Isode Ltd

14 Castle Mews Hampton Middlesex TW12 2NP United Kingdom Alexey.Melnikov@isode.com

MMHS S/MIME MIXER email 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.

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 header field and optionally protected using S/MIME or DomainKeys Identified Mail (DKIM) . 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 ) 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.

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

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.

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 . 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.

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): 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 ) and that it matches the sender of the message, as described in . Another example is by verifying a DKIM signature (added by the Drafter's Mail User Agent (MUA) or MSA) that covers the From/Sender header fields. 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. Allow the authorizing user to review the content of the message. Some of the checks can be automated (for example, search for keywords). (See 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 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. 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. 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.

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 preforma 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.

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 ), 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 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.

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.

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.

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.

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

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 . If the MMHS-Authorizing-Users header field is present: 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. 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 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 and also specified in 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 is used. For military messages, the following mapping is used: IPMS.Heading.originator is mapped to the From header field. The IPMS.Heading.authorizing-users is mapped to the MMHS-Authorizing-Users header field.

IANA has added the MMHS-Authorizing-Users header field specified in to the "Provisional Message Header Field Names" registry, defined by "Registration Procedures for Message Header Fields" . The registration template is as follows: Header field name: MMHS-Authorizing-Users Applicable protocol: mail () Status: provisional Author/Change controller: Alexey Melnikov <alexey.melnikov@isode.com> Specification document(s): RFC 7912 Related information:

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.

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.

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.

CCEB NATO

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.