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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 7912

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                         November 12, 2013
Expires: May 16, 2014


                 Draft and Release using Internet Email
                draft-melnikov-mmhs-authorizing-users-04

Abstract

   This document describes a procedure for when an Military Message
   Handling System (MMHS) message is composed by one user and is only
   released to the mail transfer system when one or more authorizing
   users authorize release of the message by adding the MMHS-
   Authorizing-Users header field.  The resulting message can be
   optionally countersigned, allowing recipients to verify both the
   original signature (if any) and countersignatures.

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 http://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 May 16, 2014.

Copyright Notice

   Copyright (c) 2013 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
   (http://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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Draft and Release procedure . . . . . . . . . . . . . . . . .   3
     3.1.  Handling of Initial Message Submission by MSA . . . . . .   3
     3.2.  Review by Authorizing User(s) . . . . . . . . . . . . . .   3
       3.2.1.  Processing of Encrypted Messages  . . . . . . . . . .   5
       3.2.2.  S/MIME countersignatures  . . . . . . . . . . . . . .   5
     3.3.  Role of other Messaging Agents  . . . . . . . . . . . . .   5
       3.3.1.  Border MTA at the sender's domain . . . . . . . . . .   5
       3.3.2.  MDA at the sender's domain  . . . . . . . . . . . . .   5
   4.  MMHS-Authorizing-Users header field . . . . . . . . . . . . .   5
   5.  Updated MIXER mapping . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Mapping from RFC 5322/MIME to X.400 . . . . . . . . . . .   6
     5.2.  Mapping from X.400 to RFC 5322/MIME . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     7.1.  Forged Header Fields  . . . . . . . . . . . . . . . . . .   7
     7.2.  Intentionally Malformed Header Fields . . . . . . . . . .   8
   8.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   In some secure environments email messages can't be released to the
   MTS (Mail Transfer System) and, thus delivered to recipients, unless
   they are authorized by one or more authorizing users (e.g. Releasing
   Officers or Release Authorities).  This document describes how this
   mechanism can be realized by an additional Internet Email [RFC5322]
   header field and optionally protected using S/MIME [RFC5750]
   [RFC5751] or DKIM [RFC6376].

   This document describes a procedure for how an email message composed
   by one user can be released to the MTS when one or more authorizing
   users authorize and optionally countersign the message.  The header
   communicates which users authorized the message.  If signed, the
   resulting message allows recipients to verify both the original (if
   any) and counter S/MIME signatures.  The list of authorizing users is
   specified in the MMHS-Authorizing-Users header field Section 4.  The
   original S/MIME signature generated by the sender (if any) should be
   unaffected by additional S/MIME countersignatures [RFC5652].



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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].

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation including the core rules defined in Appendix B of
   RFC 5234 [RFC5234].  Terms not defined in this document are taken
   from [RFC5322].

3.  Draft and Release procedure

3.1.  Handling of Initial Message Submission by MSA

   The original email message to be sent doesn't include the MMHS-
   Authorizing-Users header field.  It may or may not include sender's S
   /MIME signature.

   The message to be sent is first submitted over SMTP [RFC6409].  The
   specific mechanism for how it arrives to authorizing user(s) is not
   specified in this document.  One possibility is for the Message
   Submission Agent (MSA) to redirect all email messages not addressed
   to authorizing users and not submitted by authorizing users to a
   preconfigured mailbox that can be accessed by authorizing user(s).
   Another possibility is for the MSA to redirect all email messages
   without the MMHS-Authorizing-Users header field and/or corresponding
   S/MIME countersignatures to a preconfigured mailbox that can be
   accessed by authorizing user(s).

   In order to prevent a malicious sender from bypassing or altering
   Draft and Release procedure, MSA MUST check that MMHS-Authorizing-
   Users header field (if present) is syntactically valid, contains
   email addresses of entities authorized to act as authorizing users
   and, when countersignatures are used, that every entity listed has
   one or more matching countersignature which is valid.

3.2.  Review by Authorizing User(s)

   Each user agent that is used by an authorized user MUST perform the
   following steps (if there are multiple authorizing users, these steps
   are repeated for each):









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   1.  Verify the origination of the message.  The exact mechanism to do
       that is out of scope for this document, but one example is by
       verifying the S/MIME signature and making sure that it matches
       the sender of the message, as described in [RFC5750] [RFC5751].
       Another example is by verifying a DKIM signature [RFC6376] that
       covers From/Sender header fields.

