The Messaging Layer Security (MLS) Protocol

1.1. Change Log

RFC EDITOR PLEASE DELETE THIS SECTION.¶

draft-18¶

• Make the document standards track¶
• Make the ratchet tree non-malleable (*)¶
• Use ExpandWithLabel to derive welcome key (*)¶
• Change MLS-Exporter label from "exporter" to "exported" (*)¶
• Loosen chain requirements (*)¶
• Clarify transcript hash initialization¶
• GREASE for MLS registries¶
• Move pseudocode out of KDFLabel definition.¶
• Rename PrivateContentTBE to PrivateMessageContent¶
• Fix DecryptWithLabel argument order for Welcome¶
• Responses to IESG reviews¶
• Describe varint length check more clearly¶

draft-17¶

• Rename MLSCiphertext and MLSPlaintext to PrivateMessage and PublicMesssage respectively (*)¶
• Add label and context to public-key encryption (*)¶
• Include leaf index in LeafNodeTBS for better parent-hash guarantees (*)¶
• Make ProtocolVersion two bytes (*)¶
• Clarify group creation (*)¶
• Validate additional properties of unmerged leaves (*)¶
• Clarify that the AS needs to vet the signature key¶
• Remove "MLS" prefix on structs¶
• Credentials should be replaced before expiring¶
• Add a section discussing the security of the sender data protection¶
• Minor fixes in presentation language.¶
• Allow multiple welcomes per commit¶
• Remove reference to BasicCredential.¶
• Client aware of its own removal in group¶
• Create IANA registries for signature and export labels¶
• Complete IANA media type registration¶
• Make more vendor code points available¶
• Update Recommended column definition to match 8447bis¶
• Responses to early ARTART review¶
• Responses to early OPSDIR review¶
• Responses to early TSV-ART review¶

draft-16¶

• Fix GroupInfoTBS (*)¶
• Make reference to h2 informative¶

draft-15¶

• Include ciphersuite in group context (*)¶
• Add new new_proposal_member SenderType (*)¶
• Always use a full tree (*)¶
• Change KeyPackage identifier extension to be LeafNode identifier (*)¶
• Use new tree for context in path secret encryption (*)¶
• Use a hash function for hash identifiers (*)¶
• Add a marker byte to tree hash input structs (*)¶
• Recommend that group ids are generated randomly (*)¶
• Update external senders extension to have SignaturePublicKey and Credential (*)¶
• Replace LeafNodeRef with leaf index (*)¶
• Remove AppAck proposal (*)¶
• Make padding arbitrary-size and all-zero (*)¶
• Require that unmerged_leaves be ordered¶
• Derive the commit secret from the end of the UpdatePath, not the root¶
• Specify the precise points in the protocol where credential validation must be done¶
• Make PSK provisions more uniform, e.g., always generating a fresh random nonce¶
• Improve parent hash guarantees with stricter checks on tree correctness¶
• Streamline some structs, e.g., folding GroupContext into GroupInfo¶
• Provide clearer rules for validating and applying commits¶
• Clarify tree hash and parent hash, and correct examples¶
• Clean up struct names and references to outdated structs¶
• Cite AEAD limits draft¶

draft-14¶

• Ensure that a signature public key is always intelligible (*)¶
• Clean up terminology of derived secrets/keys¶
• Fix parent hash (*)¶
• Specify compatibility behavior around new credentials¶
• Add Path Required to Proposal Type template¶
• Sub-group branching requires fresh key packages for each member¶
• Use aasvg and typed code blocks¶
• Require init key and leaf key to be different¶
• Preconfigured senders extension and removal of signature key indirection¶

draft-13¶

• TLS syntax updates (including variable-header-length vectors) (*)¶
• Stop generating redundant PKE key pairs. (*)¶
• Move validation of identity change to the AS¶
• Add message/mls MIME type registration¶
• Split LeafNode from KeyPackage (*)¶
• Remove endpoint_id (*)¶
• Reorganize to make section layout more sane¶
• Forbid proposals by reference in external commits (*)¶
• Domain separation for KeyPackage and Proposal references (*)¶
• Downgrade MUST to SHOULD for commit senders including all valid commits¶
• Stronger parent hashes for authenticated identities (*)¶
• Move wire_format to a separate tagged-union structure MLSMessage¶
• Generalize tree extend/truncate algorithms¶
• Add algorithms for link-based trees¶
• Forbid self-Update entirely (*)¶
• Consolidate resumption PSK cases (*)¶
• 384 Ciphersuite Addition¶
• Remove explicit version pin on HPKE (*)¶
• Remove the requirement for Add in external commit (*)¶
• Use smaller, fixed-size hash-based identifiers (*)¶
• Be explicit that Credentials can attest to multiple identities (*)¶

draft-12¶

• Use the GroupContext to derive the joiner_secret (*)¶
• Make PreSharedKeys non optional in GroupSecrets (*)¶
• Update name for this particular key (*)¶
• Truncate tree size on removal (*)¶
• Use HPKE draft-08 (*)¶
• Clarify requirements around identity in MLS groups (*)¶
• Signal the intended wire format for MLS messages (*)¶
• Inject GroupContext as HPKE info instead of AAD (*)¶
• Clarify extension handling and make extension updatable (*)¶
• Improve extensibility of Proposals (*)¶
• Constrain proposal in External Commit (*)¶
• Remove the notion of a 'leaf index' (*)¶
• Add group_context_extensions proposal ID (*)¶
• Add RequiredCapabilities extension (*)¶
• Use cascaded KDF instead of concatenation to consolidate PSKs (*)¶
• Use key package hash to index clients in message structs (*)¶
• Don't require PublicGroupState for external init (*)¶
• Make ratchet tree section clearer.¶
• Handle non-member sender cases in MLSPlaintextTBS¶
• Clarify encoding of signatures with NIST curves¶
• Remove OPEN ISSUEs and TODOs¶
• Normalize the description of the zero vector¶

draft-11¶

• Include subtree keys in parent hash (*)¶
• Pin HPKE to draft-07 (*)¶
• Move joiner secret to the end of the first key schedule epoch (*)¶
• Add an AppAck proposal¶
• Make initializations of transcript hashes consistent¶

draft-10¶

• Allow new members to join via an external Commit (*)¶
• Enable proposals to be sent inline in a Commit (*)¶
• Re-enable constant-time Add (*)¶
• Change expiration extension to lifetime extension (*)¶
• Make the tree in the Welcome optional (*)¶
• PSK injection, re-init, sub-group branching (*)¶
• Require the initial init_secret to be a random value (*)¶
• Remove explicit sender data nonce (*)¶
• Do not encrypt to joiners in UpdatePath generation (*)¶
• Move MLSPlaintext signature under the confirmation tag (*)¶
• Explicitly authenticate group membership with MLSPLaintext (*)¶
• Clarify X509Credential structure (*)¶
• Remove unneeded interim transcript hash from GroupInfo (*)¶
• IANA considerations¶
• Derive an authentication secret¶
• Use Extract/Expand from HPKE KDF¶
• Clarify that application messages MUST be encrypted¶

draft-09¶

• Remove blanking of nodes on Add (*)¶
• Change epoch numbers to uint64 (*)¶
• Add PSK inputs (*)¶
• Add key schedule exporter (*)¶
• Sign the updated direct path on Commit, using "parent hashes" and one signature per leaf (*)¶
• Use structured types for external senders (*)¶
• Redesign Welcome to include confirmation and use derived keys (*)¶
• Remove ignored proposals (*)¶
• Always include an Update with a Commit (*)¶
• Add per-message entropy to guard against nonce reuse (*)¶
• Use the same hash ratchet construct for both application and handshake keys (*)¶
• Add more ciphersuites¶
• Use HKDF to derive key pairs (*)¶
• Mandate expiration of ClientInitKeys (*)¶
• Add extensions to GroupContext and flesh out the extensibility story (*)¶
• Rename ClientInitKey to KeyPackage¶

draft-08¶

• Change ClientInitKeys so that they only refer to one ciphersuite (*)¶
• Decompose group operations into Proposals and Commits (*)¶
• Enable Add and Remove proposals from outside the group (*)¶
• Replace Init messages with multi-recipient Welcome message (*)¶
• Add extensions to ClientInitKeys for expiration and downgrade resistance (*)¶
• Allow multiple Proposals and a single Commit in one MLSPlaintext (*)¶

draft-07¶

• Initial version of the Tree based Application Key Schedule (*)¶
• Initial definition of the Init message for group creation (*)¶
• Fix issue with the transcript used for newcomers (*)¶
• Clarifications on message framing and HPKE contexts (*)¶

draft-06¶

• Reorder blanking and update in the Remove operation (*)¶
• Rename the GroupState structure to GroupContext (*)¶
• Rename UserInitKey to ClientInitKey¶
• Resolve the circular dependency that draft-05 introduced in the confirmation MAC calculation (*)¶
• Cover the entire MLSPlaintext in the transcript hash (*)¶

draft-05¶

• Common framing for handshake and application messages (*)¶
• Handshake message encryption (*)¶
• Convert from literal state to a commitment via the "tree hash" (*)¶
• Add credentials to the tree and remove the "roster" concept (*)¶
• Remove the secret field from tree node values¶

draft-04¶

• Updating the language to be similar to the Architecture document¶
• ECIES is now renamed in favor of HPKE (*)¶
• Using a KDF instead of a Hash in TreeKEM (*)¶

draft-03¶

• Added ciphersuites and signature schemes (*)¶
• Re-ordered fields in UserInitKey to make parsing easier (*)¶
• Fixed inconsistencies between Welcome and GroupState (*)¶
• Added encryption of the Welcome message (*)¶

draft-02¶

• Removed ART (*)¶
• Allowed partial trees to avoid double-joins (*)¶
• Added explicit key confirmation (*)¶

draft-01¶

• Initial description of the Message Protection mechanism. (*)¶
• Initial specification proposal for the Application Key Schedule using the per-participant chaining of the Application Secret design. (*)¶
• Initial specification proposal for an encryption mechanism to protect Application Messages using an AEAD scheme. (*)¶
• Initial specification proposal for an authentication mechanism of Application Messages using signatures. (*)¶
• Initial specification proposal for a padding mechanism to improving protection of Application Messages against traffic analysis. (*)¶
• Inversion of the Group Init Add and Application Secret derivations in the Handshake Key Schedule to be ease chaining in case we switch design. (*)¶
• Removal of the UserAdd construct and split of GroupAdd into Add and Welcome messages (*)¶
• Initial proposal for authenticating handshake messages by signing over group state and including group state in the key schedule (*)¶
• Added an appendix with example code for tree math¶
• Changed the ECIES mechanism used by TreeKEM so that it uses nonces generated from the shared secret¶

draft-00¶

• Initial adoption of draft-barnes-mls-protocol-01 as a WG item.¶

2.1. Presentation Language

We use the TLS presentation language [RFC8446] to describe the structure of protocol messages. In addition to the base syntax, we add two additional features, the ability for fields to be optional and the ability for vectors to have variable-size length headers.¶

struct {
    uint8 present;
    select (present) {
        case 0: struct{};
        case 1: T value;
    };
} optional<T>;

struct {
    uint32 fixed<0..255>;
    opaque variable<V>;
} StructWithVectors;

