Header Protection for S/MIME
draft-ietf-lamps-header-protection-03
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draft-ietf-lamps-header-protection-03
LAMPS Working Group D.K. Gillmor
Internet-Draft American Civil Liberties Union
Intended status: Standards Track B. Hoeneisen
Expires: 26 August 2021 pEp Foundation
A. Melnikov
Isode Ltd
22 February 2021
Header Protection for S/MIME
draft-ietf-lamps-header-protection-03
Abstract
S/MIME version 3.1 has introduced a feasible standardized option to
accomplish Header Protection. However, few implementations generate
messages using this structure, and several legacy and non-legacy
implementations have revealed rendering issues at the receiving side.
Clearer specifications regarding message processing, particularly
with respect to header sections, are needed in order to resolve these
rendering issues. Some mail user agents are also sending and
receiving cryptographically-protected message headers using a
different structure.
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.
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 26 August 2021.
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Copyright Notice
Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Two Schemes of Protected Headers . . . . . . . . . . . . 4
1.2. Problems with Wrapped Messages . . . . . . . . . . . . . 4
1.3. Motivation . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Other Protocols to Protect Email Headers . . . . . . . . 5
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 6
1.6. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2. Security . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Usability . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4. Interoperability . . . . . . . . . . . . . . . . . . . . 9
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Interactions . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1. Main Use Case . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Backward Compatibility Use Cases . . . . . . . . . . 10
3.2. Protection Levels . . . . . . . . . . . . . . . . . . . . 12
3.2.1. In-Scope . . . . . . . . . . . . . . . . . . . . . . 12
3.2.2. Out-of-Scope . . . . . . . . . . . . . . . . . . . . 12
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Main Use Case . . . . . . . . . . . . . . . . . . . . . . 13
4.1.1. MIME Format . . . . . . . . . . . . . . . . . . . . . 13
4.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . 16
4.1.3. Default Header Confidentiality Policy . . . . . . . . 21
4.1.4. Receiving Side . . . . . . . . . . . . . . . . . . . 22
4.2. Backward Compatibility Use Cases . . . . . . . . . . . . 30
4.2.1. Receiving Side MIME-Conformant . . . . . . . . . . . 31
4.2.2. Receiving Side Not MIME-Conformant . . . . . . . . . 31
5. Usability Considerations . . . . . . . . . . . . . . . . . . 32
5.1. Mixed Protections Within a Message Are Hard To
Understand . . . . . . . . . . . . . . . . . . . . . . . 32
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5.2. Users Should Not Have To Choose a Header Confidentiality
Policy . . . . . . . . . . . . . . . . . . . . . . . . . 32
6. Security Considerations . . . . . . . . . . . . . . . . . . . 32
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 32
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 35
A.1. Wrapped Message examples . . . . . . . . . . . . . . . . 35
A.1.1. Wrapped Message: signed-only, with PKCS7
signedData . . . . . . . . . . . . . . . . . . . . . 35
A.1.2. Wrapped Message: signed-only, using multipart/
signed . . . . . . . . . . . . . . . . . . . . . . . 35
A.1.3. Wrapped Message: signed-and-encrypted . . . . . . . . 35
A.2. Injected Headers examples . . . . . . . . . . . . . . . . 35
A.2.1. Injected Headers: signed-only, with PKCS7
signedData . . . . . . . . . . . . . . . . . . . . . 35
A.2.2. Injected Headers: signed-only, using multipart/
signed . . . . . . . . . . . . . . . . . . . . . . . 36
A.2.3. Injected Headers: signed-and-encrypted with Legacy
Display part . . . . . . . . . . . . . . . . . . . . 36
A.2.4. Injected Headers: signed-and-encrypted without Legacy
Display part . . . . . . . . . . . . . . . . . . . . 36
A.3. Messages without Header Protection . . . . . . . . . . . 36
A.3.1. Unprotected Headers: signed-only, with PKCS7
signedData . . . . . . . . . . . . . . . . . . . . . 36
A.3.2. Unprotected Headers: signed-only, using multipart/
signed . . . . . . . . . . . . . . . . . . . . . . . 36
A.3.3. Unprotected Headers: signed-and-encrypted . . . . . . 36
Appendix B. Additional information . . . . . . . . . . . . . . . 36
B.1. Stored Variants of Messages with Bcc . . . . . . . . . . 36
Appendix C. Text Moved from Above . . . . . . . . . . . . . . . 37
C.1. MIME Format . . . . . . . . . . . . . . . . . . . . . . . 37
C.1.1. S/MIME Specification . . . . . . . . . . . . . . . . 37
C.1.2. Sending Side . . . . . . . . . . . . . . . . . . . . 40
Appendix D. Document Considerations . . . . . . . . . . . . . . 44
Appendix E. Document Changelog . . . . . . . . . . . . . . . . . 45
Appendix F. Open Issues . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
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1. Introduction
Privacy and security issues regarding email Header Protection in S/
MIME have been identified for some time. Most current
implementations of cryptographically-protected electronic mail
protect only the body of the message, which leaves significant room
for attacks against otherwise-protected messages. For example, lack
of header protection allows an attacker to substitute the message
subject and/or author.
