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Network Working Group B. Hoeneisen
Internet-Draft pEp Foundation
Intended status: Informational A. Melnikov
Expires: December 28, 2020 Isode Ltd
June 26, 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 December 28, 2020.
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 . . . . . . . . . . . . . . . . . . . . 6
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Interactions . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Main Case for Header Protection . . . . . . . . . . . 7
3.1.2. Backward Compatibility . . . . . . . . . . . . . . . 7
3.2. Protection Levels . . . . . . . . . . . . . . . . . . . . 7
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Main Use Case . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. MIME Format . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. Inner Message Header Fields . . . . . . . . . . . . . 13
4.1.3. Wrapper . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.4. Outer Message Header Fields . . . . . . . . . . . . . 13
4.1.5. Sending Side Message Processing . . . . . . . . . . . 15
4.1.6. Receiving Side Message Processing . . . . . . . . . . 16
4.1.7. Header Field Flow . . . . . . . . . . . . . . . . . . 17
4.2. Backward Compatibility Use Case . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix A. Additional information . . . . . . . . . . . . . . . 21
A.1. Stored Variants of Messages with Bcc . . . . . . . . . . 21
Appendix B. Document Changelog . . . . . . . . . . . . . . . . . 22
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
A range of protocols for the protection of electronic mail (email)
exist, which allow 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
spareness 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 whatever
mechanism will be 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 to base
the upcoming discussions on. In any case, the final solution 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].
o Transport: The entity taking care of the transport of a Message
towards the receiver or from the sender. The Transport on the
sending side typically determines the destination recipients by
reading the To, Cc and Bcc Header Fields (of the Outer Message).
The Transport is typically implemented by an MTA (Mail Transfer
Agent).
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].
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 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
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payload of a Message. Typically, the Body consists of a
(multipart) MIME [RFC2045] construct.
o MIME Header Fields: Header Fields describing the MIME structure of
its body as defined in [RFC2045].
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 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 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, shortly
before wrapping and protection measures are applied to on the
sending side or the message shortly after decryption and
unwrapping has been applied to 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. 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
Transport or received from the Transport 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 after the Outer Message Header
Section has been merged with the Inner Message Header Section.
o Essential Header Fields (EHF): The Header Fields an Outer Message
Header Section SHOULD contain at least; cf. Section 4.1.4.
o Protected: Protected refers to the parts of a message where
protection measures of any Protection Levels have been applied to.
o Protected Message: A message that protection measures of any
Protection Levels have been applied to.
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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 Data Minimization: Data spareness and hiding 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 spareness and hiding all
technically concealable information whenever possible
2.2. Security
o MITM attacks (cf. [RFC4949])
2.3. Usability
o User interaction / User experience
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).
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These use cases apply independently of whether S/MIME, PGP/MIME or
any other technology is used to achieve HP.
3.1. Interactions
3.1.1. Main Case for Header Protection
Both peers (sending and receiving side) fully support Header
Protection as specified in this document; see Section 4.1.
3.1.2. Backward Compatibility
The sending side fully supports Header protection as specified in
this document, while the receiving side does not support any Header
Protection; see Section 4.2.
Note: The compatibility of legacy HP systems with this new solutions,
and how to handle issues surrounding future maintenance for these
legacy systems, will be decided by the LAMPS WG.
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 clarifies Section 3.1 of [RFC8551] (S/MIME 4.0).
This specification is likely to be integrated into S/MIME at some
later stage.
Furthermore, it is likely that PGP/MIME [RFC3156] will also take over
this specification or parts of it.
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This specification applies to the protection levels "signature &
encryption" and "signature only" (cf. Section 3.2):
Sending and receiving sides SHOULD 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 SHOULD implement "signature only".
It generally is NOT RECOMMENDED to send a message with protection
level "encryption only". On the other hand, messages with protection
level "encryption only" might arrive at the receiving side. While
not targeted to protection level "encryption only", this
specification is assumed to also function for "encryption only".
Receiving sides SHOULD implement "encryption only".
Note: It is for further study whether or not more guidance for
handling messages with protection level "encryption only" at the
receiving side is needed.
4.1. Main Use Case
The following covers the Interaction (cf. Section 3.1) if all
parties (sending and receiving side) implement this specification.
(For backward compatibility cases cf. 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.
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The MIME structure of an Email message looks as follows:
<Outer Message Header Section (unprotected)>
<Outer Message Body (protected)>
<MIME Header Section (wrapper)>
<Inner Message Header Section>
<Inner Message Body>
The following example demonstrates how header section and payload of
a protect 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@matt.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@matt.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.
There may also be an additional MIME layer with media type
"multipart/mixed" in the Outer Message Body to contain the Inner
Message (wrapped in a "message/RFC822") along with other MIME
part(s).
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 unambigously delimit the Inner
Message.
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 header section and payload of
a protect 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 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@matt.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@matt.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.
There may also be an additional MIME layer with media type
"multipart/mixed" in the Outer Message Body to contain the Inner
Message along with other MIME part(s).