ReadVarint(data):
  // The length of variable-length integers is encoded in the
  // first two bits of the first byte.
  v = data.next_byte()
  prefix = v >> 6
  if prefix == 3:
    raise Exception('invalid variable length integer prefix')

  length = 1 << prefix

  // Once the length is known, remove these bits and read any
  // remaining bytes.
  v = v & 0x3f
  repeat length-1 times:
    v = (v << 8) + data.next_byte()

  // Check if the value would fit in half the provided length.
  if prefix >= 1 && v < (1 << (8*(length/2) - 2)):
    raise Exception('minimum encoding was not used')

  return v

3.1. Cryptographic State and Evolution

The cryptographic state at the core of MLS is divided into three areas of responsibility:¶

• A ratchet tree that represents the membership of the group, providing group members a way to authenticate each other and efficiently encrypt messages to subsets of the group. Each epoch has a distinct ratchet tree. It seeds the key schedule.¶

• A key schedule that describes the chain of key derivations used to progress from epoch to epoch (mainly using the init_secret and epoch_secret), as well as the derivation of a variety of other secrets (see Table 4), for example:¶

  • An encryption secret that is used to initialize the secret tree for the epoch.¶
  • An exporter secret that allows other protocols to leverage MLS as a generic authenticated group key exchange.¶
  • A resumption secret that members can use to prove their membership in the group, e.g., when creating a subgroup or a successor group.¶
• A secret tree derived from the key schedule that represents shared secrets used by the members of the group for encrypting and authenticating messages. Each epoch has a distinct secret tree.¶

Each member of the group maintains a partial view of these components of the group's state. MLS messages are used to initialize these views and keep them in sync as the group transitions between epochs.¶

Each new epoch is initiated with a Commit message. The Commit instructs existing members of the group to update their views of the ratchet tree by applying a set of Proposals, and uses the updated ratchet tree to distribute fresh entropy to the group. This fresh entropy is provided only to members in the new epoch and not to members who have been removed. Commits thus maintain the property that the epoch secret is confidential to the members in the current epoch.¶

For each Commit that adds one or more members to the group, there are one or more corresponding Welcome messages. Each Welcome message provides new members with the information they need to initialize their views of the key schedule and ratchet tree, so that these views align with the views held by other members of the group in this epoch.¶

3.2. Example Protocol Execution

There are three major operations in the lifecycle of a group:¶

• Adding a member, initiated by a current member;¶
• Updating the keys that represent a member in the tree;¶
• Removing a member.¶

Each of these operations is "proposed" by sending a message of the corresponding type (Add / Update / Remove). The state of the group is not changed, however, until a Commit message is sent to provide the group with fresh entropy. In this section, we show each proposal being committed immediately, but in more advanced deployment cases an application might gather several proposals before committing them all at once. In the illustrations below, we show the Proposal and Commit messages directly, while in reality they would be sent encapsulated in PublicMessage or PrivateMessage objects.¶

Before the initialization of a group, clients publish KeyPackages to a directory provided by the DS (see Figure 2).¶

Figure 3 shows how these prepublished KeyPackages are used to create a group. When a client A wants to establish a group with B and C, it first initializes a group state containing only itself and downloads KeyPackages for B and C. For each member, A generates an Add and Commit message adding that member, and broadcasts them to the group. It also generates a Welcome message and sends this directly to the new member (there's no need to send it to the group). Only after A has received its Commit message back from the Delivery Service does it update its state to reflect the new member's addition.¶

Once A has updated its state, the new member has processed the Welcome, and any other group members have processed the Commit, they will all have consistent representations of the group state, including a group secret that is known only to the members the group. The new member will be able to read and send new messages to the group, but messages sent before they were added to the group will not be accessible.¶

Subsequent additions of group members proceed in the same way. Any member of the group can download a KeyPackage for a new client and broadcast Add and Commit messages that the current group will use to update their state, and a Welcome message that the new client can use to initialize its state and join the group.¶

To enforce the forward secrecy and post-compromise security of messages, each member periodically updates the keys that represent them to the group. A member does this by sending a Commit (possibly with no proposals), or by sending an Update message that is committed by another member (see Figure 4). Once the other members of the group have processed these messages, the group's secrets will be unknown to an attacker that had compromised the secrets corresponding to the sender's leaf in the tree.¶

Update messages SHOULD be sent at regular intervals of time as long as the group is active, and members that don't update SHOULD eventually be removed from the group. It's left to the application to determine an appropriate amount of time between Updates. Since the purpose of sending an Update is to proactively constrain a compromise window, the right frequency is usually on the order of hours or days, not milliseconds. For example, an application might send an Update each time a member sends an application message after receiving from other members, or daily if no application messages are sent.¶

The MLS architecture recommends that MLS be operated over a secure transport (see Section 7.1 of [I-D.ietf-mls-architecture]). Such transport protocols will typically provide functions such as congestion control that manage the impact of an MLS-using application on other applications sharing the same network. Applications should take care that they do not send MLS messages at a rate that will cause problems such as network congestion, especially if they are not following the above recommendation (e.g., sending MLS directly over UDP instead).¶

Members are removed from the group in a similar way, as shown in Figure 5. Any member of the group can send a Remove proposal followed by a Commit message. The Commit message provides new entropy to all members of the group except the removed member. This new entropy is added to the epoch secret for the new epoch so that it is not known to the removed member. Note that this does not necessarily imply that any member is actually allowed to evict other members; groups can enforce access control policies on top of these basic mechanisms.¶

Note that the flows in this section are examples; applications can arrange message flows in other ways. For example:¶

• Welcome messages don't necessarily need to be sent directly to new joiners. Since they are encrypted to new joiners, they could be distributed more broadly, say if the application only had access to a broadcast channel for the group.¶
• Proposal messages don't need to be immediately sent to all group members. They need to be available to the committer before generating a commit, and to other members before processing the commit.¶
• The sender of a Commit doesn't necessarily have to wait to receive its own Commit back before advancing its state. It only needs to know that its Commit will be the next one applied by the group, say based on a promise from an orchestration server.¶

3.3. External Joins

In addition to the Welcome-based flow for adding a new member to the group, it is also possible for a new member to join by means of an "external Commit". This mechanism can be used when the existing members don't have a KeyPackage for the new member, for example, in the case of an "open" group that can be joined by new members without asking permission from existing members.¶

Figure 6 shows a typical message flow for an external join. To enable a new member to join the group in this way, a member of the group (A, B) publishes a GroupInfo object that includes the GroupContext for the group as well as a public key that can be used to encrypt a secret to the existing members of the group. When the new member Z wishes to join, they download the GroupInfo object and use it to form a Commit of a special form that adds Z to the group (as detailed in Section 12.4.3.2). The existing members of the group process this external Commit in a similar way to a normal Commit, advancing to a new epoch in which Z is now a member of the group.¶

                                                          Group
A              B              Z          Directory        Channel
|              |              |              |              |
| GroupInfo    |              |              |              |
+------------------------------------------->|              |
|              |              | GroupInfo    |              |
|              |              |<-------------+              |
|              |              |              |              |
|              |              | Commit(ExtZ) |              |
|              |              +---------------------------->|
|              |              |              | Commit(ExtZ) |
|<----------------------------------------------------------+
|              |<-------------------------------------------+
|              |              |<----------------------------+
|              |              |              |              |

3.4. Relationships Between Epochs

A group has a single linear sequence of epochs. Groups and epochs are generally independent of one another. However, it can sometimes be useful to link epochs cryptographically, either within a group or across groups. MLS derives a resumption pre-shared key (PSK) from each epoch to allow entropy extracted from one epoch to be injected into a future epoch. A group member that wishes to inject a PSK issues a PreSharedKey proposal (Section 12.1.4) describing the PSK to be injected. When this proposal is committed, the corresponding PSK will be incorporated into the key schedule as described in Section 8.4.¶

Linking epochs in this way guarantees that members entering the new epoch agree on a key if and only if they were members of the group during the epoch from which the resumption key was extracted.¶

MLS supports two ways to tie a new group to an existing group, illustrated in Figure 7 and Figure 8. Reinitialization closes one group and creates a new group comprising the same members with different parameters. Branching starts a new group with a subset of the original group's participants (with no effect on the original group). In both cases, the new group is linked to the old group via a resumption PSK.¶

Applications may also choose to use resumption PSKs to link epochs in other ways. For example, Figure 9 shows a case where a resumption PSK from epoch n is injected into epoch n+k. This demonstrates that the members of the group at epoch n+k were also members at epoch n, irrespective of any changes to these members' keys due to Updates or Commits.¶

4.1. Ratchet Tree Terminology

Trees consist of nodes. A node is a leaf if it has no children, and a parent otherwise; note that all parents in our trees have precisely two children, a left child and a right child. A node is the root of a tree if it has no parents, and intermediate if it has both children and parents. The descendants of a node are that node's children, and the descendants of its children, and we say a tree contains a node if that node is a descendant of the root of the tree, or if the node itself is the root of the tree. Nodes are siblings if they share the same parent.¶

A subtree of a tree is the tree given by any node (the head of the subtree) and its descendants. The size of a tree or subtree is the number of leaf nodes it contains. For a given parent node, its left subtree is the subtree with its left child as head (respectively right subtree).¶

Every tree used in this protocol is a perfect binary tree, that is, a complete balanced binary tree with 2^d leaves all at the same depth d. This structure is unique for a given depth d.¶

There are multiple ways that an implementation might represent a ratchet tree in memory. A convenient property of left-balanced binary trees (including the complete trees used here) is that they can be represented as an array of nodes, with node relationships computed based on the nodes' indices in the array. A more traditional representation based on linked node objects may also be used. Appendix C and Appendix D provide some details on how to implement the tree operations required for MLS in these representations. MLS places no requirements on implementations' internal representations of ratchet trees. An implementation may use any tree representation and associated algorithms, as long as they produce correct protocol messages.¶

               ...
               /
              _
        ______|______
       /             \
      X[B]            _
    __|__           __|__
   /     \         /     \
  _       _       Y       _
 / \     / \     / \     / \
A   B   _   D   E   F   _   H

0   1   2   3   4   5   6   7

4.2. Views of a Ratchet Tree

We generally assume that each participant maintains a complete and up-to-date view of the public state of the group's ratchet tree, including the public keys for all nodes and the credentials associated with the leaf nodes.¶

No participant in an MLS group knows the private key associated with every node in the tree. Instead, each member is assigned to a leaf of the tree, which determines the subset of private keys it knows. The credential stored at that leaf is one provided by the member.¶

In particular, MLS maintains the members' views of the tree in such a way as to maintain the tree invariant:¶

The private key for a node in the tree is known to a member of
the group only if the node's subtree contains that member's leaf.