This document describes two different structures for how message
headers can be cryptographically protected, and provides guidance for
implementers of MUAs that generate and interpret such messages. It
takes particular care to ensure that messages interact reasonably
well with legacy MUAs.
1.1. Two Schemes of Protected Headers
Unfortunately, there are two different schemes for cryptographically-
protected email headers that may be in use on the Internet today.
This document addresses them both and provides guidance to
implementers.
One scheme is the form specified in S/MIME 3.1 and later, which
involves wrapping a "message/rfc822" MIME object with a Cryptographic
Envelope. This document calls this scheme "Wrapped Message", and it
is documented in more detail in [RFC8551]. Experience has shown that
this form does not interact well with some legacy MUAs (see
Section 1.2).
Consequently, another form of header protection is produced and
consumed by some MUAs, where the protected headers are placed
directly on the Cryptographic Payload, without using an intervening
"message/*" MIME object. This document calls this scheme "Injected
Headers", and it is documented in more detail in
[I-D.autocrypt-lamps-protected-headers].
1.2. Problems with Wrapped Messages
Several legacy MUAs have revealed rendering issues when dealing with
a message with headers protected by the Wrapped Message scheme. 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.
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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
1.3. Motivation
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.
1.4. 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)
[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. However, integrity
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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.
1.5. 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].
1.6. 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.
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* 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. Appendix C.1.2.5.
* 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.
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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. 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, Cryptographic
Envelope, Structural Headers, and MUA are all used as defined in
[I-D.dkg-lamps-e2e-mail-guidance]
* User-Facing Headers are defined in
[I-D.autocrypt-lamps-protected-headers].
* Legacy MUA: a MUA that does not understand protected headers as
described in this document. A Legacy Non-Crypto MUA is incapable
of doing any end-to-end cryptographic operations. A Legacy Crypto
MUA is capable of doing cryptographic operations, but does not
understand or generate protected headers.
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* Wrapped Message: The protected headers scheme that uses the
mechanism described in [RFC8551], where the Cryptographic Payload
is a "message/rfc822" or "message/global" MIME object.
* Injected Headers: The protected headers scheme that uses the
mechanism described in [I-D.autocrypt-lamps-protected-headers],
where the protected headers are inserted on the Cryptographic
Payload directly.
* Header Confidentiality Policy: documented in Section 4.1.2.2
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
* (Technical) Data Minimization, which includes data sparseness and
hiding all technically concealable information whenever possible
2.2. Security
* Prevent MITM attacks (cf. [RFC4949])
2.3. Usability
* Improved User interaction / User experience, in particular at the
receiving side
2.4. Interoperability
* Interoperability with [RFC8551] implementations
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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:
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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".
[...]
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).
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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).
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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.
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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)
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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.
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, and the Inner Message Body is
typically the same as the Original Message Body.
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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 C.1.1.1
(instead of the currently standardized and above defined format).
4.1.2. Sending Side
This section describes the process an MUA should use to apply
cryptographic protection to an e-mail message with header protection.
We start by describing the legacy message composition process as a
baseline.
4.1.2.1. Composing a Cryptographically-Protected Message Without Header
Protection
[I-D.dkg-lamps-e2e-mail-guidance] describes the typical process for a
legacy crypto MUA to apply cryptographic protections to an e-mail
message. That guidance and terminology is replicated here for
reference:
* "origbody": the traditional unprotected message body as a well-
formed MIME tree (possibly just a single MIME leaf part). As a
well-formed MIME tree, "origbody" already has structural headers
("Content-*") present.
* "origheaders": the intended non-structural headers for the
message, represented here as a list of "(h,v)" pairs, where "h" is
a header field name and "v" is the associated value. Note that
these are header fields that the MUA intends to be visible to the
recipient of the message. In particular, if the MUA uses the
"Bcc" header during composition, but plans to omit it from the
message (see section 3.6.3 of [RFC5322]), it will not be in
"origheaders".
* "crypto": The series of cryptographic protections to apply (for
example, "sign with the secret key corresponding to X.509
certificate X, then encrypt to X.509 certificates X and Y"). This
is a routine that accepts a MIME tree as input (the Cryptographic
Payload), wraps the input in the appropriate Cryptographic
Envelope, and returns the resultant MIME tree as output.
The algorithm returns a MIME object that is ready to be injected into
the mail system:
* Apply "crypto" to "origbody", yielding MIME tree "output"
* For each header name and value "(h,v)" in "origheaders":
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- Add header "h" of "output" with value "v"
* Return "output"
4.1.2.2. Header Confidentiality Policy
When composing an encrypted message with protected headers, the
composing MUA needs a Header Confidentialiy Policy. In this
document, we represent that Header Confidentiality Policy as a
function "hcp":
* "hcp(name, val_in) --> val_out": this function takes a header
field name "name" and initial value "val_in" as arguments, and
returns a replacement header value "val_out". If "val_out" is the
special value "null", it mean that the header in question should
be omitted from the set of headers visible outside the
Cryptographic Envelope.
