Internet DRAFT - draft-ietf-lamps-header-protection-requirements
draft-ietf-lamps-header-protection-requirements
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational B. Hoeneisen
Expires: April 30, 2020 Ucom.ch
October 28, 2019
Problem Statement and Requirements for Header Protection
draft-ietf-lamps-header-protection-requirements-01
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 likely to be updated
regarding header protection.
This document describes the problem statement, generic use cases, and
requirements of 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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2020.
Copyright Notice
Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Security . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Usability . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Interoperability . . . . . . . . . . . . . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Interactions . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Protection Levels . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. General Requirements . . . . . . . . . . . . . . . . . . 7
4.1.1. Sending Side . . . . . . . . . . . . . . . . . . . . 7
4.1.2. Receiving Side . . . . . . . . . . . . . . . . . . . 8
4.2. Additional Requirements for Backward-Compatibility With
Legacy Clients Unaware of Header Protection . . . . . . . 8
4.2.1. Sending side . . . . . . . . . . . . . . . . . . . . 8
4.2.2. Receiving side . . . . . . . . . . . . . . . . . . . 9
4.3. Additional Requirements for Backward-Compatibility with
Legacy Header Protection Systems (if supported) . . . . . 9
4.3.1. Sending Side . . . . . . . . . . . . . . . . . . . . 9
4.3.2. Receiving Side . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Implementation Considerations . . . . . . . . . . . 12
A.1. Options to Achieve Header Protection . . . . . . . . . . 12
A.1.1. Option 1: Memory Hole . . . . . . . . . . . . . . . . 12
A.1.2. Option 2: Wrapping with message/rfc822 or
message/global . . . . . . . . . . . . . . . . . . . 12
A.1.3. Option 2.1: Progressive Header Disclosure . . . . . . 13
A.1.4. Examples . . . . . . . . . . . . . . . . . . . . . . 14
A.2. Sending Side Considerations . . . . . . . . . . . . . . . 20
A.2.1. Candidate Header Fields for Header Protection . . . . 20
A.3. Receiving Side Considerations . . . . . . . . . . . . . . 21
A.3.1. Which Header Fields to Display to User . . . . . . . 22
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A.3.2. Mail User Agent Algorithm for deciding which version
of a header field to display . . . . . . . . . . . . 22
Appendix B. Document Changelog . . . . . . . . . . . . . . . . . 22
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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
(Pretty Good Privacy) [RFC4880] 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 requirements for Header Protection
(Section 4). In Appendix A, possible solutions to address the
challenge and some best practices are collected. 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 Header Protection (HP): cryptographic protection of email Header
Sections for signatures and encryption
o Header Field (HF): cf. [RFC5322]
o Header Section (HS): cf. [RFC5322]
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 'Signature and encryption', 'signature only' or 'encryption only'
are further explained in Section 3.2.
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
o Data Minimization, which includes data spareness and hiding all
technically concealable information whenever possible
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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 a list of the generic use cases that need to be
addressed for messages with Header Protection (HP). These use cases
apply independently of whether S/MIME, PGP/MIME or any other
technology is used to achieve HP.
3.1. Interactions
The main interaction case for Header Protection (HP) is:
1) Both peers (sending and receiving side) fully support HP
For backward compatibility of legacy clients - unaware of any HP -
the following intermediate interactions need to be considered as
well:
2) The sending side fully supports HP, while the receiving side does
not support any HP
3) The sending side does not support any HP, while the receiving
side fully supports HP
4) Neither the sending side nor the receiving side supports any HP
(trivial case)
The following intermediate use cases may need to be considered as
well for backward compatibility with legacy HP systems, such as
S/MIME version 3.1 and later (cf. [RFC8551]), in the following
designated as legacy HP:
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5) The sending side fully supports HP, while the receiving side
supports legacy HP only
6) The sending side supports legacy HP only, while the receiving side
fully supports HP
7) Both peers (sending and receiving side) support legacy HP only
8) The sending side supports legacy HP only, while the receiving side
does not support any HP
9) The sending side does not support any HP, while the receiving side
supports legacy HP only
Note: It is to be decided whether to ensure legacy HP systems do not
conflict with any new solution for HP at all or whether (and to which
degree) backward compatibility to legacy HP systems shall be
maintained.