In other words, if a node is not blank, then it holds a public key. The corresponding private key is known only to members occupying leaves below that node.¶

The reverse implication is not true: A member may not know the private key of an intermediate node above them. Such a member has an unmerged leaf at the intermediate node. Encrypting to an intermediate node requires encrypting to the node's public key, as well as the public keys of all the unmerged leaves below it. A leaf is unmerged with regard to all of its ancestors when it is first added, because the process of adding the leaf does not give it access to the private keys for all of the nodes above it in the tree. Leaves are "merged" as they receive the private keys for nodes, as described in Section 7.4.¶

For example, consider a four-member group (A, B, C, D) where the node above the right two members is blank. (This is what it would look like if A created a group with B, C, and D.) Then the public state of the tree and the views of the private keys of the tree held by each participant would be as follows, where _ represents a blank node, ? represents an unknown private key, and pk(X) represents the public key corresponding to the private key X:¶

         Public Tree
============================
            pk(ABCD)
          /          \
    pk(AB)            _
     / \             / \
pk(A)   pk(B)   pk(C)   pk(D)


 Private @ A       Private @ B       Private @ C       Private @ D
=============     =============     =============     =============
     ABCD              ABCD              ABCD              ABCD
    /   \             /   \             /   \             /   \
  AB      _         AB      _         ?       _         ?       _
 / \     / \       / \     / \       / \     / \       / \     / \
A   ?   ?   ?     ?   B   ?   ?     ?   ?   C   ?     ?   ?   ?   D

Note how the tree invariant applies: Each member knows only their own leaf, the private key AB is known only to A and B, and the private key ABCD is known to all four members. This also illustrates another important point: it is possible for there to be "holes" on the path from a member's leaf to the root in which the member knows the key both above and below a given node, but not for that node, as in the case with D.¶

5.1. Ciphersuites

Each MLS session uses a single ciphersuite that specifies the following primitives to be used in group key computations:¶

• HPKE parameters:¶

  • A Key Encapsulation Mechanism (KEM)¶
  • A Key Derivation Function (KDF)¶
  • An Authenticated Encryption with Associated Data (AEAD) encryption algorithm¶
• A hash algorithm¶
• A MAC algorithm¶
• A signature algorithm¶

MLS uses HPKE for public-key encryption [RFC9180]. The DeriveKeyPair function associated to the KEM for the ciphersuite maps octet strings to HPKE key pairs. As in HPKE, MLS assumes that an AEAD algorithm produces a single ciphertext output from AEAD encryption (aligning with [RFC5116]), as opposed to a separate ciphertext and tag.¶

Ciphersuites are represented with the CipherSuite type. The ciphersuites are defined in Section 17.1.¶

opaque HPKEPublicKey<V>;

opaque SignaturePublicKey<V>;

SignWithLabel(SignatureKey, Label, Content) =
    Signature.Sign(SignatureKey, SignContent)

VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) =
    Signature.Verify(VerificationKey, SignContent, SignatureValue)

struct {
    opaque label<V>;
    opaque content<V>;
} SignContent;

label = "MLS 1.0 " + Label;
content = Content;

EncryptWithLabel(PublicKey, Label, Context, Plaintext) =
  SealBase(PublicKey, EncryptContext, "", Plaintext)

DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) =
  OpenBase(KEMOutput, PrivateKey, EncryptContext, "", Ciphertext)

struct {
  opaque label<V>;
  opaque context<V>;
} EncryptContext;

label = "MLS 1.0 " + Label;
context = Context;

5.2. Hash-Based Identifiers

Some MLS messages refer to other MLS objects by hash. For example, Welcome messages refer to KeyPackages for the members being welcomed, and Commits refer to Proposals they cover. These identifiers are computed as follows:¶

opaque HashReference<V>;

HashReference KeyPackageRef;
HashReference ProposalRef;

MakeKeyPackageRef(value) = RefHash("MLS 1.0 KeyPackage Reference", value)
MakeProposalRef(value)   = RefHash("MLS 1.0 Proposal Reference", value)

RefHash(label, value) = Hash(RefHashInput)

Where RefHashInput is defined as:¶

struct {
  opaque label<V>;
  opaque value<V>;
} RefHashInput;

And its fields set to:¶

label = label;
value = value;

For a KeyPackageRef, the value input is the encoded KeyPackage, and the ciphersuite specified in the KeyPackage determines the KDF used. For a ProposalRef, the value input is the AuthenticatedContent carrying the proposal. In the latter two cases, the KDF is determined by the group's ciphersuite.¶

5.3. Credentials

Each member of a group presents a credential that provides one or more identities for the member, and associates them with the member's signing key. The identities and signing key are verified by the Authentication Service in use for a group.¶

It is up to the application to decide which identifier or identifiers to use at the application level. For example, a certificate in an X509Credential may attest to several domain names or email addresses in its subjectAltName extension. An application may decide to present all of these to a user, or if it knows a "desired" domain name or email address, it can check that the desired identifier is among those attested. Using the terminology from [RFC6125], a Credential provides "presented identifiers", and it is up to the application to supply a "reference identifier" for the authenticated client, if any.¶

// See IANA registry for registered values
uint16 CredentialType;

struct {
    opaque cert_data<V>;
} Certificate;

struct {
    CredentialType credential_type;
    select (Credential.credential_type) {
        case basic:
            opaque identity<V>;

        case x509:
            Certificate certificates<V>;
    };
} Credential;

A "basic" credential is a bare assertion of an identity, without any additional information. The format of the encoded identity is defined by the application.¶

For an X.509 credential, each entry in the certificates field represents a single DER-encoded X.509 certificate. The chain is ordered such that the first entry (certificates[0]) is the end-entity certificate. The public key encoded in the subjectPublicKeyInfo of the end-entity certificate MUST be identical to the signature_key in the LeafNode containing this credential. A chain MAY omit any non-leaf certificates that supported peers are known to already possess.¶

opaque application_id<V>;

6.1. Content Authentication

FramedContent is authenticated using the FramedContentAuthData structure.¶

struct {
    ProtocolVersion version = mls10;
    WireFormat wire_format;
    FramedContent content;
    select (FramedContentTBS.content.sender.sender_type) {
        case member:
        case new_member_commit:
            GroupContext context;
        case external:
        case new_member_proposal:
            struct{};
    };
} FramedContentTBS;

opaque MAC<V>;

struct {
    /* SignWithLabel(., "FramedContentTBS", FramedContentTBS) */
    opaque signature<V>;
    select (FramedContent.content_type) {
        case commit:
            /*
              MAC(confirmation_key,
                  GroupContext.confirmed_transcript_hash)
            */
            MAC confirmation_tag;
        case application:
        case proposal:
            struct{};
    };
} FramedContentAuthData;

The signature is computed using SignWithLabel with label "FramedContentTBS" and with a content that covers the message content and the wire format that will be used for this message. If the sender's sender_type is member, the content also covers the GroupContext for the current epoch so that signatures are specific to a given group and epoch.¶

The sender MUST use the private key corresponding to the following signature key depending on the sender's sender_type:¶

• member: The signature key contained in the LeafNode at the index indicated by leaf_index in the ratchet tree.¶
• external: The signature key at the index indicated by sender_index in the external_senders group context extension (see Section 12.1.8.1). The content_type of the message MUST be proposal and the proposal_type MUST be a value that is allowed for external senders.¶
• new_member_commit: The signature key in the LeafNode in the Commit's path (see Section 12.4.3.2). The content_type of the message MUST be commit.¶
• new_member_proposal: The signature key in the LeafNode in the KeyPackage embedded in an External Add Proposal. The content_type of the message MUST be proposal and the proposal_type of the Proposal MUST be add.¶

Recipients of an MLSMessage MUST verify the signature with the key depending on the sender_type of the sender as described above.¶

The confirmation tag value confirms that the members of the group have arrived at the same state of the group. A FramedContentAuthData is said to be valid when both the signature and confirmation_tag fields are valid.¶

6.2. Encoding and Decoding a Public Message

Messages that are authenticated but not encrypted are encoded using the PublicMessage structure.¶

struct {
    FramedContent content;
    FramedContentAuthData auth;
    select (PublicMessage.content.sender.sender_type) {
        case member:
            MAC membership_tag;
        case external:
        case new_member_commit:
        case new_member_proposal:
            struct{};
    };
} PublicMessage;

The membership_tag field in the PublicMessage object authenticates the sender's membership in the group. For messages sent by members, it MUST be set to the following value:¶

struct {
  FramedContentTBS content_tbs;
  FramedContentAuthData auth;
} AuthenticatedContentTBM;

membership_tag = MAC(membership_key, AuthenticatedContentTBM)

When decoding an PublicMessage into an AuthenticatedContent, the application MUST check membership_tag and MUST check that the FramedContentAuthData is valid.¶

6.3. Encoding and Decoding a Private Message

Authenticated and encrypted messages are encoded using the PrivateMessage structure.¶

struct {
    opaque group_id<V>;
    uint64 epoch;
    ContentType content_type;
    opaque authenticated_data<V>;
    opaque encrypted_sender_data<V>;
    opaque ciphertext<V>;
} PrivateMessage;

encrypted_sender_data and ciphertext are encrypted using the AEAD function specified by the ciphersuite in use, using as input the structures SenderData and PrivateMessageContent.¶

struct {
    select (PrivateMessage.content_type) {
        case application:
          opaque application_data<V>;

        case proposal:
          Proposal proposal;

        case commit:
          Commit commit;
    };

    FramedContentAuthData auth;
    opaque padding[length_of_padding];
} PrivateMessageContent;

+-+-+-+-+---------...---+
|   Key Schedule Nonce  |
+-+-+-+-+---------...---+
           XOR
+-+-+-+-+---------...---+
| Guard |       0       |
+-+-+-+-+---------...---+
           ===
+-+-+-+-+---------...---+
| Encrypt/Decrypt Nonce |
+-+-+-+-+---------...---+

struct {
    opaque group_id<V>;
    uint64 epoch;
    ContentType content_type;
    opaque authenticated_data<V>;
} PrivateContentAAD;

struct {
    uint32 leaf_index;
    uint32 generation;
    opaque reuse_guard[4];
} SenderData;

ciphertext_sample = ciphertext[0..KDF.Nh-1]

sender_data_key = ExpandWithLabel(sender_data_secret, "key",
                      ciphertext_sample, AEAD.Nk)
sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
                      ciphertext_sample, AEAD.Nn)

struct {
    opaque group_id<V>;
    uint64 epoch;
    ContentType content_type;
} SenderDataAAD;

7.1. Parent Node Contents

As discussed in Section 4.1.1, the nodes of a ratchet tree contain several types of data describing individual members (for leaf nodes) or subgroups of the group (for parent nodes). Parent nodes are simpler:¶

struct {
    HPKEPublicKey encryption_key;
    opaque parent_hash<V>;
    uint32 unmerged_leaves<V>;
} ParentNode;

The encryption_key field contains an HPKE public key whose private key is held only by the members at the leaves among its descendants. The parent_hash field contains a hash of this node's parent node, as described in Section 7.9. The unmerged_leaves field lists the leaves under this parent node that are unmerged, according to their indices among all the leaves in the tree. The entries in the unmerged_leaves vector MUST be sorted in increasing order.¶

7.2. Leaf Node Contents

A leaf node in the tree describes all the details of an individual client's appearance in the group, signed by that client. It is also used in client KeyPackage objects to store the information that will be needed to add a client to a group.¶

enum {
    reserved(0),
    key_package(1),
    update(2),
    commit(3),
    (255)
} LeafNodeSource;

struct {
    ProtocolVersion versions<V>;
    CipherSuite ciphersuites<V>;
    ExtensionType extensions<V>;
    ProposalType proposals<V>;
    CredentialType credentials<V>;
} Capabilities;

struct {
    uint64 not_before;
    uint64 not_after;
} Lifetime;

