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Network Working Group B. Hoeneisen
Internet-Draft pEp Foundation
Intended status: Informational A. Melnikov
Expires: January 10, 2021 Isode Ltd
July 09, 2020
Header Protection for S/MIME
draft-ietf-lamps-header-protection-00
Abstract
Privacy and security issues with email header protection in S/MIME
have been identified for some time. However, the desire to fix these
issues has only recently been expressed in the IETF LAMPS Working
Group. The existing S/MIME specification is to be updated regarding
header protection.
This document describes the problem statement, generic use cases, and
the S/MIME specification for header protection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 10, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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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
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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
(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 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
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.
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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. It typically determines
the destination recipients by reading the To, Cc and Bcc Header
Fields (of the Outer Message).
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 or for
mixnet networks, i.e. "onion routing" for email (e.g.
[pEp.mixnet]).
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 peers (sending and receiving side) (fully) support Header
Protection as specified in this document.
The main use case is specified in Section 4.1.
3.1.2. Backward Compatibility Use Cases
Regarding backwards compatibility the main distinction is based on
whether or not the receiving side conforms to MIME according to
[RFC2046], ff., which in particular also includes Section 2 of
[RFC2049] on "MIME Conformance". In the following an excerpt of
paragraphs relevant in this context:
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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.
Note: The compatibility of legacy HP systems with this new solution,
and how to handle issues surrounding future maintenance for these
legacy systems, will be decided by the LAMPS WG.
3.1.2.1. Receiving Side MIME-Conformant
The sending side (fully) supports Header Protection as specified in
this document, while the receiving side does not support this
specification. The receiving side is MIME-conformant according to
[RFC2045], ff. (cf. Section 3.1.2),
This use case is specified in Section 4.2.1.
Note: This case is expected to just work fine, if the sending side
applies specification for the main use case Section 4.1.
[[TODO: Verify once solution is stable and update last sentence ]]
3.1.2.2. Receiving Side Not MIME-Conformant
The sending side (fully) supports Header Protection as specified in
this document, while the receiving side does not support this
specification. The receiving side is *not* MIME-conformant according
to [RFC2045], ff. (cf. Section 3.1.2) either.
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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.
4. Specification
This section contains the specification for Header Protection in
S/MIME to update and it clarifies Section 3.1 of [RFC8551] (S/MIME
4.0).
Furthermore, it is likely that PGP/MIME [RFC3156] will also
incorporate this specification or parts of it.
This specification applies to the protection levels "signature &
encryption" and "signature only" (cf. Section 3.2):
Sending and receiving sides MUST implement "signature and
encryption", which is the default to use on the sending side.
Certain implementations MAY decide to send "signature only" messages,
depending on the circumstances and customer requirements. Sending
side MAY and receiving sides MUST implement "signature only".
It generally is NOT RECOMMENDED to send a message with protection
level "encryption only". On the other hand, messages with protection
level "encryption only" might arrive at the receiving side. While
not targeted to protection level "encryption only", this
specification is assumed to also function for "encryption only".
Receiving sides SHOULD implement "encryption only".
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Note: It is for further study whether or not more guidance for
handling messages with protection level "encryption only" at the
receiving side is needed.
4.1. Main Use Case
This section applies to the main use case, where both peers (sending
and receiving side) (fully) support Header Protection as specified
herein (cf. Section 3.1.1).
Note: The sending side specification of the main use case is also
applicable to the cases, where the sending side (fully) supports
Header Protection as specified herein, while the receiving side does
not support this specification, but is MIME-conformant according to
[RFC2045], ff. (cf. Section 3.1.2) and Section 3.1.2.1)
Further backward compatibility cases are defined in Section 4.2.