[[ TODO: Decide in which form legacy HP requirements should remain in
this document. ]]
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.
Sending and receiving side SHOULD implement 'signature and
encryption', which is the default to use on the sending side.
b) Signature only
Messages containing a cryptographic signature, but which no
encryption is applied to.
Certain implementations MAY decide to send 'signature only' messages,
depending on the circumstances and customer requirements. Sending
and Receiving sides SHOULD implement 'signature only'.
c) Encryption only
Messages that encryption is applied to which do not contain a
cryptographic signature.
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'Encryption only' is NOT RECOMMENDED on the sending side, however the
receiving side needs certain guidelines on how to process received
'encrypted only' messages
4. Requirements
The following is a list of requirements that need to be addressed
independently of whether S/MIME, PGP/MIME or any other technology is
used to apply HP to.
4.1. General Requirements
Note: This subsection lists the requirements to address use case 1)
(cf. Section 3.1).
G1: Define the HP format for all protection levels (cf. above), which
includes MIME structure, Content-Type (including all parameters,
such as "charset" and "name"), Content-Disposition (including all
parameters, such as "filename"), and Content-Transfer-Encoding.
G2: To foster wide implementation of the new solution, it shall be
easily implementable. Unless needed for maximizing protection and
privacy, existing implementations shall not require substantial
changes in the existing code base. In particular also MIME
libraries widely used shall not need to be changed to comply with
the new mechanism for HP.
G3: There SHOULD be a single format that covers all protection levels
(cf. above).
[[ TODO: Should this one remain in the document?]]
G4: Ensure that man-in-the-middle attack (MITM, cf. [RFC4949]), in
particular downgrade attacks, are mitigated to the greatest extent
possible.
4.1.1. Sending Side
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GS1: Determine which Header Fields (HFs) should or must be protected
for 'signature only' emails at a minimum.
GS2: Determine which HFs should or must be sent in clear text (i.e.,
included in the outer header) for emails with (signature and)
encryption applied.
GS3: Determine which HFs should not or must not be sent in clear text
(i.e., not be included in the outer header) of an email with
(signature and) encryption applied.
GS4: Determine which HFs to not include to any HP part (e.g. Bcc).
4.1.2. Receiving Side
GR1: Determine how HFs should be displayed to the user in case of
conflicting information between the protected and unprotected
HFs.
GR2: Ensure that man-in-the-middle attacks (MITM, cf. [RFC4949]), in
particular downgrade attacks, can be detected.
GR3: Define how emails that 'encryption only' was applied to
are to be treated.
4.2. Additional Requirements for Backward-Compatibility With Legacy
Clients Unaware of Header Protection
Note: This sub-section addresses the use cases 2) - 4) (cf.
Section 3.1)
B1: Define a means to distinguish between forwarded emails and
encapsulated emails using new HP mechanism.
4.2.1. Sending side
BS1: Define how full HP support can be indicated to outgoing
emails.
BS2: Define how full HP support of the receiver can be detected or
derived.
BS3: Ensure a HP-unaware receiving side easily can display the
"Subject" HF to the user.
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4.2.2. Receiving side
BR1: Define how full HP support can be detected in incoming emails.
4.3. Additional Requirements for Backward-Compatibility with Legacy
Header Protection Systems (if supported)
Note: This sub-section addresses the use cases 5) - 9) (cf.
Section 3.1).
LS1: Depending on the solution, define a means to distinguish between
forwarded emails, legacy encapsulated emails, and
encapsulated emails using new HP mechanism.
LS2: The solution should be backward compatible to existing solutions
and aim to minimize the implementation effort to include support
for existing solutions.
4.3.1. Sending Side
LSS1: Determine how legacy HP support can be indicated to outgoing
emails.
LSS2: Determine how legacy HP support of the receiver can be detected
or derived.
4.3.2. Receiving Side
LSR1: Determine how legacy HP support can be detected in incoming
emails.
5. Security Considerations
This document talks about UI considerations, including security
considerations, when processing messages protecting Header Fields.
One of the goals of this document is to specify UI for displaying
such messages which is less confusing/misleading for the end-user and
thus more secure.
