Considerations for protecting Email header with S/MIME
draft-melnikov-smime-header-signing-05

Versions: 00 01 02 03 04 05                                             
Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                           October 1, 2017
Expires: April 4, 2018


         Considerations for protecting Email header with S/MIME
                 draft-melnikov-smime-header-signing-05

Abstract

   This document describes best practices for handling of Email header
   protected by S/MIME.  It also adds a new Content-Type parameter to
   help distinguish an S/MIME protected forwarded message from an S/MIME
   construct protecting message header.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Recommendations for handling of S/MIME protected header . . .   3
   4.  New Content-Type header field parameter: forwarded  . . . . .   4
   5.  Example message with S/MIME header protection . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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 disclosure or
   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.  This
   document describes best UI and other practices for handling of
   messages wrapped inside message/rfc822 body parts.  The goal of this
   document is to improve interoperability and minimize damage caused by
   possible differences between inner and outer headers.




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2.  Conventions Used in This Document

   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].

3.  Recommendations for handling of S/MIME protected header

   When generating S/MIME messages which protect header fields by
   wrapping a message inside message/rfc822 wrapper:

   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.

   2.  The outer header SHOULD be minimal in order to avoid disclosure
       of confidential information.  It is recommended that the outer
       header only contains "Date" (set to the same value as in the
       inner header, 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 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 only apply to non MIME header
   fields (header fields with names not starting with "Content-"
   prefix).

   Note that the above recommendations can also negatively affect
   antispam processing.




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   When displaying S/MIME messages which protect header fields by
   wrapping a message inside message/rfc822 wrapper:

   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(*).

       (*) - 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.

4.  New Content-Type header field parameter: forwarded

   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 encrypted body parts.  The value "yes" means that the message
   nested inside message/rfc822 is a forwarded message and not a
   construct created solely to protect the inner header.

   Instructions in [RFC5751] describing how to protect the Email message
   header [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 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 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 merging issues as
      previously discussed.





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5.  Example message with S/MIME header protection

   The following example demonstrates a message generated to protect
   original message header.  For example, this will be the first body
   part of a multipart/signed message or the payload of the application/
   pkcs7-mime body part.


Content-Type: message/rfc822; forwarded=no

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: Secret meeting at my place
To: somebody@example.net
X-Mailer: Isode Harrier Web Server
content-type: text/plain; charset=us-ascii

This is a secret message worth protecting.

   [[CREF1: Extend the example to show different inner and outer header
   fields and clarify what should be displayed?]]

6.  IANA Considerations

   This document requests no action from IANA.  RFC Editor should delete
   this section before publication.

7.  Security Considerations

   This document talks about UI considerations, including security
   considerations, when processing wrapped message/rfc822 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.

8.  References








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8.1.  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>.

   [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>.

8.2.  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>.





















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Appendix A.  Acknowledgements

   Thank you to Wei Chuang, Steve Kille, David Wilson and Robert
   Williams for suggestions, comments and corrections on this document.
   Some ideas were also taken from Daniel Kahn Gillmor's email on the
   OpenPGP mailing list.

   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































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