   2.  Check if the message already contains the MMHS-Authorizing-Users
       header field with the email address of the authorizing user.
       (This can happen if email system is misconfigured and thus
       contains a loop, or if a malicious sender or attacker is trying
       to affect authorization procedure.)  If the message doesn't
       contain the email address of the authorizing user in the MMHS-
       Authorizing-Users header field, then go to the next step.  If
       MMHS-Authorizing-Users header field contains the email address of
       the authorizing user, verify validity of the header field (for
       example by checking for S/MIME countersignature or for DKIM
       signature).  If the validity of the MMHS-Authorizing-Users header
       field can be verified, go to step 5 below.  Otherwise strip the
       MMHS-Authorizing-Users header field or return the message to
       sender (bounce).

   3.  Allow the authorizing user to review content of the message.
       Some of the checks can be automated (for example search for
       keywords).  (See Section 3.2.1 for additional considerations.)
       If based on the check the authorizing user is happy to release
       the message to MTS (or to the next authorizing user, if multiple
       authorizations are required), the UA should enable the
       authorizing user to protect additions to the MMHS-Authorizing-
       Users header field, for example by allowing to add S/MIME
       countersignature (if S/MIME is used for protecting MMHS-
       Authorizing-Users header field.  See Section 3.2.2 for more
       details).  If the authorizing user wants to block the message, it
       can be discarded or returned to sender.  The authorizing user can
       also choose to forward the message to another authorizing user
       for approval.

   4.  If there is an existing MMHS-Authorizing-Users header field
       containing the email address of the authorizing user, skip this
       step.  Otherwise, insert a new MMHS-Authorizing-Users header
       field (if absent) containing the email address of the authorizing
       user or append the email address of the authorizing user to the
       end of the existing MMHS-Authorizing-Users header field.

   5.  The (possibly) updated email message is either released to the
       MTS, or to the next authorizing user, as per email system
       configuration.




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3.2.1.  Processing of Encrypted Messages

   Any encrypted message sent in an environment where Draft and Release
   procedure is in force needs to be also encrypted to all authorizing
   users, so that they can perform review of the message.  A message
   that can't be decrypted by an authorizing user MUST be returned to
   sender.

3.2.2.  S/MIME countersignatures

   If a message is signed multiple times (for example using different
   cryptographic algorithms), all of the signatures that can be verified
   by an authorizing user SHOULD be countersigned.  A recipient of the
   message should consider any chain of countersignatures that matches
   MMHS-Authorizing-Users header field values as valid, only if all
   signatures in the chain verify.

   When triple wrapping [RFC2634] is used, countersignature SHOULD be
   applied to the outer level, so that it can be verified by MTAs
   without the need to decrypt content.

3.3.  Role of other Messaging Agents

3.3.1.  Border MTA at the sender's domain

   Sender's domain border MTAs are responsible for ensuring that all
   messages that leave sender's domain were properly authorized by
   authorizing user(s), as determined by the sender's domain email
   system configuration.  They verify presence and validity of MMHS-
   Authorizing-Users header field in outgoing messages, as well as
   validity of associated signatures on the message.

3.3.2.  MDA at the sender's domain

   If a message being sent is to be delivered within the sender's
   domain, Message Delivery Agents (MDAs) are responsible for ensuring
   that the message was properly authorized by authorizing user(s), as
   determined by the sender's domain email system configuration.  They
   verify presence and validity of MMHS-Authorizing-Users header field
   in the message, as well as validity of associated signatures on the
   message.

4.  MMHS-Authorizing-Users header field








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   The MMHS-Authorizing-Users header field specifies the list of
   authorizing users (or entities(*)) that countersigned this email
   message (for example using S/MIME) before it was authorized for
   release to MTS.  Each user/entity is described by her/his/its email
   address.

   (*) Note that in some environments identities of authorizing users
   are required to be hidden from recipients of email messages, so upon
   receipt MMHS-Authorizing-Users might contain email address associated
   with a group of possible users.