// See IANA registry for registered values
uint16 ExtensionType;

struct {
    ExtensionType extension_type;
    opaque extension_data<V>;
} Extension;

struct {
    HPKEPublicKey encryption_key;
    SignaturePublicKey signature_key;
    Credential credential;
    Capabilities capabilities;

    LeafNodeSource leaf_node_source;
    select (LeafNode.leaf_node_source) {
        case key_package:
            Lifetime lifetime;

        case update:
            struct{};

        case commit:
            opaque parent_hash<V>;
    };

    Extension extensions<V>;
    /* SignWithLabel(., "LeafNodeTBS", LeafNodeTBS) */
    opaque signature<V>;
} LeafNode;

struct {
    HPKEPublicKey encryption_key;
    SignaturePublicKey signature_key;
    Credential credential;
    Capabilities capabilities;

    LeafNodeSource leaf_node_source;
    select (LeafNodeTBS.leaf_node_source) {
        case key_package:
            Lifetime lifetime;

        case update:
            struct{};

        case commit:
            opaque parent_hash<V>;
    };

    Extension extensions<V>;

    select (LeafNodeTBS.leaf_node_source) {
        case key_package:
            struct{};

        case update:
            opaque group_id<V>;
            uint32 leaf_index;

        case commit:
            opaque group_id<V>;
            uint32 leaf_index;
    };
} LeafNodeTBS;

The encryption_key field contains an HPKE public key whose private key is held only by the member occupying this leaf (or in the case of a LeafNode in a KeyPackage object, the issuer of the KeyPackage). The signature_key field contains the member's public signing key. The credential field contains information authenticating both the member's identity and the provided signing key, as described in Section 5.3.¶

The capabilities field indicates what protocol versions, ciphersuites, extensions, credential types, and non-default proposal types are supported by a client. Proposal and extension types defined in this document are considered "default" and thus need not be listed, while any credential types the application wishes to use MUST be listed. Extensions that appear in the extensions field of a LeafNode MUST be included in the extensions field of the capabilities field, and the credential type used in the LeafNode MUST be included in the credentials field of the capabilities field. As discussed in Section 13, unknown values in capabilities MUST be ignored, and the creator of a capabilities field SHOULD include some random GREASE values to help ensure that other clients correctly ignore unknown values.¶

The leaf_node_source field indicates how this LeafNode came to be added to the tree. This signal tells other members of the group whether the leaf node is required to have a lifetime or parent_hash, and whether the group_id is added as context to the signature. Whether these fields can be computed by the client represented by the LeafNode depends on when the LeafNode was created. For example, a KeyPackage is created before the client knows which group it will be used with, so its signature can't bind to a group_id.¶

In the case where the leaf was added to the tree based on a pre-published KeyPackage, the lifetime field represents the times between which clients will consider a LeafNode valid. These times are represented as absolute times, measured in seconds since the Unix epoch (1970-01-01T00:00:00Z). Applications MUST define a maximum total lifetime that is acceptable for a LeafNode, and reject any LeafNode where the total lifetime is longer than this duration. In order to avoid disagreements about whether a LeafNode has a valid lifetime, the clients in a group SHOULD maintain time synchronization (e.g., using the Network Time Protocol [RFC5905]).¶

In the case where the leaf node was inserted into the tree via a Commit message, the parent_hash field contains the parent hash for this leaf node (see Section 7.9).¶

The LeafNodeTBS structure covers the fields above the signature in the LeafNode. In addition, when the leaf node was created in the context of a group (the update and commit cases), the group ID of the group is added as context to the signature.¶

LeafNode objects stored in the group's ratchet tree are updated according to the evolution of the tree. Each modification of LeafNode content MUST be reflected by a change in its signature. This allows other members to verify the validity of the LeafNode at any time, particularly in the case of a newcomer joining the group.¶

7.3. Leaf Node Validation

The validity of a LeafNode needs to be verified at a few stages:¶

• When a LeafNode is downloaded in a KeyPackage, before it is used to add the client to the group¶
• When a LeafNode is received by a group member in an Add, Update, or Commit message¶
• When a client validates a ratchet tree, e.g., when joining a group or after processing a commit¶

The client verifies the validity of a LeafNode using the following steps:¶

• Verify that the credential in the LeafNode is valid as described in Section 5.3.1.¶
• Verify that the signature on the LeafNode is valid using signature_key.¶
• Verify that the LeafNode is compatible with the group's parameters. If the GroupContext has a required_capabilities extension, then the required extensions, proposals, and credential types MUST be listed in the LeafNode's capabilities field.¶
• Verify that the credential type is supported by all members of the group, as specified by the capabilities field of each member's LeafNode, and that the capabilities field of this LeafNode indicates support for all the credential types currently in use by other members.¶

• Verify the lifetime field:¶

  • If the LeafNode appears in a message being sent by the client, e.g., a proposal or a commit, then the client MUST verify that the current time is within the range of the lifetime field.¶
  • If instead the LeafNode appears in a message being received by the client, e.g., a proposal, a commit, or a ratchet tree of the group the client is joining, it is RECOMMENDED that the client verifies that the current time is within the range of the lifetime field. (This check is not mandatory because the LeafNode might have expired in the time between when the message was sent and when it was received.)¶
• Verify that the extensions in the LeafNode are supported by checking that the ID for each extension in the extensions field is listed in the capabilities.extensions field of the LeafNode.¶

• Verify the leaf_node_source field:¶

  • If the LeafNode appears in a KeyPackage, verify that leaf_node_source is set to key_package.¶
  • If the LeafNode appears in an Update proposal, verify that leaf_node_source is set to update and that encryption_key represents a different public key than the encryption_key in the leaf node being replaced by the Update proposal.¶
  • If the LeafNode appears in the leaf_node value of the UpdatePath in a Commit, verify that leaf_node_source is set to commit.¶

• Verify that the following fields are unique among the members of the group:¶

  • signature_key¶
  • encryption_key¶

7.4. Ratchet Tree Evolution

Whenever a member initiates an epoch change (i.e., commits; see Section 12.4), they may need to refresh the key pairs of their leaf and of the nodes on their leaf's direct path in order to maintain forward secrecy and post-compromise security.¶

The member initiating the epoch change generates the fresh key pairs using the following procedure. The procedure is designed in a way that allows group members to efficiently communicate the fresh secret keys to other group members, as described in Section 7.6.¶

A member updates the nodes along its direct path as follows:¶

• Blank all the nodes on the direct path from the leaf to the root.¶
• Generate a fresh HPKE key pair for the leaf.¶
• Generate a sequence of path secrets, one for each node on the leaf's filtered direct path, as follows. In this setting, path_secret[0] refers to the first parent node in the filtered direct path, path_secret[1] to the second parent node, and so on.¶

path_secret[0] is sampled at random
path_secret[n] = DeriveSecret(path_secret[n-1], "path")

• Compute the sequence of HPKE key pairs (node_priv,node_pub), one for each node on the leaf's direct path, as follows.¶

node_secret[n] = DeriveSecret(path_secret[n], "node")
node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])

The node secret is derived as a temporary intermediate secret so that each secret is only used with one algorithm: The path secret is used as an input to DeriveSecret and the node secret is used as an input to DeriveKeyPair.¶

For example, suppose there is a group with four members, with C an unmerged leaf at Z:¶

If member B subsequently generates an UpdatePath based on a secret "leaf_secret", then it would generate the following sequence of path secrets:¶

After applying the UpdatePath, the tree will have the following structure:¶

7.5. Synchronizing Views of the Tree

After generating fresh key material and applying it to ratchet forward their local tree state as described in the Section 7.4, the generator broadcasts this update to other members of the group in a Commit message, who apply it to keep their local views of the tree in sync with the sender's. More specifically, when a member commits a change to the tree (e.g., to add or remove a member), it transmits an UpdatePath containing a set of public keys and encrypted path secrets for intermediate nodes in the filtered direct path of its leaf. The other members of the group use these values to update their view of the tree, aligning their copy of the tree to the sender's.¶

An UpdatePath contains the following information for each node in the filtered direct path of the sender's leaf, including the root:¶

• The public key for the node¶
• One or more encrypted copies of the path secret corresponding to the node¶

The path secret value for a given node is encrypted to the subtree rooted at the parent's non-updated child, i.e., the child on the copath of the sender's leaf node. There is one encryption of the path secret to each public key in the resolution of the non-updated child.¶

A member of the group updates their direct path by computing new values for their leaf node and the nodes along their filtered direct path:¶

1. Blank all nodes along the direct path of the sender's leaf.¶

2. Compute updated path secrets and public keys for the nodes on the sender's filtered direct path.¶

   • Generate a sequence of path secrets of the same length as the filtered direct path, as defined in Section 7.4¶
   • For each node in the filtered direct path, replace the node's public key with the node_pub[n] value derived from the corresponding path secret path_secret[n].¶
3. Compute the new parent hashes for the nodes along the filtered direct path and the sender's leaf node.¶

4. Update the leaf node for the sender.¶

   • Set the leaf_node_source to commit.¶
   • Set the encryption_key to the public key of a freshly sampled key pair¶
   • Set the parent hash to the parent hash for the leaf.¶
   • Re-sign the leaf node with its new contents¶

Since the new leaf node effectively updates an existing leaf node in the group, it MUST adhere to the same restrictions as LeafNodes used in Update proposals (aside from leaf_node_source). The application MAY specify other changes to the leaf node, e.g., providing a new signature key, updated capabilities, or different extensions.¶

The member then encrypts path secrets to the group. For each node in the member's filtered direct path, the member takes the following steps:¶

1. Compute the resolution of the node's child that is on the copath of the sender (the child that is not in the direct path of the sender). Any new member (from an Add proposal) added in the same Commit MUST be excluded from this resolution.¶
2. For each node in the resolution, encrypt the path secret for the direct path node using the public key of the resolution node, as defined in Section 7.6¶

The recipient of an UpdatePath performs the corresponding steps. First, the recipient merges UpdatePath into the tree:¶

1. Blank all nodes on the direct path of the sender's leaf.¶

2. For all nodes on the filtered direct path of the sender's leaf,¶

   • Set the public key to the public key in the UpdatePath.¶
   • Set the list of unmerged leaves to the empty list.¶

3. Compute parent hashes for the nodes in the sender's filtered direct path, and verify that the parent_hash field of the leaf node matches the parent hash for the first node in its filtered direct path.¶

   • Note that these hashes are computed from root to leaf, so that each hash incorporates all the non-blank nodes above it. The root node always has a zero-length hash for its parent hash.¶

Second, the recipient decrypts the path secrets:¶

1. Identify a node in the filtered direct path for which the recipient is in the subtree of the non-updated child.¶
2. Identify a node in the resolution of the copath node for which the recipient has a private key.¶
3. Decrypt the path secret for the parent of the copath node using the private key from the resolution node.¶
4. Derive path secrets for ancestors of that node in the sender's filtered direct path using the algorithm described above.¶
5. Derive the node secrets and node key pairs from the path secrets.¶
6. Verify that the derived public keys are the same as the corresponding public keys sent in the UpdatePath.¶
7. Store the derived private keys in the corresponding ratchet tree nodes.¶

For example, in order to communicate the example update described in Section 7.4, the member at node B would transmit the following values:¶

In this table, the value node_pub[i] represents the public key derived from node_secret[i], pk(X) represents the current public key of node X, and E(K, S) represents the public-key encryption of the path secret S to the public key K (using HPKE).¶

