Internet DRAFT - draft-melnikov-lamps-header-protection
draft-melnikov-lamps-header-protection
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational October 18, 2018
Expires: April 21, 2019
Considerations for protecting Email header with S/MIME
draft-melnikov-lamps-header-protection-00
Abstract
This document describes best practices for handling of Email header
protected by S/MIME. Procedures described in this document are also
applicable to OpenPGP.
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 April 21, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Melnikov Expires April 21, 2019 [Page 1]
Internet-Draft Header protection with S/MIME October 2018
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Recommended algorithms for email header protection . . . . . 3
3.1. Option 1: Memory Hole . . . . . . . . . . . . . . . . . . 4
3.2. Option 2: Wrapping with message/rfc822 or message/global 5
4. Recommendations for handling of S/MIME protected header . . . 7
5. Mail User Agent Algorithm for deciding which version of a
header field to display . . . . . . . . . . . . . . . . . . . 9
6. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
S/MIME [RFC5751] is typically used to protect (sign and/or encrypt)
Email message body parts, but not header of corresponding Email
messages. Header fields may contain confidential information or
information whose validity need protecting from modification.
[RFC5751] describes how to protect the Email message header
[RFC5322], by wrapping the message inside a message/rfc822 container
[RFC2045]:
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 along with the
unprotected "outer" header.
When an S/MIME message is received, if the top-level protected
MIME entity has a Content-Type of message/rfc822, 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 merging issues as previously discussed.
While the above advice helps in protecting message header fields, it
doesn't provide enough guidance on what information should and should
not be included in outer (unprotected) header and how information
from outer and inner headers should be presented to users.
Additionally, there are very few implementations of the header
Melnikov Expires April 21, 2019 [Page 2]
Internet-Draft Header protection with S/MIME October 2018
protection mechanism specified in [RFC5751]. Email clients that
don't support this mechanism display messages with header protection
as if they are forwarded email messages. Some of them don't display
content of forwarded messages by default (e.g. they display at
attachment or an icon), so viewing them requires an extra action by
the user.
[[Alexey: Depending on WG consensus, the following text will be
updated to either suggest and alternative approach that is friendlier
to non compliant email clients or to reinforce use of message/rfc822
for header protection + recommend use of the new "forwarded"
parameter to Content-Type.]] This document describes best UI and
other practices for handling of message header protection. The goal
of this document is to improve interoperability and minimize damage
caused by possible differences between inner and outer headers.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The terms "header field" and "header section" are defined in
[RFC5322].
The following terms are defined in this document:
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 and not
encrypted + signed.
3. Recommended algorithms for email header protection
[[LAMPS WG should pick between the following 2 alternatives. They
are described in details in subsections of this section.]]
Examples in subsequent sections assume that an email client is trying
to protect (sign) the following initial message:
Melnikov Expires April 21, 2019 [Page 3]
Internet-Draft Header protection with S/MIME October 2018
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@mattingly.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@mattingly.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--
3.1. Option 1: Memory Hole
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 and/or encryption) it shares the
protections of the message body.
Melnikov Expires April 21, 2019 [Page 4]
Internet-Draft Header protection with S/MIME October 2018
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.
O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@mattingly.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@mattingly.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--
3.2. 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 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.
Since the MIME body part header section is itself covered by the
protection mechanisms (signing and/or encryption) it shares the
protections of the message body.
Melnikov Expires April 21, 2019 [Page 5]
Internet-Draft Header protection with S/MIME October 2018
The rest of this section formally defines the new "forwarded"
Content-Type header field parameter 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 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 [RFC5751] 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.
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.
Melnikov Expires April 21, 2019 [Page 6]
Internet-Draft Header protection with S/MIME October 2018
O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@mattingly.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@mattingly.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--
4. Recommendations for handling of S/MIME protected header
[[This section needs more work. Don't treat anything in it as
unchangeable.]]
For a signed-only message, it is RECOMMENDED that all "outer" header
fields are copied into the "inner" protected body part. 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.
When generating encrypted or encrypted+signed S/MIME messages which
protect header fields:
Melnikov Expires April 21, 2019 [Page 7]
Internet-Draft Header protection with S/MIME October 2018
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 RFC 5322, 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" header fields. 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, e.g. Content-Language.
Note that the above recommendations can also negatively affect
antispam processing.
When displaying S/MIME messages which protect header fields (whether
they are signed-only, encrypted or encrypted+signed):
1. The outer headers 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(*).
Melnikov Expires April 21, 2019 [Page 8]
Internet-Draft Header protection with S/MIME October 2018
(*) - 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.
5. Mail User Agent Algorithm for deciding which version of a header
field to display
[[TBD: describe how to recurse to find the innermost protected root
body part, extract header fields from it and propogate them to the
top level. This should also work for triple-wrapped messages.]]
6. Open Issues
[[This list should be empty before publication:]]
7. IANA Considerations
This document requests no action from IANA. RFC Editor can delete
this section before publication.
8. 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 and thus more
secure.
The document is not defining new protocol, so it doesn't create any
new security concerns not already covered by S/MIME [RFC5751], MIME
[RFC2045] and Email [RFC5322] in general.
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>.
Melnikov Expires April 21, 2019 [Page 9]
Internet-Draft Header protection with S/MIME October 2018
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
Melnikov Expires April 21, 2019 [Page 10]
Internet-Draft Header protection with S/MIME October 2018
Appendix A. Acknowledgements
Thank you to Wei Chuang, Steve Kille, David Wilson and Robert
Williams for suggestions, comments and corrections on this document.
Text on "Memory Hole" approach was taken from Daniel Kahn Gillmor's
emails.
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.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: alexey.melnikov@isode.com
Melnikov Expires April 21, 2019 [Page 11]
