wdiff draft-ietf-lamps-header-protection-requirements-00.txt draft-ietf-lamps-header-protection-requirements-01.txt

Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational B. Hoeneisen
Expires: January 9, April 30, 2020 Ucom.ch
July 08,
October 28, 2019

Problem Statement and Requirements for Header Protection
draft-ietf-lamps-header-protection-requirements-00
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
issue
issues has only recently been expressed in the IETF LAMPS Working Group only
recently.
Group. The existing S/MIME specification is likely to be updated
regarding header protection.

Several 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, generic use cases, and
requirements. Additionally it drafts possible solutions to address
the challenge. Finally some best practices are collected.
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
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This Internet-Draft will expire on January 9, April 30, 2020.

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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 . . . . . . . . . . . . . . . . . . . . . . . . 6 7
4.1. General Requirements . . . . . . . . . . . . . . . . . . 6 7
4.1.1. Sending Side . . . . . . . . . . . . . . . . . . . . 7
4.1.2. Receiving Side . . . . . . . . . . . . . . . . . . . 7 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 . . . . . . . . . . . . . . . . . . . 8 9
4.3. Additional Requirements for Backward-Compatibility with
Legacy Header Protection Systems (if supported) . . . . . 8 9
4.3.1. Sending Side . . . . . . . . . . . . . . . . . . . . 9
4.3.2. Receiving Side . . . . . . . . . . . . . . . . . . . 9
5. Options to Achieve Header Protection . Security Considerations . . . . . . . . . . . 9
5.1. Option 1: Memory Hole . . . . . . . . 9
6. Privacy Considerations . . . . . . . . . . 9
5.2. Option 2: Wrapping with message/rfc822 or message/global 9
5.2.1. Content-Type Parameter "forwarded" . . . . . . . . . 10
5.3. Option 2.1: Progressive Header Disclosure
7. IANA Considerations . . . . . . . . 10
5.4. Examples . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . 11
5.4.1. Option 1: Memory Hole . . . . . . . . . . . . 10
9. References . . . . 12
5.4.2. Option 2: Wrapping with message/rfc822 or
message/global . . . . . . . . . . . . . . . . . . . 13
5.4.3. Option 2.1 Progressive Header Disclosure . . 10
9.1. Normative References . . . . 14
6. Sending Side Considerations . . . . . . . . . . . . . . 10
9.2. Informative References . . . 14
6.1. Candidate Header Fields for Header Protection . . . . . . 14
7. Receiving Side Considerations . . . . . . . . 11
Appendix A. Implementation Considerations . . . . . . . . 16
7.1. Which Header Fields to Display to User . . . . . . . . . 16
7.2. Mail User Agent Algorithm for deciding which version of a
header field 12
A.1. Options to display . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . Achieve Header Protection . . 16
9. Privacy Considerations . . . . . . . . 12
A.1.1. Option 1: Memory Hole . . . . . . . . . . . 17
10. IANA Considerations . . . . . 12
A.1.2. Option 2: Wrapping with message/rfc822 or
message/global . . . . . . . . . . . . . . . . 17
11. Acknowledgments . . . 12
A.1.3. Option 2.1: Progressive Header Disclosure . . . . . . 13
A.1.4. Examples . . . . . . . . . . . . . . 17
12. References . . . . . . . . 14
A.2. Sending Side Considerations . . . . . . . . . . . . . . . 20
A.2.1. Candidate Header Fields for Header Protection . . 17
12.1. Normative References . . 20
A.3. Receiving Side Considerations . . . . . . . . . . . . . . 21
A.3.1. Which Header Fields to Display to User . . 17
12.2. Informative References . . . . . 22
A.3.2. Mail User Agent Algorithm for deciding which version
of a header field to display . . . . . . . . . . . . 18 22
Appendix A. B. Document Changelog . . . . . . . . . . . . . . . . . 18 22
Appendix B. C. Open Issues . . . . . . . . . . . . . . . . . . . . 19 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 23

1. Introduction

[[ Note: Please be advised that this document is early work-in-
progress, and will substantially change in future revisions. ]]

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 domain-
level perspective, specifically DomainKeys Identified Mail (DKIM)
[RFC6376] and Sender Policy Framework (SPF) [RFC7208] [RFC7208], and Domain-based 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 headers
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 the hiding of 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 headers header section
exists for S/MIME since version 3.1. 3.1 and later. (cf. [RFC8551]):

No mechanism for header protection (HP) has been standardized for PGP
(Pretty Good Privacy) [RFC4880] yet.

End-to-end protection

Several varying implementations of end-to-end protections for the email headers section is currently not
widely implemented - neither for messages protected by means of
S/MIME nor PGP. At least two variants of
header protection are known sections exist, though the total number of such
implementations appears to be implemented. 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, statement (Section 2), generic
use cases (Section 3) and requirements for header protection Header Protection
(Section 4).
Additionally it drafts In Appendix A, possible solutions to address the challenge.
However,
challenge and some best practices are collected. In any case, the
final solution will is to be determined by the IETF LAMPS WG.
Finally, some best practices are collected.