For example, an MUA that only obscures the "Subject" header field by
replacing it with the literal string "[...]" and does not offer
confidentiality to any other header fields would be represented as
(in pseudocode):
"hcp(name, val_in) --> val_out: if name is 'Subject': return '[...]'
else: return val_in"
Note that such a policy is only needed when the end-to-end
protections include encryption (confidentiality). No comparable
policy is needed for other end-to-end cryptographic protections
(integrity and authenticity), as they are simply uniformly applied so
that all header fields known by the sender have these protections.
This asymmetry is an unfortunate consequence of complexities in
message delivery systems, some of which may reject, drop, or delay
messages where all headers are removed from the top-level MIME
object.
This document does not mandate any particular Header Confidentiality
Policy, though it offers guidance for MUA implementers in selecting
one in Section 4.1.3. Future documents may recommend or mandate such
a policy for an MUA with specific needs. Such a recommendation might
be motivated by descriptions of metadata-derived attacks, or stem
from research about message deliverability, or describe new
signalling mechanisms, but these topics are out of scope for this
document.
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4.1.2.3. Composing with "Wrapped Message" Header Protection
To compose a message using "Wrapped Message" header protection, we
use those inputs described in Section 4.1.2.1 plus the Header
Confidentiality Policy "hcp" defined in Section 4.1.2.2. The new
algorithm is:
* For header name and value "(h,v)" in "origheaders":
- Add header "h" of "origbody" with value "v"
* If any of the header fields in "origbody", including headers in
the nested internal MIME structure, contain any 8-bit UTF-8
characters (see section section 3.7 of [RFC6532]):
- Let "payload" be a new MIME part with one header: "Content-
Type: message/global; forwarded=no", and whose body is
"origbody".
* Else:
- Let "payload" be a new MIME part with one header: "Content-
Type: message/rfc822; forwarded=no", and whose body is
"origbody".
* Apply "crypto" to "payload", yielding MIME tree "output"
* If "crypto" contains encryption:
- Create new empty list of header field names and values "newh"
- For header name and value "(h,v)" in "origheaders":
o Let "newval" be "hcp(h, v)"
o If "newval" is not "null":
+ Append "(h,newval)" to "newh"
- Set "origheaders" to "newh"
* For header name and value "(h,v)" in "origheaders":
- Add header "h" of "output" with value "v"
* Return "output"
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Note that the Header Confidentiality Policy "hcp" is ignored if
"crypto" does not contain encryption. This is by design.
4.1.2.4. Composing with "Injected Headers" Header Protection
To compose a message using "Injected Headers" header protection, the
composing MUA needs one additional input in addition to the Header
Confidentiality Policy "hcp" defined in Section 4.1.2.2.
* "legacy": a boolean value, indicating whether any recipient of the
message is believed to have a legacy client. If all recipients
are known to implement this draft, "legacy" should be set to
"false". (How a MUA determines the value of "legacy" is out of
scope for this document; an initial implementation can simply set
it to "true")
The revised algorithm for applying cryptographic protection to a
message is as follows:
* Create a new MIME leaf part "legacydisplay" with header "Content-
Type: text/plain; protected-headers="v1"" and an empty body.
* if "crypto" contains encryption, and "legacy" is "true":
- For each header name and value "(h,v)" in "origheaders":
o If "h" is user-facing (see
[I-D.autocrypt-lamps-protected-headers]):
+ If "hcp(h,v)" is not "v":
* Add "h: v" to the body of "legacydisplay". For
example, if "h" is "Subject", and "v" is "lunch
plans?", then add the line "Subject: lunch plans?" to
the body of "legacydisplay"
* If the body of "legacydisplay" is empty:
- Let "payload" be MIME part "origbody", discarding
"legacydisplay"
* Else: (body of "legacydisplay" is not empty)
- Construct a new MIME part "wrapper" with "Content-Type:
multipart/mixed"
- Give "wrapper" exactly two subparts: "legacydisplay" and
"origbody", in that order.
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- Let "payload" be MIME part "wrapper"
* For each header name and value "(h,v)" in "origheaders":
- Add header "h" of MIME part "payload" with value "v"
* Set the "protected-headers" parameter on the "Content-Type" of
"payload" to "v1"
* Apply "crypto" to "payload", producing MIME tree "output"
* If "crypto" contains encryption:
- Create new empty list of header field names and values "newh"
- For header name and value "(h,v)" in "origheaders":
o Let "newval" be "hcp(h, v)"
o If "newval" is not "null":
+ Add "newh[h]" to "newval"
- Set "origheaders" to "newh"
* For each header name and value "(h,v)" in "origheaders":
- Add header "h" of "output" with value "v"
* Return "output"
Note that both new parameters ("hcp" and "legacy") are effectively
ignored if "crypto" does not contain encryption. This is by design,
because they are irrelevant for signed-only cryptographic
protections.