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4.1.2. Inner Message Header Fields
It is RECOMMEND that the Inner Messages contains all the Header
Fields of the Original Message with the exception of the following
Header Field, which MUST NOT be included to the Inner Message nor to
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 SHOULD contain
the Content-Type header field parameter "forwarded=no" as defined in
[I-D.melnikov-iana-reg-forwarded]. The wrapper delimits unambigously
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 OrigM)
o Cc (if present in the OrigM)
o Bcc (if present in the OrigM, see also Section 4.1.2 and
Appendix A.1)
o Date
o Message-ID
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o Subject
Some of these Header Fields are needed by the Transport (e.g. to
determine the destination). Furthermore, not including certain
Header Fields may trigger spam detection to flag the message as spam
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 SHOULD be prefered 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 MIME-Version
o Content-Type
o Content-Transfer-Encoding
o Content-Disposition
The following example shows the MIME header 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 obfucated. Those may be replaced by e.g.
o To: Obfuscated anonymous@anonymous.invalid [1]
Such implementations need to ensure that the Transport has access to
these Header Fields in clear text and is capable of processing those.
A use case for obfuscation of all Outer Message Header Fields is
mixnet netwerks, 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. Sending Side Message Processing
For a protected message the following steps are applied before a
message is handed over to the Transport:
4.1.5.1. 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 Orignial 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.
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4.1.5.2. Step 2: Compose the Outer Message Header Section
Depending on the decision in Section 4.1.5.1, compose the Outer
Message Header Section. (Note that this also includes the necessary
MIME Header Section part for the following protection layer.)
Outer Header Fields that are not obfuscated SHOULD contain the same
values as in the Original Message (except for MIME Header
Section part, which depends on the protection level selected in
Section 4.1.5.1).
4.1.5.3. Step 3: Apply Protection to the Original Message
Depending on the protection level selected in Section 4.1.5.1 apply
signature and/or encryption to the original message including the
wrapper (as per [RFC8551]) and put the result to the message as Outer
Message Body.
The resulting (Outer) Message is then typically handed over to the
Transport.
[[ TODO: Example ]]
4.1.6. Receiving Side Message Processing
When a protected message is received the following steps are applied:
4.1.6.1. 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].
4.1.6.2. Step 2: Construct the Receiving User Facing Message
The Receiving User Facing Message is constructed as follows:
o The Header Section of the Receiving User Facing Message MUST
consist of the Outer Message Header Fields that are not contained
in the Inner Message Header Section, and the Inner Message Header
Fields (i.e. the Inner Message Header Field MUST always take
precedence).
o The Body of the Receiving User Facing Message Body MUST be the
same as the Inner Message Body.
The resulting message is handed over for further processing, which
typically involves rendering it to the user.
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[[ TODO: Do we need to take special care for HFs, which may appear
multiple times, e.g. Received HF? ]]
Note: It is for further study whether and, if yes, how the Outer
Message Header Section (as received from the Transport) is preserved
for the user.
4.1.7. Header Field Flow
The Following figure depicts the different message representations
(OrigM, InnerM, OuterM, RUFM) and which parts those are constructed
from:
OrigM InnerM Outer(S) OuterM(R) RUFM
<Trans-HF> > <Trans-HF>
From (OrigM) = From
To (OrigM) = To
Cc (OrigM) = Cc
Bcc (OrigM) = Bcc* > 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
processing by Transport)
o OuterM(R): Outer Message at receiving side (as received by
Transport)
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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
unambigously delimit the inner message from the rest of the
message.
o MIME-HSp: MIME Header Section part to describe the encryption or
signature as per [RFC8551]
o Trans-HF: Header Fields added in Transit (between sending and
receiving side)
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 HF 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 4.1.5.1, while '(new)' / '(OrigM)' designate
the default.
4.2. Backward Compatibility Use Case
[I-D.autocrypt-lamps-protected-headers] describes a possibility to
achieve backward compatibility with existing S/MIME (and PGP/MIME)
implementations unaware of this specification (Legacy Display). It
mainly focuses on email clients that do not render emails using
header protection (nicely) 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] applies 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 reveal, the Legacy Display format described in
[I-D.autocrypt-lamps-protected-headers] may serve as a basis to
mitigate those (backward compatibility use case).
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Another variant of backward compatibility has been implemented by pEp
[I-D.pep-email], i.e. pEp Email Format 1.0. At this time pEp has
implemented this for PGP/MIME (but not yet S/MIME).
5. Security Considerations
[[ TODO ]]
6. Privacy Considerations
[[ TODO ]]
7. IANA Considerations
This document requests no action from IANA.
[[ RFC Editor: This section may be removed before publication. ]]
8. Acknowledgments
The authors would like to thank the following people who have
provided helpful comments and suggestions for this document: Claudio
Luck, 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>.
[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>.
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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 addressees)
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.
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Appendix B. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
o draft-ietf-lamps-header-protection-00
* Initial version (text partialy 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 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 4.1.5)
o Should Outer Message Header Section (as received) be preserved for
the user? (Section 4.1.6.2)
o Do we need to take special care of HFs that may appear multiple
times, e.g. Received HF? (Section 4.1.6.2)
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
o Improve definitions in Section 3.2
o Privacy Considerations Section 6
o Security Considerations Section 5
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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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