RFC7912

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Independent Submission A. Melnikov Request for Comments: 7912 Isode Ltd Category: Informational June 2016 ISSN: 2070-1721 

   Message Authorizing Email Header Field and Its Use for the
                  Draft and Release Procedure

Abstract

This document describes a procedure for when a 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 signed by the sender and/or reviewer, allowing recipients to verify both the original signature (if any) and the review signatures.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7912.

Copyright Notice

Copyright (c) 2016 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. 

 3.4.  Role of Other Messaging Agents at the Sender's Domain . .   6

Introduction

In some secure environments, email messages can't be released to the Message Transfer System (MTS); thus, they can't be 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 DomainKeys Identified Mail (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 MMHS- Authorizing-Users header field (see Section 4) communicates which user(s) authorized the message. If S/MIME signed, the resulting message allows recipients to verify both the original (if any) and counter signatures. The original S/MIME signature generated by the sender (if any) is unaffected by additional S/MIME review signatures.

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.

Draft and Release Procedure

Terminology

Drafter: Any email user that composes a message (Draft Message) needing authorization before it is released to its intended recipients.

Authorizing User (also Releaser or Authorizer): The mailbox of a user or a group of users that must inspect and authorize the release of a Draft Message before it can be sent. An organization may require more than one Authorizing User to authorize the release of a Draft Message.

Handling of Initial Message Submission by the MSA

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

The message to be sent is first submitted over SMTP RFC6409. The specific mechanism for how it arrives to the 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(es) 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 review signatures to a preconfigured mailbox(es) that can be accessed by Authorizing User(s).

In order to prevent a malicious sender from bypassing or altering the Draft and Release procedure, the MSA MUST check that the MMHS- Authorizing-Users header field (if present) is syntactically valid, contains the email addresses of entities authorized to act as Authorizing Users, and, when review signatures are used, that every

entity listed has one or more matching review signature (or signature) that is valid.

Review by Authorizing User(s)

Each user agent (UA) that is used by an authorized user MUST perform the following steps (if there are multiple Authorizing Users, the whole sequence of steps below is repeated for each Authorizing User):

1. Verify the origination of the message (From/Sender header 

   fields).  The exact mechanism to do that is out of scope for this
   document, but one example is by verifying the S/MIME signature,
   making sure that the signature protects all header fields (i.e.,
   wrapped by message/rfc822, as described in Section 3.1 of
   RFC5751) and that it matches the sender of the message, as
   described in RFC5750.  Another example is by verifying a DKIM
   signature RFC6376 (added by the Drafter's Mail User Agent (MUA)
   or MSA) that covers the 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, for example, if the email system is
   misconfigured and thus contains a loop, or if a malicious sender
   or attacker is trying to affect the 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 the MMHS-Authorizing-Users header field contains
   the email address of the Authorizing User, verify the validity of
   the header field (for example, by checking for the S/MIME
   signature/review signature or for the DKIM signature) and also
   verify that the email address associated with the signature
   matches the email address of the Authorizing User.  If the
   validity of the MMHS-Authorizing-Users header field can be
   verified, go to step 5 below.  Otherwise, return the message to
   the sender (bounce) or redirect the message to a designated abuse
   mailbox.

3. Allow the Authorizing User to review the content of the message. 

   Some of the checks can be automated (for example, search for
   keywords).  (See Section 3.3.1 for additional considerations.)
   If, based on the check, the Authorizing User is happy to release
   the message to the 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 the addition of the
   S/MIME review signature (if S/MIME is used for protecting the
   MMHS-Authorizing-Users header field.  See Section 3.3.2 for more
   details).  If the Authorizing User wants to reject the message,

   it SHOULD be returned to the Drafter with an explanatory note or
   it MAY be discarded.  The Authorizing User can also choose to
   forward the message to another Authorizing User for additional
   approval or become a new Drafter of the message.  If the
   Authorizing User becomes the new Drafter, its UA MUST strip any
   existing email addresses from the MMHS-Authorizing-Users header
   field.

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.  Note that if the Authorizing User updates the
   message in a manner that invalidates existing S/MIME or DKIM
   signature(s), the Authorizing User becomes the Drafter and needs
   to reapply any protections.

Processing of Encrypted Messages

Any encrypted message sent in an environment where the Draft and Release procedure is in force also needs to be encrypted to all Authorizing Users, so that they can perform review of the message. If a User Agent used by an Authorizing User can't decrypt the message, it SHOULD notify the sender (which can be the Drafter or a previous Authorizing User) about the problem using a non-delivery Delivery Status Notification (DSN) or through some other means. The ciphertext that cannot be decrypted by the Authorizing User MAY be included in the notification to aid debugging. A possible reason not to notify the sender is to avoid Denial-of-Service attacks, for example, if an attacker discovers a way to inject fake messages with encryption that doesn't validate in order to overflow the sender's INBOX.

Authorizing S/MIME Signatures

If S/MIME were not used, the Authorizing User can become the original signer of the message.

If a message is signed with multiple signatures (for example, using different cryptographic algorithms, as described in RFC5752), all of the signatures that can be verified by an Authorizing User SHOULD be signed with a review signature (authorizing signatures). A recipient of the message can consider any chain of review signatures

that matches MMHS-Authorizing-Users header field values as valid, only if all signatures in the chain are verified. All of the signatures that cannot be verified MUST be stripped by the Authorizing User Agent.

When triple wrapping RFC2634 is used, authorizing signatures are applied to the outer level, so that it can be verified by Message Transfer Agents (MTAs) without the need to decrypt content.

