): Outer Message at receiving side (as received by the
      last hop, before decryption and/or signature verification is
      applied to)

   o  InnerM: Inner Message (that protection is applied to)

   o  RUFM: Receiving User Facing Message

   o  More-HF: Additional Header Fields (HF) in the Original Message
      (OrigM)

   o  Wrapper: MIME Header Section; with media type (message/RFC822) to
      unambiguously delimit the inner message from the rest of the
      message.

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   o  MIME-HSp: MIME Header Section part to describe the encryption or
      signature as per [RFC8551]

   o  Trace-HF: Header Fields added in Transit (between sending and
      receiving side) as per [RFC5322]

   o  >: taken over / copied from last column

   o  =: propagates unchanged, unless something unusual (e.g. attack)
      happens

   o  *: HF that is often not present (also further HFs, e.g.  To, may
      not be present).  If a HF is not present, naturally it can neither
      be taken over nor propagated.

   o  (new) / (OrigM): HF or MIME-HSp is generated depending on the
      decision in Section 4.1.7.1, while '(new)' / '(OrigM)' designate
      the default.

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:

   o  Which Protection Level (signature and/or encryption) is applied to
      the message?  This depends on user request and/or local policy as
      well as availability of cryptographic keys.

   o  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 4.1.4.

   o  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 4.1.4.1.

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.)

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   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)

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   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 the case 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 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 ]]

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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, juga, Krista Bennett, Kelly Bristol, Lars Rohwedder, 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", draft-ietf-lamps-
              header-protection-requirements-01 (work in progress),
              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,
              <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>.

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   [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", draft-autocrypt-lamps-
              protected-headers-02 (work in progress), December 2019.

   [I-D.melnikov-iana-reg-forwarded]
              Melnikov, A. and B. Hoeneisen, "IANA Registration of
              Content-Type Header Field Parameter 'forwarded'", draft-
              melnikov-iana-reg-forwarded-00 (work in progress),
              November 2019.

   [I-D.pep-email]
              Marques, H., "pretty Easy privacy (pEp): Email Formats and
              Protocols", draft-pep-email-00 (work in progress), July
              2020.

   [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>.

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   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [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 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.

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Appendix B.  Document Changelog

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

   o  draft-ietf-lamps-header-protection-00

      *  Initial version (text partially taken over from
         [I-D.ietf-lamps-header-protection-requirements]

Appendix C.  Open Issues

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

   o  Ensure "protected header" (Ex-Memory-Hole) option is (fully)
      compliant with the MIME standard, in particular also [RFC2046],
      Section 5.1.  (Multipart Media Type) Section 4.1.1.2.

   o  Decide on format of obfuscated HFs, in particular Date HF
      (Section 4.1.4.1)

   o  Impact on spam filtering, if HFs are obfuscated (Section 4.1.4.1)

   o  More examples (e.g. in Section 4.1.7)

   o  Should Outer Message Header Section (as received) be preserved for
      the user?  (Section 4.1.8.2)

   o  Decide on whether or not merge requirements from
      [I-D.ietf-lamps-header-protection-requirements] into this
      document.

   o  Decide what parts of [I-D.autocrypt-lamps-protected-headers] to
      merge into this document.

   o  Enhance Introduction Section 1 and Problem Statement (Section 2).

   o  Decide on whether or not specification for more legacy HP
      requirements should be added to this document (Section 3.1.2).

   o  Verify simple backward compatibility case (Receiving Side MIME-
      Conformant) is working; once solution is stable and update
      paragraphs in Section 4.1, Section 3.1.2.1 and Section 4.2.1
      accordingly.

   o  Verify ability to distinguish between Messages with Header
      Protection as specified in this document and legacy clients and
      update Section 3.1 accordingly.

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   o  Improve definitions of Protection Levels and enhance list of
      Protection Levels (Section 3.2, Section 4).

   o  Privacy Considerations Section 6

   o  Security Considerations Section 5

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

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