4.1.2.5. Choosing Between Wrapped Message and Injected Headers
When composing a message with end-to-end cryptographic protections,
an MUA SHOULD protect the headers of that message as well as the
body.
An MUA MAY protect the headers of any outbound message using either
the "Wrapped Message" or the "Injected Headers" style of protection.
See Section 4.2 for more discussion about reasons to choose one
mechanism or another.
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[[ TODO: this document should recommend generation of one particular
scheme by default for new implementers ]]
4.1.3. Default Header Confidentiality Policy
An MUA SHOULD have a sensible default Header Confidentiality Policy,
and SHOULD NOT require the user to select one.
The default Header Confidentiality Policy SHOULD provide
confidentiality for the "Subject" header field by replacing it with
the literal string "[...]". Most users treat the Subject of a
message the same way that they treat the body, and they are surprised
to find that the Subject of an encrypted message is visible.
[[ TODO: select one of the two policies below the recommended default
]]
4.1.3.1. Minimalist Header Confidentiality Policy
Accordingly, the most conservative recommended Header Confidentiality
Policy only protects the "Subject":
"hcp_minimal(name, val_in) --> val_out: if name is 'Subject': return
'[...]' else: return val_in"
4.1.3.2. Strong Header Confidentiality Policy
Alternately, a more aggressive (and therefore more privacy-
preserving) Header Confidentiality Policy only leaks a handful of
fields whose absence is known to increase rates of delivery failure,
and simultaneously obscures the "Message-ID" behind a random new one:
"hcp_strong(name, val_in) --> val_out: if name in ['From', 'To',
'Cc', 'Date']: return val_in else if name is 'Subject': return
'[...]' else if name is 'Message-ID': return
generate_new_message_id() else: return null"
The function "generate_new_message_id()" represents whatever process
the MUA typically uses to generate a "Message-ID" for a new outbound
message.
4.1.3.3. Offering Stronger Header Confidentiality
A MUA MAY offer even stronger confidentiality for headers of an
encrypted message than described in Section 4.1.3.2. For example, it
might implement an HCP that obfuscates the "From" field, or omits the
"Cc" field, or ensures "Date" is represented in "UTC" (obscuring the
local timezone).
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The authors of this document hope that implementers with deployment
experience will document their chosen Header Confidentiality Policy
and the rationale behind their choice.
4.1.4. Receiving Side
An MUA that receives a cryptographically-protected e-mail will render
it for the user.
The receiving MUA will render the message body, a selected subset of
header fields, and (as described in
[I-D.dkg-lamps-e2e-mail-guidance]) provide a summary of the
cryptographic properties of the message.
Most MUAs only render a subset of header fields by default. For
example, few MUAs typically render "Message-Id" or "Received" header
fields for the user, but most do render "From", "To", "Cc", "Date",
and "Subject".
A MUA that knows how to handle a message with protected headers makes
the following two changes to its behavior when rendering a message:
* If it detects that an incoming message had protected headers, it
renders header fields for the message from the protected headers,
ignoring the external (unprotected) headers.
* It includes information in the message's cryptographic summary to
indicate the types of protection that applied to each rendered
header field (if any).
A MUA that handles protected headers does _not_ need to render any
new header fields that it did not render before.
4.1.4.1. Identifying that a Message has Protected Headers
An incoming message can be identified as having protected headers
based on one of two signals:
* The Cryptographic Payload has "Content-Type: message/rfc822" or
"Content-Type: message/global" and the parameter "forwarded" has a
value of "no". See Section 4.1.4.3 for rendering guidance.
* The Cryptographic Payload has some other "Content-Type" and it has
parameter "protected-headers" set to "v1". See Section 4.1.4.4
for rendering guidance.
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Messages of both types exist in the wild, and a sensible MUA should
be able to handle them both. They provide the same semantics and the
same meaning.
4.1.4.2. Updating the Cryptographic Summary
Regardless of whether a cryptographically-protected message has
protected headers, the cryptographic summary of the message should be
modified to indicate what protections the headers have.
Each header individually has exactly one the following protections:
* "unprotected" (this is the case for all headers in messages that
have no protected headers)
* "signed-only" (bound into the same validated signature as the
enclosing message, but also visible in transit)
* "encrypted-only" (only appears within the cryptographic payload;
the corresponding external header was either omitted or
obfuscated)
* "encrypted-and-signed" (same as encrypted, but additionally is
under a validatd signature)
Note that while the message itself may be "encrypted-and-signed",
some headers may be replicated on the outside of the message (e.g.
"Date") Those headers would be "signed-only", despite the message
itself being "encrypted-and-signed".
Rendering this information is likely to be complex and messy ---
users may not understand it. It is beyond the scope of this document
to suggest any specific graphical affordances or user experience.