[[ TODO: enhance this section ]]

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 Field:: cf. [RFC5322]

o Protection (HP): cryptographic protection of email Header Section: cf. [RFC5322]

o Signed-only message: a multipart/signed or application/pkcs7-mime
containing SignedData message which doesn't contain any encrypted
layer. I.e. this is a message which is not encrypted
Sections for signatures and not
encrypted + signed. 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 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, we show 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 for which Header Protection (HP) is to be applied
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 (trivial case)

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 since version 3.1 and later (cf. [RFC8551]), in the following
designated as legacy HP:

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 (trivial case)

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 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 [[ TODO: verify whether relevant ]]

Messages that encryption is applied to which do not contain a
cryptographic signature.

'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

In the

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 is listing lists the requirements to address use case 1)
(cf. Section 3.1).

G1: Define the format for 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 only one a single format that covers all Protection Levels protection levels
(cf. {{protection-levels}})) above).

[[ TODO: Should this one remain in the document?
If yes, consider improve / rewrite sentence
]] document?]]

G4: Ensure that man-in-the-middle attack (MITM) (MITM, cf. {{RFC4949}}, [RFC4949]), in
particular downgrade attacks, are mitigated as good as to the greatest extent
possible.

4.1.1. Sending Side
GS1: Determine which Header Fields (HFs) should or must be protected
at least
for signed only email. 'signature only' emails at a minimum.

GS2: Determine which HFs should or must be sent in clear of an
encrypted email. text (i.e.,
included in the outer header) for emails with (signature and)
encryption applied.

GS3: Determine which HF HFs should not or must not be sent in clear text
(i.e., not be included in the
visible header (for transport) outer header) of an encrypted email, email with the
default being that whatever is not needed from GS2 is not put
into the unencrypted transport headers, thus fulfilling data
minimization requirements (including data spareness and hiding
of all information that technically can be hidden).
(signature and) encryption applied.

GS4: Determine which HF HFs to not to include to any HP part (e.g. Bcc).

4.1.2. Receiving Side

GR1: Determine how HF HFs should be displayed to the user in case of
conflicting information between the protected and unprotected
headers.
HFs.

GR2: Ensure that man-in-the-middle attack (MITM) attacks (MITM, cf. {{RFC4949}}, [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: Depending on the solution, define Define a means to distinguish between forwarded messages emails and
encapsulated messages emails using new HP mechanism.

4.2.1. Sending side

BS1: Define how full HP support can be indicated to outgoing
messages.
emails.

BS2: Define how full HP support of the receiver can be detected or
guessed.
derived.

BS3: Ensure a HP unaware HP-unaware receiving side easily can display the
"Subject" HF to the user.

4.2.2. Receiving side

BR1: Define how full HP support can be detected in incoming messages. 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 messages, emails, legacy encapsulated messages, emails, and
encapsulated messages 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
messages.
emails.

LSS2: Determine how legacy HP support of the receiver can be detected
or guessed. derived.

4.3.2. Receiving Side

LSR1: Determine how legacy HP support can be detected in incoming
messages.
emails.

5. Options to Achieve Security Considerations

This document talks about UI considerations, including security
considerations, when processing messages protecting Header Protection

In Fields.
One of the following a set goals of Options this document is to achieve Email Header Protection.
It specify UI for displaying
such messages which is expected that less confusing/misleading for the IETF LAMPS WG chooses an option to update
[RFC8551] wrt. Header Protection.

5.1. Option 1: Memory Hole

Memory Hole approach works 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 copying S/MIME [RFC8551],
MIME [RFC2045] and Email [RFC5322] in general.

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 normal message header
fields 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 the MIME header 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>.

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>.

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 (signing (signature and/or encryption) it shares the
protections of the message body.

[[ TODO: add more information on memory hole ]]

5.2.

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. LF
(see also Appendix A.1.2.1). Since the MIME body part header section
is itself covered by the protection mechanisms (signing (signature and/or
encryption) it shares the protections of the message body.

5.2.1.

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:

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") ("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.

5.3.

A.1.3. Option 2.1: Progressive Header Disclosure

This option is similar to Option 2 (cf. Section 5.2). Appendix A.1.2). It also
makes use the Content-Type parameter "forwarded" (cf. Section 5.2.1).
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
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 Section 5.2.1) 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].

5.4.

A.1.4. Examples

Examples in subsequent sections assume that an email client is trying
to protect (sign) the following initial message:

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.)

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:
Content-Type: application/pkcs7-signature

[[base-64 encoded signature]]

--.cbe16d2a-e1a3-4220-b821-38348fc97237--

5.4.1.

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
prepended by "O: " are the outer header section. Lines prepended by
"I: " are the inner header section.