Role of Other Messaging Agents at the Sender's Domain

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 the presence and validity of the MMHS-Authorizing-Users header field in the message, as well as the validity of associated signatures on the message.

Note that the above requirements don't apply to direct delivery to any user designated as an Authorizing User.

Border MTA at the Sender's Domain

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

MMHS-Authorizing-Users Header Field

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 the MTS. Each user/entity is described by the 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 an email address associated with a group of possible users. Such email addresses need to have signatures that don't disclose group membership.

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 a malformed message SHOULD either return it to the sender (if possible) or fix the message so that it contains only one copy of the header field. 

   MMHS-Authorizing-Users = "MMHS-Authorizing-Users:"
                      mailbox-list CRLF

   mailbox-list = <Defined in RFC 5322>

Updated MIXER Mapping

This section provides an updated version of the MIXER mapping specified in RFC2156 for MMHS applications.

Mapping from RFC 5322/MIME to X.400

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

If the MMHS-Authorizing-Users header field is present:

1. If the Sender header field is present, it is mapped to 

   IPMS.Heading.originator; otherwise, the first From header field
   address is mapped to IPMS.Heading.originator.

2. Map the From header field address(es) and the MMHS-Authorizing- 

   Users header field address(es) to IPMS.Heading.authorizing-users,
   skipping the first From header field address if it was mapped to
   IPMS.Heading.originator.

Mapping from X.400 to RFC 5322/MIME

Mapping from X.400 to the Internet is controlled by whether or not a particular message is considered a military message. A message is considered 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 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 the From header field.

2. The IPMS.Heading.authorizing-users is mapped to the MMHS- 

   Authorizing-Users header field.

IANA Considerations

IANA has added the MMHS-Authorizing-Users header field specified in Section 4 to the "Provisional Message Header Field Names" registry, 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: provisional

Author/Change controller: Alexey Melnikov <[email protected]>

Specification document(s): RFC 7912

Related information:

Security Considerations

In some military environments, the identities of Authorizing Users are required to be hidden from recipients of email messages. This can be accomplished by using a group address for the MMHS- Authorizing-Users. In this way, the recipient will know that it was released by an Authorizing User in that group, but the recipient will not know which one of them took the action.

For those organizations that do not wish to disclose the Authorizing Users' group membership, care must also be taken to ensure that the information included in the certificate used for signing email messages does not disclose individuals in the group.

Further security considerations are described in subsections of this section.

Forged Header Fields

A malicious sender may add/change an MMHS-Authorizing-Users header field to bypass or alter the message authorization procedure invoked for messages with no MMHS-Authorizing-Users header field. For this reason, it is important for agents and clients that rely on the validity of the MMHS-Authorizing-Users header field to also verify the review signature (or a similar protection mechanism) that confirms that a particular person or entity authorized release of a message.

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 the header field parsing code. Implementations MUST thoroughly verify all such header fields received from MTAs and be robust against intentionally as well as unintentionally malformed header fields.

References

Normative References

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

          (B), May 2009.

RFC2119 Bradner, S., "Key words for use in RFCs to Indicate 

          Requirement Levels", BCP 14, RFC 2119,
          DOI 10.17487/RFC2119, March 1997,
          <http://www.rfc-editor.org/info/rfc2119>.

RFC2156 Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 

          Mapping between X.400 and RFC 822/MIME", RFC 2156,
          DOI 10.17487/RFC2156, January 1998,
          <http://www.rfc-editor.org/info/rfc2156>.

RFC2634 Hoffman, P., Ed., "Enhanced Security Services for S/MIME", 

          RFC 2634, DOI 10.17487/RFC2634, June 1999,
          <http://www.rfc-editor.org/info/rfc2634>.

RFC5234 Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 

          Specifications: ABNF", STD 68, RFC 5234,
          DOI 10.17487/RFC5234, January 2008,
          <http://www.rfc-editor.org/info/rfc5234>.

RFC5322 Resnick, P., Ed., "Internet Message Format", RFC 5322, 

          DOI 10.17487/RFC5322, October 2008,
          <http://www.rfc-editor.org/info/rfc5322>.

RFC5750 Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 

          Mail Extensions (S/MIME) Version 3.2 Certificate
          Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
          <http://www.rfc-editor.org/info/rfc5750>.

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, <http://www.rfc-editor.org/info/rfc5751>.

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,
          <http://www.rfc-editor.org/info/rfc6376>.

RFC6409 Gellens, R. and J. Klensin, "Message Submission for Mail", 

          STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
          <http://www.rfc-editor.org/info/rfc6409>.

Informative References

RFC3864 Klyne, G., Nottingham, M., and J. Mogul, "Registration 

          Procedures for Message Header Fields", BCP 90, RFC 3864,
          DOI 10.17487/RFC3864, September 2004,
          <http://www.rfc-editor.org/info/rfc3864>.

RFC5752 Turner, S. and J. Schaad, "Multiple Signatures in 

          Cryptographic Message Syntax (CMS)", RFC 5752,
          DOI 10.17487/RFC5752, January 2010,
          <http://www.rfc-editor.org/info/rfc5752>.

[STANAG-4406] 

          NATO, "STANAG 4406 Edition 2: Military Message Handling
          System", STANAG 4406 Ed. 2, March 2005.

Acknowledgements

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

Some text in this document was copied from RFC 7001.

Author's Address

Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex TW12 2NP United Kingdom

Email: [email protected]

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