Future work should include examples of successful rendering of this
information.
4.1.4.3. Rendering a Wrapped Message
When the Cryptographic Payload has "Content-Type" of "message/rfc822"
or "message/global", and the parameter "forwarded" is set to "no",
the values of the protected headers are drawn from the headers of the
Cryptographic Payload, and the body that is rendered is the body of
the Cryptographic Payload.
4.1.4.3.1. Example Signed-Only Wrapped Message
Consider a message with this structure, where the MUA is able to
validate the cryptographic signature:
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A └─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
B └┬╴message/rfc822 [Cryptographic Payload]
C └┬╴multipart/alternative [Rendered Body]
D ├─╴text/plain
E └─╴text/html
The message body should be rendered the same way as this message:
C └┬╴multipart/alternative
D ├─╴text/plain
E └─╴text/html
It should render header fields taken from part "C".
Its cryptographic summary should indicates that the message was
signed and all rendered header fields were included in the signature.
The MUA SHOULD ignore header fields from part "A" for the purposes of
rendering.
4.1.4.3.2. Example Encrypted-and-Signed Wrapped Message
Consider a message with this structure, where the MUA is able to
validate the cryptographic signature:
F └─╴application/pkcs7-mime; smime-type="enveloped-data"
↧ (decrypts to)
G └─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
H └┬╴message/rfc822 [Cryptographic Payload]
I └┬╴multipart/alternative [Rendered Body]
J ├─╴text/plain
K └─╴text/html
The message body should be rendered the same way as this message:
I └┬╴multipart/alternative
J ├─╴text/plain
K └─╴text/html
It should render headers taken from part "I".
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Its cryptographic summary should indicates that the message was
signed and encrypted. Each rendered header field found in "I" should
be compared against the header field of the same name from "F". If
the value found in "F" matches the value found in "I", the header
field should be marked as "signed-only". If no matching header field
was found in "F", or the value found did not match the value from
"I", the header field should be marked as "signed-and-encrypted".
4.1.4.4. Rendering a Message with Injected Headers
When the Cryptographic Payload does not have a "Content-Type" of
"message/rfc822" or "message/global", and the parameter "protected-
headers" is set to "v1", the values of the protected headers are
drawn from the headers of the Cryptographic Payload, and the body
that is rendered is the Cryptographic Payload itself.
4.1.4.4.1. Example Signed-only Message with Injected Headers
L └─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
M └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
N ├─╴text/plain
O └─╴text/html
The message body should be rendered the same way as this message:
M └┬╴multipart/alternative
N ├─╴text/plain
O └─╴text/html
It should render header fieldss taken from part "M".
Its cryptographic summary should indicates that the message was
signed and all rendered header fields were included in the signature.
The MUA SHOULD ignore header fields from part "L" for the purposes of
rendering.
4.1.4.4.2. Example Signed-and-Encrypted Message with Injected Headers
Consider a message with this structure, where the MUA is able to
validate the cryptographic signature:
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P └─╴application/pkcs7-mime; smime-type="enveloped-data"
↧ (decrypts to)
Q └─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
R └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
S ├─╴text/plain
T └─╴text/html
The message body should be rendered the same way as this message:
R └┬╴multipart/alternative
S ├─╴text/plain
T └─╴text/html
It should render headers taken from part "R".
Its cryptographic summary should indicates that the message was
signed and encrypted. As in Section 4.1.4.3.2, each rendered header
field found in "R" should be compared against the header field of the
same name from "P". If the value found in "P" matches the value
found in "R", the header field should be marked as "signed-only". If
no matching header field was found in "P", or the value found did not
match the value from "R", the header field should be marked as
"signed-and-encrypted".
4.1.4.4.3. Do Not Render Legacy Display Part
As described [I-D.autocrypt-lamps-protected-headers], a message with
cryptographic confidentiality protection MAY include a "Legacy
Display" part for backward-compatibility with legacy MUAs
The receiving MUA SHOULD avoid rendering the Legacy Display part to
the user at all, since it is aware of and can render the actual
Protected Headers.
If a Legacy Display part is detected, it and its enclosing
"multipart/mixed" wrapper should be discarded before rendering.
4.1.4.4.3.1. Legacy Display Detection Algorithm
A receiving MUA acting on a message SHOULD detect the presence of a
Legacy Display part and the corresponding "original body" with the
following simple algorithm:
* Check that all of the following are true for the message:
* The Cryptographic Envelope must contain an encrypting
Cryptographic Layer
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* The Cryptographic Payload must have a "Content-Type" of
"multipart/mixed"
* The Cryptographic Payload must have exactly two subparts
* The first subpart of the Cryptographic Payload must have a
"Content-Type" of "text/plain" or "text/rfc822-headers"
* The first subpart of the Cryptographic Payload's "Content-Type"
must contain a property of "protected-headers", and its value must
be "v1".
* If all of the above are true, then the first subpart is the Legacy
Display part, and the second subpart is the "original body".