The document does not define a new protocol, and thus does not create
any new security concerns not already covered by S/MIME [RFC8551],
MIME [RFC2045] and Email [RFC5322] in general.
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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: David
Wilson, Kelly Bristol, Robert Williams, Steve Kille, and Wei Chuang.
Essential parts of [I-D.luck-lamps-pep-header-protection] have been
merged into this document. Special thanks to its author Claudio
Luck. For further Acknowledgments, please refer to Acknowledgments
section of [I-D.luck-lamps-pep-header-protection].
David Wilson came up with the idea of defining a new Content-Type
header field parameter to distinguish forwarded messages from inner
header field protection constructs.
9. References
9.1. Normative References
[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>.
[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>.
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9.2. Informative References
[I-D.luck-lamps-pep-header-protection]
Luck, C., "pretty Easy privacy (pEp): Progressive Header
Disclosure", draft-luck-lamps-pep-header-protection-03
(work in progress), July 2019.
[I-D.marques-pep-email]
Marques, H., "pretty Easy privacy (pEp): Email Formats and
Protocols", draft-marques-pep-email-02 (work in progress),
October 2018.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[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>.
[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>.
[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>.
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Appendix A. Implementation Considerations
[[ Note: Please be advised that this part of the document is early
work-in-progress. ]]
This Appendix A contains additional information and considerations
regarding the implementation. Although not (strictly) part of the
requirements, this is useful to better understand them. Parts of the
text in this Appendix A will likely be moved to the upcoming
implementation document.
A.1. Options to Achieve Header Protection
The following are current options for addressing Email Header
Protection. The IETF LAMPS WG may choose from these options in order
to update [RFC8551].
A.1.1. Option 1: Memory Hole
The Memory Hole approach works by copying the normal message header
fields into the MIME header section of the top level protected body
part. Since the MIME body part header section is itself covered by
the protection mechanisms (signature and/or encryption) it shares the
protections of the message body.
[[ TODO: add more information on memory hole ]]
A.1.2. Option 2: Wrapping with message/rfc822 or message/global
Wrapping with message/rfc822 (or message/global) works by copying the
normal message header fields into the MIME header section of the top
level protect body part
[[ TODO: consider rephrasing, as not only the header fields is
copied, but also the content.]]
and then prepending them with "Content-Type: message/rfc822;
forwarded=no\r\n" or "Content-Type: message/global;
forwarded=no\r\n", where \r\n is US-ASCII CR followed by US-ASCII LF
(see also Appendix A.1.2.1). Since the MIME body part header section
is itself covered by the protection mechanisms (signature and/or
encryption) it shares the protections of the message body.
A.1.2.1. Content-Type Parameter "forwarded"
This section outlines how the new "forwarded" Content-Type header
field parameter could be defined (probably in a separate document)
and how header section wrapping works:
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This document defines a new Content-Type header field parameter
[RFC2045] with name "forwarded". The parameter value is case-
insensitive and can be either "yes" or "no". (The default value
being "yes"). The parameter is only meaningful with media type
"message/rfc822" and "message/global" [RFC6532] when used within
S/MIME or PGP/MIME signed or encrypted body parts. The value "yes"
means that the message nested inside "message/rfc822" ("message/
global") is a forwarded message and not a construct created solely to
protect the inner header section.
Instructions in [RFC8551] describing how to protect the Email message
header section [RFC5322], by wrapping the message inside a message/
rfc822 container [RFC2045] are thus updated to read:
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. It is up to the receiving client to
decide how to present this "inner" header section along with the
unprotected "outer" header section.
When an S/MIME message is received, if the top-level protected
MIME entity has a Content-Type of message/rfc822 or message/global
without the "forwarded" parameter or with the "forwarded"
parameter set to "no", it can be assumed that the intent was to
provide header protection. This entity SHOULD be presented as the
top-level message, taking into account header section merging
issues as previously discussed.
A.1.3. Option 2.1: Progressive Header Disclosure
This option is similar to Option 2 (cf. Appendix A.1.2). It also
makes use the Content-Type parameter "forwarded" (cf.