A recipient at node A would decrypt E(pk(A), path_secret\[0\]) to obtain path_secret\[0\], then use it to derive path_secret[1] and the resulting node secrets and key pairs. Thus, A would have the private keys to nodes X' and Y', in accordance with the tree invariant.¶

Similarly, a recipient at node D would decrypt E(pk(Z), path_secret[1]) to obtain path_secret[1], then use it to derive the node secret and key pair for the node Y'. As required to maintain the tree invariant, node D does not receive the private key for the node X', since X' is not an ancestor of D.¶

After processing the update, each recipient MUST delete outdated key material, specifically:¶

• The path secrets and node secrets used to derive each updated node key pair.¶
• Each outdated node key pair that was replaced by the update.¶

7.6. Update Paths

As described in Section 12.4, each Commit message may optionally contain an UpdatePath, with a new LeafNode and set of parent nodes for the sender's filtered direct path. For each parent node, the UpdatePath contains a new public key and encrypted path secret. The parent nodes are kept in the same order as the filtered direct path.¶

struct {
    opaque kem_output<V>;
    opaque ciphertext<V>;
} HPKECiphertext;

struct {
    HPKEPublicKey encryption_key;
    HPKECiphertext encrypted_path_secret<V>;
} UpdatePathNode;

struct {
    LeafNode leaf_node;
    UpdatePathNode nodes<V>;
} UpdatePath;

For each UpdatePathNode, the resolution of the corresponding copath node MUST exclude all new leaf nodes added as part of the current Commit. The length of the encrypted_path_secret vector MUST be equal to the length of the resolution of the copath node (excluding new leaf nodes), with each ciphertext being the encryption to the respective resolution node.¶

The HPKECiphertext values are encrypted and decrypted as follows:¶

(kem_output, ciphertext) =
  EncryptWithLabel(node_public_key, "UpdatePathNode",
                   group_context, path_secret)

path_secret =
  DecryptWithLabel(node_private_key, "UpdatePathNode",
                   group_context, kem_output, ciphertext)

Here node_public_key is the public key of the node for which the path secret is encrypted, group_context is the provisional GroupContext object for the group, and the EncryptWithLabel function is as defined in Section 5.1.3.¶

7.7. Adding and Removing Leaves

In addition to the path-based updates to the tree described above, it is also necessary to add and remove leaves of the tree in order to reflect changes to the membership of the group (see Section 12.1.1 and Section 12.1.3). Since the tree is always full, adding or removing leaves corresponds to increasing or decreasing the depth of the tree, resulting in the number of leaves being doubled or halved. These operations are also known as extending and truncating the tree.¶

Leaves are always added and removed at the right edge of the tree. When the size of the tree needs to be increased, a new blank root node is added, whose left subtree is the existing tree and right subtree is a new all-blank subtree. This operation is typically done when adding a member to the group.¶

                  _ <-- new blank root                    _
                __|__                                   __|__
               /     \                                 /     \
  X    ===>   X       _ <-- new blank subtree ===>    X       _
 / \         / \     / \                             / \     / \
A   B       A   B   _   _                           A   B   C   _
                                                            ^
                                                            |
                                                            +-- new member

When the right subtree of the tree no longer has any non-blank nodes, it can be safely removed. The root of the tree and the right subtree are discarded (whether or not the root node is blank). The left child of the root becomes the new root node, and the left subtree becomes the new tree. This operation is typically done after removing a member from the group.¶

               Y                  Y
             __|__              __|__
            /     \            /     \
           X       _   ===>   X       _   ==>   X <-- new root
          / \     / \        / \     / \       / \
         A   B   C   _      A   B   _   _     A   B
                 ^
                 |
removed member --+

Concrete algorithms for these operations on array-based and link-based trees are provided in Appendix C and Appendix D. The concrete algorithms are non-normative. An implementation may use any algorithm that produces the correct tree in its internal representation.¶

7.8. Tree Hashes

MLS hashes the contents of the tree in two ways to authenticate different properties of the tree. Tree hashes are defined in this section, and parent hashes are defined in Section 7.9.¶

Each node in a ratchet tree has a tree hash that summarizes the subtree below that node. The tree hash of the root is used in the GroupContext to confirm that the group agrees on the whole tree. Tree hashes are computed recursively from the leaves up to the root.¶

The tree hash of an individual node is the hash of the node's TreeHashInput object, which may contain either a LeafNodeHashInput or a ParentNodeHashInput depending on the type of node. LeafNodeHashInput objects contain the leaf_index and the LeafNode (if any). ParentNodeHashInput objects contain the ParentNode (if any) and the tree hash of the node's left and right children. For both parent and leaf nodes, the optional node value MUST be absent if the node is blank and present if the node contains a value.¶

enum {
    reserved(0),
    leaf(1),
    parent(2),
    (255)
} NodeType;

struct {
  NodeType node_type;
  select (TreeHashInput.node_type) {
    case leaf:   LeafNodeHashInput leaf_node;
    case parent: ParentNodeHashInput parent_node;
  };
} TreeHashInput;

struct {
    uint32 leaf_index;
    optional<LeafNode> leaf_node;
} LeafNodeHashInput;

struct {
    optional<ParentNode> parent_node;
    opaque left_hash<V>;
    opaque right_hash<V>;
} ParentNodeHashInput;

The tree hash of an entire tree corresponds to the tree hash of the root node, which is computed recursively by starting at the leaf nodes and building up.¶

7.9. Parent Hashes

While tree hashes summarize the state of a tree at point in time, parent hashes capture information about how keys in the tree were populated.¶

When a client sends a commit to change a group, it can include an UpdatePath to assign new keys to the nodes along its filtered direct path. When a client computes an UpdatePath (as defined in Section 7.5), it computes and signs a parent hash that summarizes the state of the tree after the UpdatePath has been applied. These summaries are constructed in a chain from the root to the member's leaf so that the part of the chain closer to the root can be overwritten as nodes set in one UpdatePath are reset by a later UpdatePath.¶

As a result, the signature over the parent hash in each member's leaf effectively signs the subtree of the tree that hasn't been changed since that leaf was last changed in an UpdatePath. A new member joining the group uses these parent hashes to verify that the parent nodes in the tree were set by members of the group, not chosen by an external attacker. For an example of how this works, see Appendix B.¶

Consider a ratchet tree with a non-blank parent node P and children D and S (for "parent", "direct path", and "sibling"), with D and P in the direct path of a leaf node L (for "leaf"):¶

         ...
         /
        P
      __|__
     /     \
    D       S
   / \     / \
 ... ... ... ...
 /
L

The parent hash of P changes whenever an UpdatePath object is applied to the ratchet tree along a path from a leaf L traversing node D (and hence also P). The new "Parent hash of P (with copath child S)" is obtained by hashing P's ParentHashInput struct.¶

struct {
    HPKEPublicKey encryption_key;
    opaque parent_hash<V>;
    opaque original_sibling_tree_hash<V>;
} ParentHashInput;

The field encryption_key contains the HPKE public key of P. If P is the root, then the parent_hash field is set to a zero-length octet string. Otherwise, parent_hash is the Parent Hash of the next node after P on the filtered direct path of the leaf L. This way, P's Parent Hash fixes the new HPKE public key of each non-blank node on the path from P to the root. Note that the path from P to the root may contain some blank nodes that are not fixed by P's Parent Hash. However, for each node that has an HPKE key, this key is fixed by P's Parent Hash.¶

Finally, original_sibling_tree_hash is the tree hash of S in the ratchet tree modified as follows: For each leaf L in P.unmerged_leaves, blank L and remove it from the unmerged_leaves sets of all parent nodes.¶

Observe that original_sibling_tree_hash does not change between updates of P. This property is crucial for the correctness of the protocol.¶

Note that original_sibling_tree_hash is the tree hash of S, not the parent hash. The parent_hash field in ParentHashInput captures information about the nodes above P. the original_sibling_tree_hash captures information about the subtree under S that is not being updated (and thus the subtree to which a path secret for P would be encrypted according to Section 7.5).¶

For example, in the following tree:¶

              W [F]
        ______|_____
       /             \
      U               Y [F]
    __|__           __|__
   /     \         /     \
  T       _       _       _
 / \     / \     / \     / \
A   B   C   D   E   F   G   _

With P = W and S = Y, original_sibling_tree_hash is the tree hash of the following tree:¶

      Y
    __|__
   /     \
  _       _
 / \     / \
E   _   G   _

Because W.unmerged_leaves includes F, F is blanked and removed from Y.unmerged_leaves.¶

Note that no recomputation is needed if the tree hash of S is unchanged since the last time P was updated. This is the case for computing or processing a Commit whose UpdatePath traverses P, since the Commit itself resets P. (In other words, it is only necessary to recompute the original sibling tree hash when validating a group's tree on joining.) More generally, if none of the entries in P.unmerged_leaves is in the subtree under S (and thus no leaves were blanked), then the original tree hash at S is the tree hash of S in the current tree.¶

If it is necessary to recompute the original tree hash of a node, the efficiency of recomputation can be improved by caching intermediate tree hashes, to avoid recomputing over the subtree when the subtree is included in multiple parent hashes. A subtree hash can be reused as long as the intersection of the parent's unmerged leaves with the subtree is the same as in the earlier computation.¶

8.1. Group Context

Each member of the group maintains a GroupContext object that summarizes the state of the group:¶

struct {
    ProtocolVersion version = mls10;
    CipherSuite cipher_suite;
    opaque group_id<V>;
    uint64 epoch;
    opaque tree_hash<V>;
    opaque confirmed_transcript_hash<V>;
    Extension extensions<V>;
} GroupContext;

The fields in this state have the following semantics:¶

• The cipher_suite is the cipher suite used by the group.¶
• The group_id field is an application-defined identifier for the group.¶
• The epoch field represents the current version of the group.¶
• The tree_hash field contains a commitment to the contents of the group's ratchet tree and the credentials for the members of the group, as described in Section 7.8.¶
• The confirmed_transcript_hash field contains a running hash over the messages that led to this state.¶
• The extensions field contains the details of any protocol extensions that apply to the group.¶

When a new member is added to the group, an existing member of the group provides the new member with a Welcome message. The Welcome message provides the information the new member needs to initialize its GroupContext.¶

Different changes to the group will have different effects on the group state. These effects are described in their respective subsections of Section 12.1. The following general rules apply:¶

• The group_id field is constant.¶
• The epoch field increments by one for each Commit message that is processed.¶
• The tree_hash is updated to represent the current tree and credentials.¶
• The confirmed_transcript_hash field is updated with the data for an AuthenticatedContent encoding a Commit message as described below.¶
• The extensions field changes when a GroupContextExtensions proposal is committed.¶

8.2. Transcript Hashes

The transcript hashes computed in MLS represent a running hash over all Proposal and Commit messages that have ever been sent in a group. Commit messages are included directly. Proposal messages are indirectly included via the Commit that applied them. Both types of message are included by hashing the AuthenticatedContent object in which they were sent.¶

The transcript hash comprises two individual hashes:¶

• A confirmed_transcript_hash that represents a transcript over the whole history of Commit messages, up to and including the signature of the most recent Commit.¶
• An interim_transcript_hash that covers the confirmed transcript hash plus the confirmation_tag of the most recent Commit.¶