Otherwise, the message does not have a Legacy Display part.
4.1.4.4.3.2. Legacy Display Example
Consider a message with this structure, where the MUA is able to
validate the cryptographic signature:
U └─╴application/pkcs7-mime; smime-type="enveloped-data"
↧ (decrypts to)
V └─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
W └┬╴multipart/mixed [Cryptographic Payload]
X ├─╴text/plain [Legacy Display]
Y └┬╴multipart/alternative [Rendered Body]
Z ├─╴text/plain
A' └─╴text/html
The message body should be rendered the same way as this message,
effectively hiding the Legacy Display part ("X") and its wrapper:
Y └┬╴multipart/alternative
Z ├─╴text/plain
A' └─╴text/html
It should render headers taken from part "W", following the same
guidance as in Section 4.1.4.4.2 and Section 4.1.4.3.2 about the
cryptographic status of each rendered header field.
4.1.4.5. Affordances for Debugging and Troubleshooting
Note that advanced users of an MUA may need access to the original
message, for example to troubleshoot problems with the MUA itself, or
problems with the SMTP transport path taken by the message.
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A MUA that applies these rendering guidelines SHOULD ensure that the
full original source of the message as it was received remains
available to such a user for debugging and troubleshooting.
4.1.4.6. Composing a Reply to an Encrypted Message with Protected
Headers
When composing a reply to an encrypted message with protected
headers, the MUA is acting both as a receiving MUA and as a sending
MUA. Special guidance applies here, as things can go wrong in at
least two ways: leaking previously-confidential information, and
replying to the wrong party.
4.1.4.6.1. Avoid Leaking Encrypted Headers in Reply
As noted in [I-D.dkg-lamps-e2e-mail-guidance], an MUA in this
position MUST NOT leak previously-encrypted content in the clear in a
followup message. The same is true for protected headers.
Values from any header field that was identified as either
"encrypted" or "signed-and-encrypted" based on the steps outlined
above MUST NOT be placed in cleartext output when generating a
message.
In particular, if "Subject" was encrypted, and it is copied into the
draft encrypted reply, the replying MUA MUST obfuscate the "Subject"
field in the cleartext header as described above.
[[ TODO: formally describe how a replying MUA should generate a
message-specific Header Protection policy based on the cryptographic
status of the headers of the incoming message ]]
4.1.4.6.2. Avoid Misdirected Replies to Encrypted Messages with
Protected Headers
When replying to a message, the Composing MUA typically decides who
to send the reply to based on:
* the "Reply-To", "Mail-Followup-To", or "From" headers
* optionally, the other "To" or "Cc" headers (if the user chose to
"reply all")
When a message has protected headers, the replying MUA MUST populate
the destination fields of the draft message using the protected
headers, and ignore any unprotected headers.
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This mitigates against an attack where Mallory gets a copy of an
encrypted message from Alice to Bob, and then replays the message to
Bob with an additional "Cc" to Mallory's own e-mail address in the
message's outer header.
If Bob knows Mallory's certificate already, and he replies to such a
message without following the guidance in this section, it's likely
that his MUA will encrypt the cleartext of the message directly to
Mallory.
4.1.4.7. Implicitly-rendered Header Fields
While "From" and "To" and "Cc" and "Subject" and "Date" are often
explicitly rendered to the user, some header fields do affect message
display, without being explicitly rendered.
For example, "Message-Id", "References", and "In-Reply-To" header
fields may collectively be used to place a message in a "thread" or
series of messages.
In another example, Section 4.1.4.6.2 observes that the value of the
"Reply-To" field can influence the draft reply message. So while the
user may never see the "Reply-To" header directly, it is implicitly
"rendered" when the user interacts with the message by replying to
it.
An MUA that depends on any implicitly-rendered header field in a
message with protected headers SHOULD use the value from the
protected header, and SHOULD NOT use any value found outside the
cryptographic protection.
4.1.4.8. Unprotected Headers Added in Transit
Some headers are legitimately added in transit, and could not have
been known to the sender at message composition time.
The most common of these headers are "Received" and "DKIM-Signature",
neither of which are typically rendered, either explicitly or
implicitly.
If a receiving MUA has specific knowledge about a given header field,
including that:
* the header field would not have been known to the original sender,
and
* the header field might be rendered explicitly or implicitly,
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then the MUA MAY decide to operate on the value of that header field
from the unprotected header section, even though the message has
protected headers.
The MUA MAY prefer to verify that the headers in question have
additional transit-derived cryptographic protections (e.g., to test
whether they are covered by a valid "DKIM-Signature") before
rendering or acting on them.
Specific examples appear below.
4.1.4.8.1. Mailing list headers: List-* and Archived-At
If the message arrives through a mailing list, the list manager
itself may inject headers (most of which start with "List-") in the
message:
* "List-Archive"
* "List-Subscribe"
* "List-Unsubscribe"
* "List-Id"
* "List-Help"
* "List-Post"
* "Archived-At"
For some MUAs, these headers are implicitly rendered, by providing
buttons for actions like "Subscribe", "View Archived Version", "Reply
List", "List Info", etc.
