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Independent Submission                                       A. MelnikovRequest for Comments: 7912                                     Isode LtdCategory: Informational                                        June 2016ISSN: 2070-1721Message Authorizing Email Header Field and Its Use for theDraft and Release ProcedureAbstract   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; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Melnikov                      Informational                     [Page 1]

RFC 7912            Message Authorizing Header Field           June 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .33.  Draft and Release Procedure . . . . . . . . . . . . . . . . .33.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .33.2.  Handling of Initial Message Submission by the MSA . . . .33.3.  Review by Authorizing User(s) . . . . . . . . . . . . . .43.3.1.  Processing of Encrypted Messages  . . . . . . . . . .53.3.2.  Authorizing S/MIME Signatures . . . . . . . . . . . .53.4.  Role of Other Messaging Agents at the Sender's Domain . .63.4.1.  MDA at the Sender's Domain  . . . . . . . . . . . . .63.4.2.  Border MTA at the Sender's Domain . . . . . . . . . .64.  MMHS-Authorizing-Users Header Field . . . . . . . . . . . . .65.  Updated MIXER Mapping . . . . . . . . . . . . . . . . . . . .75.1.  Mapping fromRFC 5322/MIME to X.400 . . . . . . . . . . .75.2.  Mapping from X.400 toRFC 5322/MIME . . . . . . . . . . .76.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .87.  Security Considerations . . . . . . . . . . . . . . . . . . .87.1.  Forged Header Fields  . . . . . . . . . . . . . . . . . .97.2.  Intentionally Malformed Header Fields . . . . . . . . . .98.  References  . . . . . . . . . . . . . . . . . . . . . . . . .98.1.  Normative References  . . . . . . . . . . . . . . . . . .98.2.  Informative References  . . . . . . . . . . . . . . . . .10   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .11   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .111.  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 (seeSection 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.Melnikov                      Informational                     [Page 2]

RFC 7912            Message Authorizing Header Field           June 20162.  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 inAppendix B of   RFC 5234 [RFC5234].  Terms not defined in this document are taken   from [RFC5322].3.  Draft and Release Procedure3.1.  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.3.2.  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 everyMelnikov                      Informational                     [Page 3]

RFC 7912            Message Authorizing Header Field           June 2016   entity listed has one or more matching review signature (or   signature) that is valid.3.3.  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 inSection 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).  (SeeSection 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.  SeeSection 3.3.2 for more       details).  If the Authorizing User wants to reject the message,Melnikov                      Informational                     [Page 4]

RFC 7912            Message Authorizing Header Field           June 2016       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.3.3.1.  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.3.3.2.  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 signaturesMelnikov                      Informational                     [Page 5]

RFC 7912            Message Authorizing Header Field           June 2016   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.3.4.  Role of Other Messaging Agents at the Sender's Domain3.4.1.  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.3.4.2.  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.4.  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.Melnikov                      Informational                     [Page 6]

RFC 7912            Message Authorizing Header Field           June 2016   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 inRFC 5322>5.  Updated MIXER Mapping   This section provides an updated version of the MIXER mapping   specified in [RFC2156] for MMHS applications.5.1.  Mapping fromRFC 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.5.2.  Mapping from X.400 toRFC 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.Melnikov                      Informational                     [Page 7]

RFC 7912            Message Authorizing Header Field           June 2016   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.6.  IANA Considerations   IANA has added the MMHS-Authorizing-Users header field specified inSection 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 <alexey.melnikov@isode.com>   Specification document(s):RFC 7912   Related information:7.  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.Melnikov                      Informational                     [Page 8]

RFC 7912            Message Authorizing Header Field           June 20167.1.  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.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 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.8.  References8.1.  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 andRFC 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>.Melnikov                      Informational                     [Page 9]

RFC 7912            Message Authorizing Header Field           June 2016   [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>.8.2.  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.Melnikov                      Informational                    [Page 10]

RFC 7912            Message Authorizing Header Field           June 2016Acknowledgements   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 fromRFC 7001.Author's Address   Alexey Melnikov   Isode Ltd   14 Castle Mews   Hampton, Middlesex  TW12 2NP   United Kingdom   Email: Alexey.Melnikov@isode.comMelnikov                      Informational                    [Page 11]