| Internet-Draft | Multi MLS | March 2025 |
| Xue, et al. | Expires 9 September 2025 | [Page] |
The Messaging Layer Security (MLS) protocol enables a group of participants to negotiate a common cryptographic state for messaging, providing Forward Secrecy (FS) and Post-Compromise Security (PCS). A group of participants may find it difficult to agree on a common state - or that reaching eventual consistency is impractical for the application. This document describes Multi-MLS (MMLS), a protocol for using MLS sessions to protect messages among participants without negotiating a common group state.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://germ-mark.github.io/multi-mls-id/draft-xue-multi-mls.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-xue-multi-mls/.¶
Discussion of this document takes place on the Messaging Layer Security Working Group mailing list (mailto:mls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/mls/. Subscribe at https://www.ietf.org/mailman/listinfo/mls/.¶
Source for this draft and an issue tracker can be found at https://github.com/germ-mark/multi-mls-id.¶
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 https://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 9 September 2025.¶
Copyright (c) 2025 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 (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
Participants operating in peer to peer or partitioned network topologies may find it impractical to access a centralized Delivery Service, or reach consensus on message sequencing to arrive at a consistent commit for each MLS epoch.¶
Multi-MLS is a protocol for users to encrypt messages to multiple recipients and update participant's keys to provide FS and PCS. It facilitates group messaging by using an MLS group per participant as that participant's send group. This allows each participant to locally and independently determine their view of the group membership, and encrypt messages using MLS to that set of recipients.¶
Send Group: An MLS group where one designated member authors all messages and other members use the group only to receive from the designated sender.¶
Universe: A set of MLS participants that comprise a MMLS group.¶
Within a group $G$ of distributed participants, we can resolve state conflict by assigning each member local state that only they control. In Multi-MLS, we assign each member an MLS group to operate as a Send Group. Through the export and import of epoch secrets across these send groups, the members of $G$ can accomplish PCS and FS without reaching consensus on global state.¶
The MLS Architecture Guide specifies two requirements for an abstract Delivery Service related to message ordering. First, Proposal messages should all arrive before the Commit that references them. Second, members of an MLS group must agree on a single MLS Commit message that ends each epoch and begins the next one.¶
An honest centralized DS, in the form of a message queuing server or content distribution network, can guarantee these requirements to be met. By controlling the order of messages delivered to MLS participants, for example, it can guarantee that Commit messages always follow their associated Proposal messages. By filtering Commit messages based on some pre-determined criteria, it can ensure that only a single Commit message per epoch is delivered to participants.¶
A decentralized DS, on the other hand, can take the form of a message queuing server without specialized logic for handling MLS messages or, prehaps, simply a local area network. These DS instantiations cannot offer any such guarantees.¶
The MLS Architecture Guide highlights the risk of two MLS participants generating different Commits in the same epoch and then sending them at the same time. The impact of this risk is inconsistency of MLS group state among participants. This perhaps leads to inability of some authorized participants to read other authorized participants' messages, i.e., a loss of availability of the message-passing service provided by MLS. A decentralized DS offers no mitigation strategy for this risk, so the participants themselves must agree on strategies, or in our terminology, operating constraints. We could say that the full weight of the CAP theorem is thus levied directly on the MLS participants in this case. However, use cases exist that benefit from, or even necessitate, MLS and its accompanying security guarantees for group message passing.¶
The MMLS operating constraints specified above allow honest participants to form a distributed system that satisfies these requirements despite a decentralized DS.¶
An MLS Send Group operates in the following constrained way: * The creator of the group, occupying leaf index 0, is the designated sender * The creator may accept update messages from any participant * Other participants only accept messages from the creator * Participants only accept messages as defined in Group Operations¶
To facilitate binding Send Groups together, we define the following exported values:
* derived groupid: MLS-Exporter("derivedGroupId", leafNodePublicSigningKey, Length)¶
This is a unique value for each participant derived from the group's current epoch * exportPSK: `MLS-Exporter("exportPSK", "Universe identifier", Length)`
¶
Similar to MLS, MMLS provides a participant application programming interface (API) with the following functions:¶
INIT¶
Given a list of MMLS participants, initialize an MMLS context by (1) creating an MLS group, (2) adding all other participants (generating a set of Welcome messages and a GroupInfo message), and (3) It is the responsibility of an MMLS implementation to define the Universe of participants and the mechanism of generating the individual send groups. "MMLS Requirements" sketches one such approach.¶
UPDATE¶
A member Alice of $U$ can update their leafNode in the universe $U$ by authoring a full or empty commit in Alice's send group, which provides PCS with regard to the committer.¶
This update commit is also an opportunity to update Alice's credential, in which case Alice should also distribute corresponding update messages in all other send groups.¶
COMMIT¶
When Bob receives Alice's MMLS update (as a full or empty commit in Alice's send group), Bob can incorporate PCS from Alice's commit by importing a PSK from Alice's send group. Precisely, Bob: * Creates a PSK proposal in Bob's send group using the exportPSK from the epoch of Alice's send group after Alice's MMLS update * If Alice's commit updated Alice's credential, Bob should have received an accompanying update proposal in Bob's send group. * Bob generates a commit covering the PSK proposal (for each send group in which he has observed a new MMLS update), and any update proposals he received.¶
The psk_group_id for this PSK is more specifically defined as follows:
psk_group_id = (opaque<8>) groupEpoch | groupId
where epoch_bytes is the byte-vector representation of the epoch in which the exporter was generated, in network byte order.