Appendix A.1.2.1).
pEp for email [I-D.marques-pep-email] defines a fixed MIME structure
for its innermost message structure. Security comes just next after
privacy in pEp, for which reason the application of signatures
without encryption to messages in transit is not considered
purposeful. pEp for email, either expects to transfer messages in
cleartext without signature or encryption, or transfer them encrypted
and with enclosed signature and necessary public keys so that replies
can be immediately upgraded to encrypted messages.
The pEp message format is equivalent to the S/MIME standard in
ensuring header protection, in that the whole message is protected
instead, by wrapping it and providing cryptographic services to the
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whole original message. However, for the purpose of allowing the
insertion of public keys, the root entity of the protected message is
thus nested once more into an additional multipart/mixed MIME entity.
The current pEp proposal is for PGP/MIME, while an extension to
S/MIME is also on the roadmap.
pEp has also implemented the above (in Appendix A.1.2.1) described
Content-Type parameter "forwarded" to distinguish between
encapsulated and forwarded emails.
More information on progressive header disclosure can be found in
[I-D.luck-lamps-pep-header-protection].
A.1.4. Examples
Examples in subsequent sections assume that an email client is trying
to protect (sign) the following initial message:
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Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
From: "Alexey Melnikov" <alexey.melnikov@example.net>
Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@matt.example.net>
MIME-Version: 1.0
MMHS-Primary-Precedence: 3
Subject: Meeting at my place
To: somebody@example.net
X-Mailer: Isode Harrier Web Server
Content-Type: text/plain; charset=us-ascii
This is an important message that I don't want to be modified.
Without message header protection the corresponding signed message
might look like this. (Lines prepended by "O: " are the outer
header.)
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
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--
A.1.4.1. Option 1: Memory Hole
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
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prepended by "O: " are the outer header section. Lines prepended by
"I: " are the inner header section.
O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@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--
[[ TODO (AM): HB: Not sure whether the Outer Subject HF is replaced
by "Encrypted Message" (or alike). Please verify. ]]
A.1.4.2. Option 2: Wrapping with message/rfc822 or message/global
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. Lines prepended by "W: " are the
wrapper.
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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
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--
A.1.4.3. Option 2.1 Progressive Header Disclosure
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
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. Lines prepended by "W: " are the
wrapper.
The main difference compared to Option 2 is an additional multipart/
mixed Content-Type containing the original message (as a whole) and
the public key (of the sender).
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Note: This example is derived from the pEp's PGP/MIME implementation
and adjusted to the above S/MIME examples. The pEp implementations
do not support S/MIME yet; therefore the following can serve no more
as for illustrative purpose. Specific examples can be found in
[I-D.luck-lamps-pep-header-protection].
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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>
W: MIME-Version: 1.0
W: Content-Type: multipart/mixed;
W: boundary="6b8b4567327b23c6643c986966334873"
W:
W: --6b8b4567327b23c6643c986966334873
W: Content-Type: message/rfc822; forwarded="no"
W:
I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@matt.example.net>
I: Subject: Meeting at my place
I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
I: MIME-Version: 1.0
I: Content-Type: multipart/signed; charset=us-ascii; micalg=sha1;
I: protocol="application/pkcs7-signature";
I: boundary=.cbe16d2a-e1a3-4220-b821-38348fc97237
This is a multipart message in MIME format.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
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--
W: --6b8b4567327b23c6643c986966334873
W: Content-Type: application/pgp-keys
W: Content-Disposition: attachment; filename="pEpkey.asc"
W:
-----BEGIN PGP PUBLIC KEY BLOCK-----
...
-----END PGP PUBLIC KEY BLOCK-----
W:
W: --6b8b4567327b23c6643c986966334873--
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A.2. Sending Side Considerations
A.2.1. Candidate Header Fields for Header Protection
[[ TODO: This section is very early stage and needs more work. ]]
For a 'signature only' (cf. Section 3.2) message, it is RECOMMENDED
that all "outer" header fields are identical to the "inner" protected
header fields. This would mean that all header fields are signed.
In this case, the "outer" header fields simply match the protected
header fields. And in the case that the "outer" header fields
differ, they can simply be replaced with their protected versions
when displayed to the user.