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:
Content-Type: application/pkcs7-signature

[[base-64 encoded signature]]

--.cbe16d2a-e1a3-4220-b821-38348fc97237--

5.4.2.

[[ 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

This is a multipart message in MIME format.
--.cbe16d2a-e1a3-4220-b821-38348fc97237
W: Content-Type: message/rfc822; forwarded=no 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).

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].

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: 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 From: "Alexey Melnikov" <alexey.melnikov@example.net>
I: X-Mailer: Isode Harrier Web Server 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:
Content-Type: application/pkcs7-signature

[[base-64 encoded signature]]

--.cbe16d2a-e1a3-4220-b821-38348fc97237--

5.4.3. Option 2.1 Progressive Header Disclosure

This looks similar as in option 2. Specific examples can be found in
[I-D.luck-lamps-pep-header-protection].

[[ TODO: include an example of the same style. ]]

6.
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--

A.2. Sending Side Considerations

6.1.

A.2.1. Candidate Header Fields for Header Protection

[[ TODO: This section is very early stage and needs more work. ]]

For a signed-only '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 encrypted or encrypted+signed S/MIME messages which 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"/"Bcc" "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.

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 exception, exceptions, e.g. Content-Language.

Note that the above recommendations can also negatively affect anti-
spam processing.

7. Receiving Side Considerations

7.1. Which Header Fields to Display to User

When displaying S/MIME messages which protect header fields (whether
they are signed-only, encrypted or encrypted+signed):

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

Messages containing at least one recipient address in the inner header,
only the inner header field value MUST be displayed (and the
corresponding outer value must be ignored). If a particular Bcc header
field is only present may appear in up to three different variants:

1. The message for the outer header, it MAY be
ignored (not displayed) recipient addresses listed in To or it MAY be displayed with a clear
indicator that it is Cc header
fields, which must not trustworthy(_).

(*) - this only applies if include the Bcc header field is not protected is
some other way, neither for example with a DKIM
signature that validates
and is trusted.

7.2. Mail User Agent Algorithm for deciding which version of a header
field to display

[[ TODO: describe how to recurse calculation nor for encryption.

2. The message(s) sent to find the innermost protected root
body part, extract recipient addresses in the Bcc header fields from it and propagate them to
field, which depends on the
top level. This should also work implementation:

a) One message for triple-wrapped messages.]]

8. Security Considerations

This document talks about UI considerations, including security
considerations, when processing messages protecting each recipient in the Bcc header fields.
One of field
separately with a Bcc header field containing only the goals address of this document
the recipient it is sent to specify UI

b) The same message for displaying each recipient in the Bcc header field
with a Bcc header field containing an indication such messages 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 less confusing/misleading and thus more
secure.
identical to 1. / cf. above)

3. The document is not defining new protocol, so it doesn't create any
new security concerns not already covered by S/MIME [RFC8551], MIME
[RFC2045] and Email [RFC5322] message stored in general.

9. Privacy Considerations

[[ TODO ]]

10. IANA Considerations

This document requests no action from IANA.

[[ RFC Editor: This section may be removed before publication. ]]

11. Acknowledgments

The authors would like to thank the following people who have
provided helpful comments and suggestions for this document: David
Wilson, Steve Kille, Wei Chuang, and Robert Williams

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 'Sent'-Folder of [I-D.luck-lamps-pep-header-protection].

David Wilson came up 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 idea of defining a new Content-Type outer header field parameter section.

A.3. Receiving Side Considerations
A.3.1. Which Header Fields to distinguish forwarded Display to User

When displaying S/MIME messages from inner which protect header field fields
(independent of which protection constructs.

12. References

12.1. Normative References

[RFC2045] Freed, N. level 'signature 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 encryption',
'signature only' or 'encryption only' is applied 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>.

12.2. 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 (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 progress), July 2019.

[I-D.marques-pep-email]
Marques, H., "pretty Easy privacy (pEp): Email Formats and
Protocols", draft-marques-pep-email-02 (work some
other way(*). If a header field is present 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>.

[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., 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 M. Kucherawy, Ed.,
"DomainKeys Identified is trusted.

A.3.2. 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) User Agent Algorithm for
Authorizing Use deciding which version 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, a header
field to display

[[ TODO: describe how to recurse to find the innermost protected root
body part, extract header fields from it and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>. propagate them to the
top level. This should also work for triple-wrapped messages.]]

Appendix A. 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 B. (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)
* 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 Signed-only protection needs further study

* pEp only does header protection by applying both signing and
encryption. Technically it is also possible to sign, but not
encrypt the protected messages. This needs further study.
Feedback from IETF-104: Probably no need to specify it, but
need to document the case. 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 Section 5.2

o Add example to Section 5.4.3 Appendix A.1.2

o Resolve question regarding Bcc in Section 6.1 Appendix A.2.1

o Rewrite Section 6.1 Appendix A.2.1

o Write Section 7.2 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
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/