Since epoch is of type uint64, this results in a fixed 8-byte vector.
groupId, which is of type opaque<V>, is then appended to epoch_bytes.
When a exportPskId is received as part of an incoming PSK proposal, it can then be processed as follows:
groupId = exportPskId[8..]
epoch = (uint64) exportPskId[0..7]
¶
Per [RFC9420], the psk_nonce must be a fresh random value of length KDF.Nh when the PSK proposal is generated.
This ensures key separation between a PSK generated by, for example, (1) a PSK generated by Bob from Alice's group and (2) a PSK generated by Charlie from Alice's group.¶
PROTECT # or encrypt? or create_message? A member Bob protects a ciphertext message and encrypting it to $U$ by encrypting it as an application message in their send group. As in MLS, before encrypting an application message, Bob should incorporate any MMLS updates he has received.¶
Each of the 3 MLS configurations of commit are possible: If Bob has no MMLS update to issue, and has seen no credential updates, Bob generates a partial commit covering PSK proposals from each updated send group.¶
If Bob has seen credential updates, Bob generates a full commit, and for each sender that has issued a MMLS update since Bob's last commit, * Bob injects a PSK for that sender's send group * Bob incorporates any updates they've observed.¶
If Bob has observed no updates but wishes to provide PCS, they can author an empty commit.¶
Members are not required to inject a PSK from a send group if they have only observed partial commits. (This allows the distibuted state to stabilize, if incorporating changes with a partial commit doesn't induce other members to commit. Injecting a PSK from a partial commit covering some PSK proposals doesn't add any benefit over importing the same PSK's coverered by the partial commit. )¶
UNPROTECT # or decrypt? or process_message? On receipt of an MLS message, a member can look up the corresponding send group by the MLS groupId in the message metadata and attempt to decrypt it with that send group.¶
(proposal) * REJOIN¶
A member of MMLS needs to receive all commits from all other send groups to continue to receive messages. A member that has been offline or otherwise fails to receive some commits still has the ability to encrypt messages to (possibly stale) credentials of the Universe in their own send group. This offline member Charlie can request to be re-join the other send groups by broadcasting an MMLS message in their own send group that indicates they are requesting a re-join. Other members should have access to a keyPackage message for Charlie. This could be attached to the re-join message if necessary.¶
The psk proposals in each of Charlie's commit serve as an ack of the commits of other send groups that Charlie has received, so other members can infer from Charlie's send group epoch, the newest epoch Charlie has observed for each send group.¶
Each member can then determine if Charlie is out of date in their send group. If necessary, they can author a commit removing Charlie's current leafNode in their send group and re-add Charlie using a keyPackage for Charlie.¶
The application layer over MLS has the responsibility to define * The Universe $U$ of members of this MMLS group * A universe identifier (as context for key exports) * The export key length * Additional common rules, such as accepted cipher suites¶
(nothing inherently requires the send groups to agree on a cipher suite - each sender could choose their own as long as they agree on export key length )¶
The MMLS layer should recommend a policy for issuing MMLS updates.¶
For example, $U$ can be defined over the wire by inferring it from a newly created send group.¶
Assume Alice has keypackages for some other members $M_i$¶
Alice can construct a MMLS group * with a randomly generated groupId * constructing a commit adding all other members $M_i$¶
Alice can distribute the Welcome message with an Application Message that indicates * this is a Send Group for Alice * that defines a Universe $U$ as the members of this group * with universe identifier equal to the groupId for Alice's send group * and defines a common export key length¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
TODO Security¶
This document has no IANA actions.¶
CAPBR: # Brewer, E., "Towards robust distributed systems (abstract)", ACM, Proceedings of the nineteenth annual ACM symposium on Principles of distributed computing, DOI 10.1145/343477.343502, July 2000, https://doi.org/10.1145/343477.343502.¶
TODO acknowledge.¶