4.1.1. MIME Format
Currently there are two options in discussion:
1. The option according to the current S/MIME specification (cf.
[RFC8551])
2. An alternative option that is based on the former "memory hole"
approach (cf. [I-D.autocrypt-lamps-protected-headers])
4.1.1.1. S/MIME Specification
As per S/MIME version 3.1 and later (cf. [RFC8551]), the sending
client MAY wrap a full MIME message in a message/RFC822 wrapper in
order to apply S/MIME security services to these header fields.
To help the receiving side to distinguish between forwarded and
wrapped message, a Content-Type header field parameter "forwarded" is
added as defined in [I-D.melnikov-iana-reg-forwarded]. Certain
mailing applications might display the Inner Message as attachment
otherwise.
The MIME structure of an Email message looks as follows:
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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 header section and payload of
a protected body part might look like. For example, this will be the
first body part of a multipart/signed message or the signed and/or
encrypted payload of the application/pkcs7-mime body part. Lines
prepended by "O: " are the Outer Message Header Section. Lines
prepended by "I: " are the Inner Message Header Section. Lines
prepended by "W: " are the wrapper (MIME Header Section):
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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=.cbe16d2a-e1a3-4220-b821-38348fc97237
This is a multipart message in MIME format.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
W: Content-Type: message/RFC822; forwarded=no
W:
I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
I: MIME-Version: 1.0
I: MMHS-Primary-Precedence: 3
I: Subject: Meeting at my place
I: To: somebody@example.net
I: X-Mailer: Isode Harrier Web Server
I: Content-Type: text/plain; charset=us-ascii
This is an important message that I don't want to be modified.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature
[[base-64 encoded signature]]
--.cbe16d2a-e1a3-4220-b821-38348fc97237--
The Outer Message Header Section is unprotected, while the remainder
(Outer Message Body) is protected. The Outer Message Body consists
of the wrapper (MIME Header Section) and the Inner Message (Header
Section and Body).
The wrapper is a simple MIME Header Section with media type "message/
RFC822" containing a Content-Type header field parameter
"forwarded=no" followed by an empty line.
The Inner Message Header Section is the same as (or a subset of) the
Original Message Header Section (cf. Section 4.1.2).
The Inner Message Body is the same as the Original Message Body.
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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, 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 the header section and payload
of a protected body part might appear. For example, this will be the
first body part of a multipart/signed message or the signed and/or
encrypted payload of the application/pkcs7-mime body part. Lines
prepended by "O: " are the outer header section. Lines prepended by
"I: " are the inner header section.
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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=.cbe16d2a-e1a3-4220-b821-38348fc97237
This is a multipart message in MIME format.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
I: MIME-Version: 1.0
I: MMHS-Primary-Precedence: 3
I: Subject: Meeting at my place
I: To: somebody@example.net
I: X-Mailer: Isode Harrier Web Server
I: Content-Type: text/plain; charset=us-ascii
This is an important message that I don't want to be modified.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
Content-Transfer-Encoding: base64
Content-Type: application/pkcs7-signature
[[base-64 encoded signature]]
--.cbe16d2a-e1a3-4220-b821-38348fc97237--
The Outer Message Header Section is unprotected, while the remainder
(Outer Message Body) is protected. The Outer Message Body consists
of the Inner Message (Header Section and Body).
The Inner Message Header Section is the same as (or a subset of) the
Original Message Header Section (cf. Section 4.1.2).
The Inner Message Body is the same as the Original Message Body.
The Original Message itself may contain any MIME structure.
4.1.2. Inner Message Header Fields
It is RECOMMENDED that the Inner Messages contains all the Header
Fields of the Original Message with the exception of the following
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Header 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 delimits
unambiguously the Inner Message from the rest of the message.
4.1.4. Outer Message Header Fields
To maximize Privacy, it is strongly RECOMMENDED to follow the
principle of Data Minimization (cf. Section 2.1).
However, the Outer Message Header Section SHOULD contain the
Essential Header Fields and, in addition, MUST contain the Header
Fields of the MIME Header Section part to describe the encryption or
signature as per [RFC8551].