An MUA that receives a message with protected headers that contains
these header fields in the unprotected section, and that has reason
to believe the message is coming through a mailing list MAY decide to
render them to the user (explicitly or implicitly) even though they
are not protected.
FIXME: other examples of unprotected transit headers?
4.2. Backward Compatibility Use Cases
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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.
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5. Usability Considerations
This section describes concerns for MUAs that are interested in easy
adoption of header protection by normal users.
While they are not protocol-level artifacts, these concerns motivate
the protocol features described in this document.
See also the Usability section in [I-D.dkg-lamps-e2e-mail-guidance].
5.1. Mixed Protections Within a Message Are Hard To Understand
[[ TODO ]]
5.2. Users Should Not Have To Choose a Header Confidentiality Policy
[[ TODO ]]
6. Security Considerations
[[ TODO ]]
7. Privacy Considerations
[[ TODO ]]
8. IANA Considerations
This document requests no action from IANA.
[[ RFC Editor: This section may be removed before publication. ]]
9. Acknowledgments
The authors would like to thank the following people who have
provided helpful comments and suggestions for this document: Berna
Alp, Claudio Luck, David Wilson, Hernani Marques, juga, Krista
Bennett, Kelly Bristol, Lars Rohwedder, Robert Williams, Russ
Housley, Sofia Balicka, Steve Kille, Volker Birk, and Wei Chuang.
10. References
10.1. Normative References
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[I-D.dkg-lamps-e2e-mail-guidance]
Gillmor, D. K., "Guidance on End-to-End E-mail Security",
Work in Progress, Internet-Draft, draft-dkg-lamps-e2e-
mail-guidance-01, 22 February 2021,
<https://www.ietf.org/archive/id/draft-dkg-lamps-e2e-mail-
guidance-01.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,
<https://www.ietf.org/archive/id/draft-ietf-lamps-header-
protection-requirements-01.txt>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, 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>.
10.2. Informative References
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[I-D.autocrypt-lamps-protected-headers]
Einarsson, B. R., juga, and D. K. Gillmor, "Protected
Headers for Cryptographic E-mail", Work in Progress,
Internet-Draft, draft-autocrypt-lamps-protected-headers-
02, 20 December 2019, <https://www.ietf.org/archive/id/
draft-autocrypt-lamps-protected-headers-02.txt>.
[I-D.dkg-lamps-samples]
Gillmor, D. K., "S/MIME Example Keys and Certificates",
Work in Progress, Internet-Draft, draft-dkg-lamps-samples-
05, 18 February 2021, <https://www.ietf.org/archive/id/
draft-dkg-lamps-samples-05.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,
<https://www.ietf.org/archive/id/draft-melnikov-iana-reg-
forwarded-00.txt>.
[I-D.pep-email]
Marques, H., "pretty Easy privacy (pEp): Email Formats and
Protocols", Work in Progress, Internet-Draft, draft-pep-
email-01, 2 November 2020,
<https://www.ietf.org/archive/id/draft-pep-email-01.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>.
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[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <https://www.rfc-editor.org/info/rfc6532>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
Appendix A. Test Vectors
This section contains sample messages using the different schemes
described in this document. Each sample contains a MIME object, and
examples of how an MUA might render it.
The cryptographic protections used in this document use the S/MIME
standard, and keying material and certificates come from
[I-D.dkg-lamps-samples].
For the signed-and-encrypted messages, only the "Subject" header is
obscured.
A.1. Wrapped Message examples
The examples in this subsection use the "Wrapped Message" header
protection scheme.
A.1.1. Wrapped Message: signed-only, with PKCS7 signedData
[[ TODO ]]
A.1.2. Wrapped Message: signed-only, using multipart/signed
[[ TODO ]]
A.1.3. Wrapped Message: signed-and-encrypted
[[ TODO ]]
A.2. Injected Headers examples
The examples in this subsection use the "Injected Headers" header
protection scheme.
A.2.1. Injected Headers: signed-only, with PKCS7 signedData
[[ TODO ]]
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A.2.2. Injected Headers: signed-only, using multipart/signed
[[ TODO ]]
A.2.3. Injected Headers: signed-and-encrypted with Legacy Display part
[[ TODO ]]
A.2.4. Injected Headers: signed-and-encrypted without Legacy Display
part
[[ TODO ]]
A.3. Messages without Header Protection
The examples in this subsection have cryptographic protection, but no
header protection. They are provided in this document as a
counterexample. An MUA implementer can use these messages to verify
that the reported cryptographic summary of the message indicates no
header protection.
A.3.1. Unprotected Headers: signed-only, with PKCS7 signedData
[[ TODO ]]
A.3.2. Unprotected Headers: signed-only, using multipart/signed
[[ TODO ]]
A.3.3. Unprotected Headers: signed-and-encrypted
[[ TODO ]]
Appendix B. Additional information
B.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:
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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 C. 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.