New members compute the interim transcript hash using the confirmation_tag field of the GroupInfo struct, while existing members can compute it directly.¶

Each Commit message updates these hashes by way of its enclosing AuthenticatedContent. The AuthenticatedContent struct is split into ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The former is used to update the confirmed transcript hash and the latter to update the interim transcript hash.¶

struct {
    WireFormat wire_format;
    FramedContent content; /* with content_type == commit */
    opaque signature<V>;
} ConfirmedTranscriptHashInput;

struct {
    MAC confirmation_tag;
} InterimTranscriptHashInput;

confirmed_transcript_hash_[0] = ""; /* zero-length octet string */
interim_transcript_hash_[0] = ""; /* zero-length octet string */

confirmed_transcript_hash_[epoch] =
    Hash(interim_transcript_hash_[epoch - 1] ||
        ConfirmedTranscriptHashInput_[epoch]);

interim_transcript_hash_[epoch] =
    Hash(confirmed_transcript_hash_[epoch] ||
        InterimTranscriptHashInput_[epoch]);

In this notation, ConfirmedTranscriptHashInput_[epoch] and InterimTranscriptHashInput_[epoch] are based on the Commit that initiated the epoch with epoch number epoch. (Note that the epoch field in this Commit will be set to epoch - 1`, since it is sent within the previous epoch.)¶

The transcript hash ConfirmedTranscriptHashInput_[epoch] is used as the confirmed_transcript_hash input to the confirmation_tag field for this Commit. Each Commit thus confirms the whole transcript of Commits up to that point, except for the latest Commit's confirmation tag.¶

8.3. External Initialization

In addition to initializing a new epoch via KDF invocations as described above, an MLS group can also initialize a new epoch via an asymmetric interaction using the external key pair for the previous epoch. This is done when a new member is joining via an external commit.¶

In this process, the joiner sends a new init_secret value to the group using the HPKE export method. The joiner then uses that init_secret with information provided in the GroupInfo and an external Commit to initialize their copy of the key schedule for the new epoch.¶

kem_output, context = SetupBaseS(external_pub, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)

Members of the group receive the kem_output in an ExternalInit proposal and perform the corresponding calculation to retrieve the init_secret value.¶

context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)

In both cases, the info input to HPKE is set to the GroupInfo for the previous epoch, encoded using the TLS serialization.¶

8.4. Pre-Shared Keys

Groups that already have an out-of-band mechanism to generate shared group secrets can inject those into the MLS key schedule to seed the MLS group secrets computations by this external entropy.¶

Injecting an external PSK can improve security in the case where having a full run of Updates across members is too expensive, or if the external group key establishment mechanism provides stronger security against classical or quantum adversaries.¶

Note that, as a PSK may have a different lifetime than an Update, it does not necessarily provide the same Forward Secrecy (FS) or Post-Compromise Security (PCS) guarantees as a Commit message. Unlike the key pairs populated in the tree by an Update or Commit, which are always freshly generated, PSKs may be pre-distributed and stored. This creates the risk that a PSK may be compromised in the process of distribution and storage. The security that the group gets from injecting a PSK thus depends on both the entropy of the PSK and the risk of compromise. These factors are outside of the scope of this document, but should be considered by application designers relying on PSKs.¶

Each PSK in MLS has a type that designates how it was provisioned. External PSKs are provided by the application, while resumption PSKs are derived from the MLS key schedule and used in cases where it is necessary to authenticate a member's participation in a prior epoch.¶

The injection of one or more PSKs into the key schedule is signaled in two ways: Existing members are informed via PreSharedKey proposals covered by a Commit, and new members added in the Commit are informed by the GroupSecrets object in the Welcome message corresponding to the Commit. To ensure that existing and new members compute the same PSK input to the key schedule, the Commit and GroupSecrets objects MUST indicate the same set of PSKs, in the same order.¶

enum {
  reserved(0),
  external(1),
  resumption(2),
  (255)
} PSKType;

enum {
  reserved(0),
  application(1),
  reinit(2),
  branch(3),
  (255)
} ResumptionPSKUsage;

struct {
  PSKType psktype;
  select (PreSharedKeyID.psktype) {
    case external:
      opaque psk_id<V>;

    case resumption:
      ResumptionPSKUsage usage;
      opaque psk_group_id<V>;
      uint64 psk_epoch;
  };
  opaque psk_nonce<V>;
} PreSharedKeyID;

Each time a client injects a PSK into a group, the psk_nonce of its PreSharedKeyID MUST be set to a fresh random value of length KDF.Nh, where KDF is the KDF for the ciphersuite of the group into which the PSK is being injected. This ensures that even when a PSK is used multiple times, the value used as an input into the key schedule is different each time.¶

Upon receiving a Commit with a PreSharedKey proposal or a GroupSecrets object with the psks field set, the receiving Client includes them in the key schedule in the order listed in the Commit, or in the psks field respectively. For resumption PSKs, the PSK is defined as the resumption_psk of the group and epoch specified in the PreSharedKeyID object. Specifically, psk_secret is computed as follows:¶

struct {
    PreSharedKeyID id;
    uint16 index;
    uint16 count;
} PSKLabel;

psk_extracted_[i] = KDF.Extract(0, psk_[i])
psk_input_[i] = ExpandWithLabel(psk_extracted_[i], "derived psk",
                  PSKLabel, KDF.Nh)

psk_secret_[0] = 0
psk_secret_[i] = KDF.Extract(psk_input_[i-1], psk_secret_[i-1])
psk_secret     = psk_secret_[n]

Here 0 represents the all-zero vector of length KDF.Nh. The index field in PSKLabel corresponds to the index of the PSK in the psk array, while the count field contains the total number of PSKs. In other words, the PSKs are chained together with KDF.Extract invocations (labeled "Extract" for brevity in the diagram), as follows:¶

In particular, if there are no PreSharedKey proposals in a given Commit, then the resulting psk_secret is psk_secret_[0], the all-zero vector.¶

8.5. Exporters

The main MLS key schedule provides an exporter_secret which can be used by an application to derive new secrets for use outside of MLS.¶

MLS-Exporter(Label, Context, Length) =
       ExpandWithLabel(DeriveSecret(exporter_secret, Label),
                         "exported", Hash(Context), Length)

Applications SHOULD provide a unique label to MLS-Exporter that identifies the secret's intended purpose. This is to help prevent the same secret from being generated and used in two different places. To help avoid the same label being used in different applications, an IANA registry for these labels has been defined in Section 17.8.¶

The exported values are bound to the group epoch from which the exporter_secret is derived, and hence reflect a particular state of the group.¶

It is RECOMMENDED for the application generating exported values to refresh those values after a Commit is processed.¶

8.6. Resumption PSK

The main MLS key schedule provides a resumption_psk that is used as a PSK to inject entropy from one epoch into another. This functionality is used in the reinitialization and branching processes described in Section 11.2 and Section 11.3, but may be used by applications for other purposes.¶

Some uses of resumption PSKs might call for the use of PSKs from historical epochs. The application SHOULD specify an upper limit on the number of past epochs for which the resumption_psk may be stored.¶

8.7. Epoch Authenticators

The main MLS key schedule provides a per-epoch epoch_authenticator. If one member of the group is being impersonated by an active attacker, the epoch_authenticator computed by their client will differ from those computed by the other group members.¶

This property can be used to construct defenses against impersonation attacks that are effective even if members' signature keys are compromised. As a trivial example, if the users of the clients in an MLS group were to meet in person and reliably confirm that their epoch authenticator values were equal (using some suitable user interface), then each user would be assured that the others were not being impersonated in the current epoch. As soon as the epoch changed, though, they would need to re-do this confirmation. The state of the group would have changed, possibly introducing an attacker.¶

More generally, in order for the members of an MLS group to obtain concrete authentication protections using the epoch_authenticator, they will need to use it in some secondary protocol (such as the face-to-face protocol above). The details of that protocol will then determine the specific authentication protections provided to the MLS group.¶

9.1. Encryption Keys

As described in Section 6, MLS encrypts three different types of information:¶

• Metadata (sender information)¶
• Handshake messages (Proposal and Commit)¶
• Application messages¶

The sender information used to look up the key for content encryption is encrypted with an AEAD where the key and nonce are derived from both sender_data_secret and a sample of the encrypted message content.¶

For handshake and application messages, a sequence of keys is derived via a "sender ratchet". Each sender has their own sender ratchet, and each step along the ratchet is called a "generation".¶

The following figure shows a secret tree for a four-member group, with the handshake and application ratchets that member D will use for sending and the first two application keys and nonces.¶

A sender ratchet starts from a per-sender base secret derived from a Secret Tree, as described in Section 9. The base secret initiates a symmetric hash ratchet which generates a sequence of keys and nonces. The sender uses the j-th key/nonce pair in the sequence to encrypt (using the AEAD) the j-th message they send during that epoch. Each key/nonce pair MUST NOT be used to encrypt more than one message.¶

Keys, nonces, and the secrets in ratchets are derived using DeriveTreeSecret. The context in a given call consists of the current position in the ratchet.¶

DeriveTreeSecret(Secret, Label, Generation, Length) =
    ExpandWithLabel(Secret, Label, Generation, Length)

Where Generation is encoded as a big endian uint32.¶

Here, AEAD.Nn and AEAD.Nk denote the lengths in bytes of the nonce and key for the AEAD scheme defined by the ciphersuite.¶

9.2. Deletion Schedule

It is important to delete all security-sensitive values as soon as they are consumed. A sensitive value S is said to be consumed if¶

• S was used to encrypt or (successfully) decrypt a message, or if¶
• a key, nonce, or secret derived from S has been consumed. (This goes for values derived via DeriveSecret as well as ExpandWithLabel.)¶

Here, S may be the init_secret, commit_secret, epoch_secret, encryption_secret as well as any secret in a Secret Tree or one of the ratchets.¶

As soon as a group member consumes a value they MUST immediately delete (all representations of) that value. This is crucial to ensuring forward secrecy for past messages. Members MAY keep unconsumed values around for some reasonable amount of time to handle out-of-order message delivery.¶

For example, suppose a group member encrypts or (successfully) decrypts an application message using the j-th key and nonce in the ratchet of leaf node L in some epoch n. Then, for that member, at least the following values have been consumed and MUST be deleted:¶

• the commit_secret, joiner_secret, epoch_secret, encryption_secret of that epoch n as well as the init_secret of the previous epoch n-1,¶
• all node secrets in the Secret Tree on the path from the root to the leaf with node L,¶
• the first j secrets in the application data ratchet of node L and¶
• application_ratchet_nonce_[L]_[j] and application_ratchet_key_[L]_[j].¶

Concretely, consider the Secret Tree shown in Figure 27. Client A, B, or C would generate the illustrated values on receiving a message from D with generation equal to 1, having not received a message with generation 0 (e.g., due to out-of-order delivery). In such a case, the following values would be consumed:¶

• The key K1 and nonce N1 used to decrypt the message¶
• The application ratchet secrets AR1 and AR0¶
• The tree secrets D, F, G (recall that G is the encryption_secret for the epoch)¶
• The epoch_secret, commit_secret, psk_secret, and joiner_secret for the current epoch¶

Other values may be retained (not consumed):¶

• K0 and N0 for decryption of an out-of-order message with generation 0¶
• AR2 for derivation of further message decryption keys and nonces¶
• HR0 for protection of handshake messages from D¶
• E and C for deriving secrets used by senders A, B, and C¶