[[ TODO: Decide whether "Bcc" header field should be excluded. Also
verify whether this requirement applies generally or just for
specific implementations. ]]
When generating S/MIME messages with applied (signature and)
encryption to protect header fields:
1. If a header field is being encrypted because it is sensitive, its
true value MUST NOT be included in the outer header. If the
header field is mandatory according to [RFC5322], a stub value
(or a value indicating that the outer value is not to be used) is
to be included in the outer header section.
2. The outer header section SHOULD be minimal in order to avoid
disclosure of confidential information. It is recommended that
the outer header section only contains "Date" (set to the same
value as in the inner header field, or, if the Date value is also
sensitive, to Monday 9am of the same week), possibly "Subject"
and "To"/"Cc" header fields. ("From", "Date", and at least one
destination header field is mandatory as per [RFC5322].) In
particular, Keywords, In-Reply-To and References header fields
SHOULD NOT be included in the outer header; "To" and "Cc" header
fields should be omitted and replaced with "Bcc: undisclosed-
recipients;".
But note that having key header fields duplicated in the outer
header is convenient for many message stores (e.g. IMAP) and
clients that can't decode S/MIME encrypted messages. In
particular, Subject/To/Cc/Bcc/Date header field values are
returned in IMAP ENVELOPE FETCH data item [RFC3501], which is
frequently used by IMAP clients in order to avoid parsing message
header.
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3. The "Subject" header field value of the outer header section
SHOULD either be identical to the inner "Subject" header field
value, or contain a clear indication that the outer value is not
to be used for display (the inner header field value would
contain the true value).
Note that recommendations listed above typically only apply to non
MIME header fields (header fields with names not starting with
"Content-" prefix), but there are exceptions, e.g. Content-Language.
Note that the above recommendations can also negatively affect anti-
spam processing.
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.
Regarding the Bcc header field there should be no difference between
the inner and the outer header section.
A.3. Receiving Side Considerations
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A.3.1. Which Header Fields to Display to User
When displaying S/MIME messages which protect header fields
(independent of which protection level 'signature and encryption',
'signature only' or 'encryption only' is applied to (cf.
Section 3.2):
1. The outer header fields might be tampered with, so a receiving
client SHOULD ignore them, unless they are protected in some
other way(*). If a header field is present in the inner header,
only the inner header field value MUST be displayed (and the
corresponding outer value must be ignored). If a particular
header field is only present in the outer header, it MAY be
ignored (not displayed) or it MAY be displayed with a clear
indicator that it is not trustworthy(*).
(*) - this only applies if the header field is not protected is
some other way, for example with a DKIM signature that validates
and is trusted.
A.3.2. Mail User Agent Algorithm for deciding which version of a header
field to display
[[ TODO: describe how to recurse to find the innermost protected root
body part, extract header fields from it and propagate them to the
top level. This should also work for triple-wrapped messages.]]
Appendix B. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
o draft-ietf-lamps-header-protection-requirements-00
* Initial version
o draft-ietf-lamps-header-protection-requirements-01
* Moved Implementation Considerations to Appendix (HB)
* Shortened abstract (HB)
* Many editorial changes, e.g., replaced "content-type" with
"Content-Type". (HB)
* Added example for Option 2.1 / pEp (HB)
* Added (short) definition of Header Protection (HB)
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* Added more information regarding Bcc (feedback IETF-105) (HB)
* Simplified GS3 (HB)
* Added GR3 (HB)
Appendix C. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication. ]]
o Enhance Introduction and Problem Statement sections
o Decide in which form legacy HP requirements should remain in this
document
o Improve definitions in Section 3.2
o Should requirement G3 remain? If you consider improve / rewrite
it.
o Add more text on Memory Hole
o Rephrase Appendix A.1.2
o Resolve question regarding Bcc in Appendix A.2.1
o Rewrite Appendix A.2.1
o Write Appendix A.3.2
o Correct terminology for Header(s) and Header Fields throughout the
document (editorial).
* Header: Whole Header Section of the message
* Header Field: Part / single Line inside a Header (Section)
o Replace "email" by "email message" as needed
Authors' Addresses
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Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
Email: alexey.melnikov@isode.com
Bernie Hoeneisen
Ucom Standards Track Solutions GmbH
CH-8046 Zuerich
Switzerland
Phone: +41 44 500 52 40
Email: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen AT ucom.ch)
URI: https://ucom.ch/
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