The following Header Fields are defined as the Essential Header
Fields:
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. ]]
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 [1]
Such implementations may need to ensure that the Submission Entity
has access to the content of these Header Fields in clear text and is
capable of processing those. This is particularly relevant, if
proprietary Submission Entities are used.
A use case for obfuscation of all Outer Message Header Fields is
mixnet networks, i.e. "onion routing" for email (e.g. [pEp.mixnet]).
Note: It is for further study to what extent Header Field obfuscation
adversely impacts spam filtering.
4.1.5. Receiving User Facing Message Header Fields
The Receiving User Facing Message SHOULD be a verbatim copy of the
Inner Message.
4.1.6. Header Field Flow
The Following figure depicts the different message representations
(OrigM, InnerM, OuterM, RUFM) and which parts those are constructed
from:
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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. {#main-use-
case}}) MUST ensure that receiving sides, that do not support this
specification, but are MIME-conformant according to [RFC2045], ff.
can still recognize and display the Inner Message (or rather the
RUFM) in such a way as to preserve any recursive structure, that is,
displaying or offering to display the encapsulated data in accordance
with its media type (cf. [RFC2049], Section 2).
[[TODO: Verify once solution is stable and update last sentence ]]
4.2.2. Receiving Side Not MIME-Conformant
This section applies to the case where the sending side (fully)
supports Header Protection as specified in this document, while the
receiving side neither supports this specification and *nor* is MIME-
conformant according to [RFC2045], ff. (cf. {{uc-ia-backward-
compatibility-use-cases}) and Section 3.1.2.2).
[I-D.autocrypt-lamps-protected-headers] describes a possible way to
achieve backward compatibility with existing S/MIME (and PGP/MIME)
implementations that predate this specification and are not MIME-
conformant (Legacy Display) either. It mainly focuses on email
clients that do not render emails using header protection (in a user
friendly manner) and may confuse the user. While this has been
observed occasionally in PGP/MIME (cf. [RFC3156]), the extent of
this problem with S/MIME implementations is still unclear. (Note: At
this time, none of the samples in
[I-D.autocrypt-lamps-protected-headers] apply header protection as
specified in Section 3.1 of [RFC8551], which is wrapping as Media
Type "message/RFC822".)
Should serious backward compatibility issues with rendering at the
receiver be discovered, the Legacy Display format described in
[I-D.autocrypt-lamps-protected-headers] may serve as a basis to
mitigate those issues (cf. Section 4.2).
Another variant of backward compatibility has been implemented by pEp
[I-D.pep-email], i.e. pEp Email Format 1.0. At this time pEp has
implemented this for PGP/MIME, but not yet S/MIME.
5. Security Considerations
[[ TODO ]]
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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, Krista Bennett, Kelly Bristol, Robert Williams, Sofia
Balicka, Steve Kille, Volker Birk, and Wei Chuang.
9. References
9.1. Normative References
[I-D.ietf-lamps-header-protection-requirements]
Melnikov, A. and B. Hoeneisen, "Problem Statement and
Requirements for Header Protection", 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-marques-pep-email-02 (work in progress),
October 2018.
[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>.
9.3. URIs
[1] mailto:anonymous@anonymous.invalid
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
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case, the message has to be cloned and adjusted depending on the
recipient.
Appendix B. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
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 Change adding Content-Type header field parameter "forwarded" from
SHOULD to MUST (Section 4.1.3)?
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 and Problem Statement sections
o Decide on whether or not specification for more legacy HP
requirements should be added to this document
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o Verify ability to distinguish between Messages with Header
Protection as specified in this document and legacy clients and
update Section 3.1 accordingly.
o Verify simple backward compatibility case (Receiving Side MIME-
Conformant) is working; once solution is stable and update
paragraphs in {#uc-ia-bc-receiving-side-mime-conformant}} and
Section 4.2.1 accordingly.
o Improve definitions in Section 3.2
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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