10.1. KeyPackage Validation

The validity of a KeyPackage needs to be verified at a few stages:¶

• When a KeyPackage is downloaded by a group member, before it is used to add the client to the group¶
• When a KeyPackage is received by a group member in an Add message¶

The client verifies the validity of a KeyPackage using the following steps:¶

• Verify that the ciphersuite and protocol version of the KeyPackage match those in the GroupContext.¶
• Verify that the leaf_node of the KeyPackage is valid for a KeyPackage according to Section 7.3.¶
• Verify that the signature on the KeyPackage is valid using the public key in leaf_node.credential.¶
• Verify that the value of leaf_node.encryption_key is different from the value of the init_key field.¶

11.1. Required Capabilities

The configuration of a group imposes certain requirements on clients in the group. At a minimum, all members of the group need to support the ciphersuite and protocol version in use. Additional requirements can be imposed by including a required_capabilities extension in the GroupContext.¶

struct {
    ExtensionType extension_types<V>;
    ProposalType proposal_types<V>;
    CredentialType credential_types<V>;
} RequiredCapabilities;

This extension lists the extensions, proposals, and credential types that must be supported by all members of the group. The "default" proposal and extension types defined in this document are assumed to be implemented by all clients, and need not be listed in RequiredCapabilities in order to be safely used. Note that this is not true for credential types.¶

For new members, support for required capabilities is enforced by existing members during the application of Add commits. Existing members should of course be in compliance already. In order to ensure this continues to be the case even as the group's extensions are updated, a GroupContextExtensions proposal is deemed invalid if it contains a required_capabilities extension that requires non-default capabilities not supported by all current members.¶

11.2. Reinitialization

A group may be reinitialized by creating a new group with the same membership and different parameters, and linking it to the old group via a resumption PSK. The members of a group reinitialize it using the following steps:¶

1. A member of the old group sends a ReInit proposal (see Section 12.1.5)¶
2. A member of the old group sends a Commit covering the ReInit proposal¶

3. A member of the old group creates an initial Commit setting up a new group that matches the ReInit and sends a Welcome message¶

   • The version, cipher_suite, group_id, and extensions fields of the GroupContext object in the Welcome message MUST be the same as the corresponding fields in the ReInit proposal. The epoch in the Welcome message MUST be 1.¶
   • The Welcome MUST specify a PreSharedKeyID of type resumption with usage reinit, where the group_id field matches the old group and the epoch field indicates the epoch after the Commit covering the ReInit.¶

Note that these three steps may be done by the same group member or different members. For example, if a group member sends a Commit with an inline ReInit proposal (steps 1 and 2) but then goes offline, another group member may recreate the group instead. This flexibility avoids situations where a group gets stuck between steps 2 and 3.¶

Resumption PSKs with usage reinit MUST NOT be used in other contexts. A PreSharedKey proposal with type resumption and usage reinit MUST be considered invalid.¶

11.3. Subgroup Branching

A new group can be formed from a subset of an existing group's members, using the same parameters as the old group.¶

A member can create a subgroup by performing the following steps:¶

1. Fetch a new KeyPackage for each group member that should be included in the subgroup.¶
2. Create an initial Commit message that sets up the new group and contains a PreSharedKey proposal of type resumption with usage branch. To avoid key re-use, the psk_nonce included in the PreSharedKeyID object MUST be a randomly sampled nonce of length KDF.Nh.¶
3. Send the corresponding Welcome message to the subgroup members.¶

A client receiving a Welcome including a PreSharedKey of type resumption with usage branch MUST verify that the new group reflects a subgroup branched from the referenced group by checking:¶

• The version and ciphersuite values in the Welcome are the same as those used by the old group.¶
• The epoch in the Welcome message MUST be 1.¶
• Each LeafNode in a new subgroup MUST match some LeafNode in the original group. In this context, a pair of LeafNodes is said to "match" if the identifiers presented by their respective credentials are considered equivalent by the application.¶

Resumption PSKs with usage branch MUST NOT be used in other contexts. A PreSharedKey proposal with type resumption and usage branch MUST be considered invalid.¶

12.1. Proposals

Proposals are included in a FramedContent by way of a Proposal structure that indicates their type:¶

// See IANA registry for registered values
uint16 ProposalType;

struct {
    ProposalType proposal_type;
    select (Proposal.proposal_type) {
        case add:                      Add;
        case update:                   Update;
        case remove:                   Remove;
        case psk:                      PreSharedKey;
        case reinit:                   ReInit;
        case external_init:            ExternalInit;
        case group_context_extensions: GroupContextExtensions;
    };
} Proposal;

On receiving a FramedContent containing a Proposal, a client MUST verify the signature inside FramedContentAuthData and that the epoch field of the enclosing FramedContent is equal to the epoch field of the current GroupContext object. If the verification is successful, then the Proposal should be cached in such a way that it can be retrieved by hash (as a ProposalOrRef object) in a later Commit message.¶

struct {
    KeyPackage key_package;
} Add;

struct {
    LeafNode leaf_node;
} Update;

struct {
    uint32 removed;
} Remove;

struct {
    PreSharedKeyID psk;
} PreSharedKey;

struct {
    opaque group_id<V>;
    ProtocolVersion version;
    CipherSuite cipher_suite;
    Extension extensions<V>;
} ReInit;

struct {
  opaque kem_output<V>;
} ExternalInit;

struct {
  Extension extensions<V>;
} GroupContextExtensions;

struct {
  SignaturePublicKey signature_key;
  Credential credential;
} ExternalSender;

ExternalSender external_senders<V>;

12.2. Proposal List Validation

A group member creating a commit and a group member processing a Commit MUST verify that the list of committed proposals is valid using one of the following procedures, depending on whether the commit is external or not. If the list of proposals is invalid, then the Commit message MUST be rejected as invalid.¶

For a regular, i.e. not external, commit the list is invalid if any of the following occurs:¶

• It contains an individual proposal that is invalid as specified in Section 12.1.¶
• It contains an Update proposal generated by the committer.¶
• It contains a Remove proposal that removes the committer.¶
• It contains multiple Update and/or Remove proposals that apply to the same leaf. If the committer has received multiple such proposals they SHOULD prefer any Remove received, or the most recent Update if there are no Removes.¶
• It contains multiple Add proposals that contain KeyPackages that represent the same client according to the application (for example, identical signature keys).¶
• It contains an Add proposal with a KeyPackage that represents a client already in the group according to the application, unless there is a Remove proposal in the list removing the matching client from the group.¶
• It contains multiple PreSharedKey proposals that reference the same PreSharedKeyID.¶
• It contains multiple GroupContextExtensions proposals.¶
• It contains a ReInit proposal together with any other proposal. If the committer has received other proposals during the epoch, they SHOULD prefer them over the ReInit proposal, allowing the ReInit to be resent and applied in a subsequent epoch.¶
• It contains an ExternalInit proposal.¶
• It contains a proposal with a non-default proposal type that is not supported by some members of the group that will process the Commit (i.e., members being added or removed by the Commit do not need to support the proposal type).¶
• After processing the commit the ratchet tree is invalid, in particular, if it contains any leaf node that is invalid according to Section 7.3.¶

An application may extend the above procedure by additional rules, for example, requiring application-level permissions to add members, or rules concerning non-default proposal types.¶

For an external commit, the list is valid if it contains only the following proposals (not necessarily in this order):¶

• Exactly one ExternalInit¶
• At most one Remove proposal, with which the joiner removes an old version of themselves. If a Remove proposal is present, then the LeafNode in the path field of the external commit MUST meet the same criteria as would the LeafNode in an Update for the removed leaf (see Section 12.1.2). In particular, the credential in the LeafNode MUST present a set of identifiers that is acceptable to the application for the removed participant.¶
• Zero or more PreSharedKey proposals.¶
• No other proposals.¶

Proposal types defined in the future may make updates to the above validation logic to incorporate considerations related to proposals of the new type.¶

12.3. Applying a Proposal List

The sections above defining each proposal type describe how each individual proposal is applied. When creating or processing a Commit, a client applies a list of proposals to the ratchet tree and GroupContext. The client MUST apply the proposals in the list in the following order:¶

• If there is a GroupContextExtensions proposal, replace the extensions field of the GroupContext for the group with the contents of the proposal. The new extensions MUST be used for evaluating other proposals in this list. For example, if a GroupContextExtensions proposal adds a required_capabilities extension, then any Add proposals need to indicate support for those capabilities.¶
• Apply any Update proposals to the ratchet tree, in any order.¶
• Apply any Remove proposals to the ratchet tree, in any order.¶
• Apply any Add proposals to the ratchet tree, in the order they appear in the list.¶
• Look up the PSK secrets for any PreSharedKey proposals, in the order they appear in the list. These secrets are then used to advance the key schedule later in Commit processing.¶
• If there is an ExternalInit proposal, use it to derive the init_secret for use later in Commit processing.¶
• If there is a ReInit proposal, note its parameters for application later in Commit processing.¶

Proposal types defined in the future MUST specify how the above steps are to be adjusted to accommodate the application of proposals of the new type.¶

12.4. Commit

A Commit message initiates a new epoch for the group, based on a collection of Proposals. It instructs group members to update their representation of the state of the group by applying the proposals and advancing the key schedule.¶

Each proposal covered by the Commit is included by a ProposalOrRef value, which identifies the proposal to be applied by value or by reference. Commits that refer to new Proposals from the committer can be included by value. Commits for previously sent proposals from anyone (including the committer) can be sent by reference. Proposals sent by reference are specified by including the hash of the AuthenticatedContent object in which the proposal was sent (see Section 5.2).¶

enum {
  reserved(0),
  proposal(1),
  reference(2),
  (255)
} ProposalOrRefType;

struct {
  ProposalOrRefType type;
  select (ProposalOrRef.type) {
    case proposal:  Proposal proposal;
    case reference: ProposalRef reference;
  };
} ProposalOrRef;

struct {
    ProposalOrRef proposals<V>;
    optional<UpdatePath> path;
} Commit;

A group member that has observed one or more valid proposals within an epoch MUST send a Commit message before sending application data. This ensures, for example, that any members whose removal was proposed during the epoch are actually removed before any application data is transmitted.¶

A sender and a receiver of a Commit MUST verify that the committed list of proposals is valid as specified in Section 12.2. A list is invalid if, for example, it includes an Update and a Remove for the same member, or an Add when the sender does not have the application-level permission to add new users.¶

The sender of a Commit SHOULD include all proposals that it has received during the current epoch, that are valid according to the rules for their proposal types and according to application policy, as long as this results in a valid proposal list.¶

Due to the asynchronous nature of proposals, receivers of a Commit SHOULD NOT enforce that all valid proposals sent within the current epoch are referenced by the next Commit. In the event that a valid proposal is omitted from the next Commit, and that proposal is still valid in the current epoch, the sender of the proposal MAY resend it after updating it to reflect the current epoch.¶

A member of the group MAY send a Commit that references no proposals at all, which would thus have an empty proposals vector. Such a Commit resets the sender's leaf and the nodes along its direct path, and provides forward secrecy and post-compromise security with regard to the sender of the Commit. An Update proposal can be regarded as a "lazy" version of this operation, where only the leaf changes and intermediate nodes are blanked out.¶