   The MMHS-Authorizing-Users header field specified in this document
   MUST NOT appear more than once in message headers.  (An email message
   that contains multiple MMHS-Authorizing-Users is malformed.  An agent
   processing such malformed message SHOULD either return it to sender
   (if possible) or fix the message so that it only contains one copy of
   the header field.)  [[An alternative is to allow for multiple copies
   of the header field and treat them as additive.  This might work
   better with DKIM!]]


       MMHS-Authorizing-Users = "MMHS-Authorizing-Users:"
                          [FWS] mailbox-list [FWS] CRLF

       mailbox-list = <Defined in RFC 5322>


5.  Updated MIXER mapping

   This section updates MIXER mapping specified in [RFC2156].

5.1.  Mapping from RFC 5322/MIME to X.400

   In the absence of the MMHS-Authorizing-Users header field, From and
   Sender header fields are mapped to their X.400 equivalents as
   specified in [RFC2156].

   If MMHS-Authorizing-Users header field is present:

   1.  The first From header field address is mapped to
       IPMS.Heading.originator if there is no Sender header field and
       the remaining From header field addresses + the MMHS-Authorizing-
       Users header field address(es) are mapped to IPMS.Heading
       .authorizing-users.  If a Sender header field is present, the
       From header field address(es) and the MMHS-Authorizing-Users
       header field address(es) are mapped to IPMS.Heading.authorizing-
       users.




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   2.  The Sender header field (if present) is mapped to
       IPMS.Heading.originator.

5.2.  Mapping from X.400 to RFC 5322/MIME

   Mapping from X.400 to Internet is controlled by whether or not a
   particular message is considered to be a military message.  A message
   is considered to be a military message (as defined by ACP 123
   [ACP123] and also specified in STANAG 4406 [STANAG-4406]) if there
   are any MMHS heading extensions present.  Alternatively, this MAY be
   done by configuration (i.e. all messages can be considered to be
   military messages).

   For non military messages, mapping from X.400 as specified in
   [RFC2156] is used.

   For military messages, the following mapping is used:

   1.  IPMS.Heading.originator is mapped to From header field.

   2.  The IPMS.Heading.authorizing-users is mapped to MMHS-Authorizing-
       Users header field.

6.  IANA Considerations

   IANA is requested to add the MMHS-Authorizing-Users header field
   specified in Section 4 to the "Permanent Message Header Field Names",
   defined by Registration Procedures for Message Header Fields
   [RFC3864].  The registration template is as follows:

   Header field name: MMHS-Authorizing-Users

   Applicable protocol: mail ([RFC5322])

   Status: Standard

   Author/Change controller: IETF

   Specification document(s): [[RFC XXXX]]

   Related information:

7.  Security Considerations

7.1.  Forged Header Fields

   A malicious sender may add/change an MMHS-Authorizing-Users header
   field to bypass or alter message authorization procedure invoked for



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   messages with no MMHS-Authorizing-Users header field.  For that
   reason it is important for agents and clients that rely on validity
   of MMHS-Authorizing-Users header field to also verify
   countersignature (or a similar protection mechanism), that confirms
   that a particular person or entity authorized release of a message.

7.2.  Intentionally Malformed Header Fields

   It is possible for an attacker to add an MMHS-Authorizing-Users
   header field that is extraordinarily large or otherwise malformed in
   an attempt to discover or exploit weaknesses in header field parsing
   code.  Implementers must thoroughly verify all such header fields
   received from MTAs and be robust against intentionally as well as
   unintentionally malformed header fields.

8.  Open Issues

   Netnews Approved header field has the same syntax and semantics as
   the one described here.  Should it be used (and be formally
   registered for email) instead?

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2156]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
              Mapping between X.400 and RFC 822/MIME", RFC 2156, January
              1998.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.





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   [RFC2634]  Hoffman, P., "Enhanced Security Services for S/MIME", RFC
              2634, June 1999.

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, January 2010.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
              September 2011.

   [ACP123]   CCEB, ., "Common Messaging strategy and procedures", ACP
              123, May 2009.

9.2.  Informative References

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [STANAG-4406]
              NATO, ., "STANAG 4406 Edition 2: Military Message Handling
              System", STANAG 4406, March 2005.

Appendix A.  Acknowledgements

   Many thanks for reviews and text provided by Steve Kille, Jim Schaad,
   Russ Housley and David Wilson.

   Some text in this document was copied from RFC 7001.

Author's Address

   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com






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