C.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])
C.1.1. S/MIME Specification
Note: This is currently described in the main part of this document.
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C.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 C.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: " Outer Message Header Section
* "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.
The Inner Message Body is the same as the Original Message Body.
The Original Message itself may contain any MIME structure.
C.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.
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C.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 B.1). Certain MUAs cannot properly decrypt Messages with
Bcc recipients. ]]
C.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.
C.1.2.3. Cryptographic Layers / Envelope
[[ TODO: Basically refer to S/MIME standards ]]
C.1.2.4. Sending Side Message Processing
For a protected Message the following steps are applied before a
Message is handed over to the Submission Entity:
C.1.2.4.1. 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. Appendix C.1.2.5.
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* 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.
Appendix C.1.2.5.
C.1.2.4.2. Step 2: Compose the Outer Message Header Section
Depending on the decision in Appendix C.1.2.4.1, 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
Appendix C.1.2.4.1).
C.1.2.4.3. Step 3: Apply Protection to the Original Message
Depending on the Protection Level selected in Appendix C.1.2.4.1, 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 ]]
C.1.2.5. Outer Message Header Fields
C.1.2.5.1. 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)
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* Cc (if present in the Original Message)
* Bcc (if present in the Original Message, see also Appendix B.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].)
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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
C.1.2.5.2. 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.
Appendix D. Document Considerations
[[ RFC Editor: This section is to be removed before publication ]]
This draft is built from markdown source, and its development is
tracked in a git repository (https://gitlab.com/dkg/lamps-header-
protection).
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While minor editorial suggestions and nit-picks can be made as merge
requests (https://gitlab.com/dkg/lamps-header-protection), please
direct all substantive discussion to the LAMPS mailing list
(https://www.ietf.org/mailman/listinfo/spasm) at "spasm@ietf.org".
Appendix E. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
* draft-ietf-lamps-header-protection-03
- dkg takes over from Bernie as primary author
- Add Usability section
- describe two distinct formats "Wrapped Message" and "Injected
Headers"
- Introduce Header Confidentiality Policy model
- Overhaul message composition guidance
- Simplify document creation workflow, move public face to gitlab
* draft-ietf-lamps-header-protection-02
- editorial changes / improve language
* draft-ietf-lamps-header-protection-01
- Add DKG as co-author
- Partial Rewrite of Abstract and Introduction [HB/AM/DKG]
- Adding definiations for Cryptographic Layer, Cryptographic
Payload, and Cryptographic Envelope (reference to
[I-D.dkg-lamps-e2e-mail-guidance]) [DKG]
- Enhanced MITM Definition to include Machine- / Meddler-in-the-
middle [HB]
- Relaxed definition of Original message, which may not be of
type "message/rfc822" [HB]
- Move "memory hole" option to the Appendix (on request by Chair
to only maintain one option in the specification) [HB]
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- Updated Scope of Protection Levels according to WG discussion
during IETF-108 [HB]
- Obfuscation recommendation only for Subject and Message-Id and
distinguish between Encrypted and Unencrypted Messages [HB]
- Removed (commented out) Header Field Flow Figure (it appeared
to be confusing as is was) [HB]
* draft-ietf-lamps-header-protection-00
- Initial version (text partially taken over from
[I-D.ietf-lamps-header-protection-requirements]
Appendix F. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication. ]]
* Ensure "protected header" (Ex-Memory-Hole) option is (fully)
compliant with the MIME standard, in particular also [RFC2046],
Section 5.1. (Multipart Media Type) Appendix C.1.1.1.
* Test Vectors! This should be a new appendix section, to avoid
injecting large blobs of unreadable data in the main text. Once
present, we can point to the relevant test vector in the main text
by reference.
* Should Outer Message Header Section (as received) be preserved for
the user? (Section 4.1.4.5)
* Decide on whether or not merge requirements from
[I-D.ietf-lamps-header-protection-requirements] into this
document.
* Decide what parts of [I-D.autocrypt-lamps-protected-headers] to
merge into this document.
* Enhance Introduction Section 1 and Problem Statement (Section 2).
* Decide on whether or not specification for more legacy HP
requirements should be added to this document (Section 3.1.2).
* 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.
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Internet-Draft Header Protection S/MIME February 2021
* Verify ability to distinguish between Messages with Header
Protection as specified in this document and legacy clients and
update Section 3.1 accordingly.
* Improve definitions of Protection Levels and enhance list of
Protection Levels (Section 3.2, Section 4).
* Privacy Considerations Section 7
* Security Considerations Section 6
Authors' Addresses
Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Email: dkg@fifthhorseman.net
Bernie Hoeneisen
pEp Foundation
Oberer Graben 4
CH- CH-8400 Winterthur
Switzerland
Email: bernie.hoeneisen@pep.foundation
URI: https://pep.foundation/
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom
Email: alexey.melnikov@isode.com
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