By default, the path field of a Commit MUST be populated. The path field MAY be omitted if (a) it covers at least one proposal and (b) none of the proposals covered by the Commit are of "path required" types. A proposal type requires a path if it cannot change the group membership in a way that requires the forward secrecy and post-compromise security guarantees that an UpdatePath provides. The only proposal types defined in this document that do not require a path are:¶

• add¶
• psk¶
• reinit¶

New proposal types MUST state whether they require a path. If any instance of a proposal type requires a path, then the proposal type requires a path. This attribute of a proposal type is reflected in the "Path Required" field of the proposal type registry defined in Section 17.4.¶

Update and Remove proposals are the clearest examples of proposals that require a path. An UpdatePath is required to evict the removed member or the old appearance of the updated member.¶

In pseudocode, the logic for validating the path field of a Commit is as follows:¶

pathRequiredTypes = [
    update,
    remove,
    external_init,
    group_context_extensions
]

pathRequired = false

for proposal in commit.proposals:
    pathRequired = pathRequired ||
                   (proposal.msg_type in pathRequiredTypes)

if len(commit.proposals) == 0 || pathRequired:
    assert(commit.path != null)

To summarize, a Commit can have three different configurations, with different uses:¶

1. An "empty" Commit that references no proposals, which updates the committer's contribution to the group and provides PCS with regard to the committer.¶
2. A "partial" Commit that references proposals that do not require a path, and where the path is empty. Such a commit doesn't provide PCS with regard to the committer.¶
3. A "full" Commit that references proposals of any type, which provides FS with regard to any removed members and PCS for the committer and any updated members.¶

struct {
    GroupContext group_context;
    Extension extensions<V>;
    MAC confirmation_tag;
    uint32 signer;
    /* SignWithLabel(., "GroupInfoTBS", GroupInfoTBS) */
    opaque signature<V>;
} GroupInfo;

struct {
    GroupContext group_context;
    Extension extensions<V>;
    MAC confirmation_tag;
    uint32 signer;
} GroupInfoTBS;

struct {
  opaque path_secret<V>;
} PathSecret;

struct {
  opaque joiner_secret<V>;
  optional<PathSecret> path_secret;
  PreSharedKeyID psks<V>;
} GroupSecrets;

struct {
  KeyPackageRef new_member;
  HPKECiphertext encrypted_group_secrets;
} EncryptedGroupSecrets;

struct {
  CipherSuite cipher_suite;
  EncryptedGroupSecrets secrets<V>;
  opaque encrypted_group_info<V>;
} Welcome;

encrypted_group_secrets = EncryptWithLabel(init_key, "Welcome",
                                           encrypted_group_info, group_secrets)

group_secrets = DecryptWithLabel(init_key_priv, "Welcome",
                                 encrypted_group_info, kem_output, ciphertext)

welcome_nonce = ExpandWithLabel(welcome_secret, "nonce", "", AEAD.Nn)
welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)

struct {
    HPKEPublicKey external_pub;
} ExternalPub;

struct {
    NodeType node_type;
    select (Node.node_type) {
        case leaf:   LeafNode leaf_node;
        case parent: ParentNode parent_node;
    };
} Node;

optional<Node> ratchet_tree<V>;

# Assuming a class Node that has left and right members
def subtree_root(nodes):
    # If there is only one node in the array return it
    if len(nodes) == 1:
        return Node(nodes[0])

    # Otherwise, the length of the array MUST be odd
    if len(nodes) % 2 == 0:
        raise Exception("Malformed node array {}", len(nodes))

    # Identify the root of the subtree
    d = 0
    while (2**(d+1)) < len(nodes):
       d += 1
    R = 2**d - 1
    root = Node(nodes[R])
    root.left = subtree_root(nodes[:R])
    root.right = subtree_root(nodes[(R+1):])
    return root

13.1. Additional Ciphersuites

As discussed in Section 5.1, MLS allows the participants in a group to negotiate the cryptographic algorithms used within the group. This extensibility is important for maintaining the security of the protocol over time [RFC7696]. It also creates a risk of interoperability failure due to clients not supporting a common ciphersuite.¶

The ciphersuite registry defined in Section 17.1 attempts to strike a balance on this point. On the one hand, the base policy for the registry is Specification Required, a fairly low bar designed to avoid the need for standards work in cases where different ciphers are needed for niche applications. There is a higher bar (Standards Action) for ciphers to set the Recommended field in the registry. This higher bar is there in part to ensure that the interoperability implications of new ciphersuites are considered.¶

MLS ciphersuites are defined independent of MLS versions, so that in principle the same ciphersuite can be used across versions. Standards work defining new versions of MLS should consider whether it is desirable for the new version to be compatible with existing ciphersuites, or whether the new version should rule out some ciphersuites. For example, a new version could follow the example of HTTP/2, which restricted the set of allowed TLS ciphers (see Section 9.2.2 of [RFC9113].¶

13.2. Proposals

Commit messages do not have an extension field because the set of proposals is extensible. As discussed in Section 12.4, Proposals with a non-default proposal type MUST NOT be included in a commit unless the proposal type is supported by all the members of the group that will process the Commit.¶

13.3. Credential Extensibility

In order to ensure that MLS provides meaningful authentication it is important that each member is able to authenticate some identity information for each other member. Identity information is encoded in Credentials, so this property is provided by ensuring that members use compatible credential types.¶

The types of credential that may be used in a group is restricted to what all members of the group support, as specified by the capabilities field of each LeafNode in the ratchet tree. An application can introduce new credential types by choosing an unallocated identifier from the registry in Section 17.5 and indicating support for the credential type in published LeafNodes, whether in Update proposals to existing groups or KeyPackages that are added to new groups. Once all members in a group indicate support for the credential type, members can start using LeafNodes with the new credential. Application may enforce that certain credential types always remain supported by adding a required_capabilities extension to the group's GroupContext, which would prevent any member from being added to the group that doesn't support them.¶

In future extensions to MLS, it may be useful to allow a member to present more than one credential. For example, such credentials might present different attributes attested by different authorities. To be consistent with the general principle stated at the beginning of this section, such an extension would need to ensure that each member can authenticate some identity for each other member. For each pair of members (Alice, Bob), Alice would need to present at least one credential of a type that Bob supports.¶

13.4. Extensions

This protocol includes a mechanism for negotiating extension parameters similar to the one in TLS [RFC8446]. In TLS, extension negotiation is one-to-one: The client offers extensions in its ClientHello message, and the server expresses its choices for the session with extensions in its ServerHello and EncryptedExtensions messages. In MLS, extensions appear in the following places:¶

• In KeyPackages, to describe additional information related to the client¶
• In LeafNodes, to describe additional information about the client or its participation in the group (once in the ratchet tree)¶
• In the GroupInfo, to tell new members of a group what parameters are being used by the group, and to provide any additional details required to join the group¶
• In the GroupContext object, to ensure that all members of the group have the same view of the parameters in use¶

In other words, an application can use GroupContext extensions to ensure that all members of the group agree on a set of parameters. Clients indicate their support for parameters in the capabilities field of their LeafNode. New members of a group are informed of the group's GroupContext extensions via the extensions field in the group_context field of the GroupInfo object. The extensions field in a GroupInfo object (outside of the group_context field) can be used to provide additional parameters to new joiners that are used to join the group.¶

This extension mechanism is designed to allow for the secure and forward-compatible negotiation of extensions. For this to work, implementations MUST correctly handle extensible fields:¶

• A client that posts a KeyPackage MUST support all parameters advertised in it. Otherwise, another client might fail to interoperate by selecting one of those parameters.¶
• A client processing a KeyPackage object MUST ignore all unrecognized values in the capabilities field of the LeafNode, and all unknown extensions in the extensions and leaf_node.extensions fields. Otherwise, it could fail to interoperate with newer clients.¶
• A client processing a GroupInfo object MUST ignore all unrecognized extensions in the extensions field.¶
• Any field containing a list of extensions MUST NOT have more than one extension of any given type.¶
• A client adding a new member to a group MUST verify that the LeafNode for the new member is compatible with the group's extensions. The capabilities field MUST indicate support for each extension in the GroupContext.¶
• A client joining a group MUST verify that it supports every extension in the GroupContext for the group. Otherwise, it MUST treat the enclosing GroupInfo message as invalid and not join the group.¶

Note that the latter two requirements mean that all MLS GroupContext extensions are mandatory, in the sense that an extension in use by the group MUST be supported by all members of the group.¶

The parameters of a group may be changed by sending a GroupContextExtensions proposal to enable additional extensions (Section 12.1.7), or by reinitializing the group (Section 11.2).¶

13.5. GREASE

As described in Section 13.4, clients are required to ignore unknown values for certain parameters. To help ensure that other clients implement this behavior, a client can follow the “Generate Random Extensions And Sustain Extensibility” or GREASE approach described in [RFC8701]. In the context of MLS, this means that a client generating a KeyPackage, LeafNode, or GroupInfo object includes random values in certain fields which would be ignored by a correctly-implemented client processing the message. A client that incorrectly rejects unknown code points will fail to process such a message, providing a signal to its implementer that the client needs to be fixed.¶

When generating the following fields, an MLS client SHOULD include a random selection of values chosen from these GREASE values:¶

• LeafNode.capabilities.ciphersuites¶
• LeafNode.capabilities.extensions¶
• LeafNode.capabilities.proposals¶
• LeafNode.capabilities.credentials¶
• LeafNode.extensions¶
• KeyPackage.extensions¶
• GroupInfo.extensions¶

For the KeyPackage and GroupInfo extensions, the extension_data for GREASE extensions MAY have any contents selected by the sender, since they will be ignored by a correctly-implemented receiver. For example, a sender might populate these extensions with a randomly-sized amount of random data.¶

GREASE values MUST NOT be sent in the following fields, because an unsupported value in one these fields (including a GREASE value), will cause the enclosing message to be rejected:¶

• Proposal.proposal_type¶
• Credential.credential_type¶
• GroupContext.extensions¶
• GroupContextExtensions.extensions¶

A set of values reserved for GREASE have been registered in the various registries in Section 17. This prevents conflict between GREASE and real future values. The following values are reserved in each registry: 0x0A0A, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A, 0x7A7A, 0x8A8A, 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, and 0xEAEA. (The value 0xFAFA falls within the private use range.) These values MUST only appear in the fields listed above, and not, for example, in the proposal_type field of a Proposal. Clients MUST NOT implement any special processing rules for how to handle these values when receiving them, since this negates their utility for detecting extensibility failures.¶

GREASE values MUST be handled using normal logic for processing unsupported values. When comparing lists of capabilities to identify mutually-supported capabilities, clients MUST represent their own capabilities with a list containing only the capabilities actually supported, without any GREASE values. In other words, lists including GREASE values are only sent to other clients; representations of a client's own capabilities MUST NOT contain GREASE values.¶

15.1. Padding

Application messages MAY be padded to provide some resistance against traffic analysis techniques over encrypted traffic [CLINIC] [HCJ16]. While MLS might deliver the same payload less frequently across a lot of ciphertexts than traditional web servers, it might still provide the attacker enough information to mount an attack. If Alice asks Bob "When are we going to the movie?", then the answer "Wednesday" could be leaked to an adversary solely by the ciphertext length.¶

The length of the padding field in PrivateMessageContent can be chosen by the sender at the time of message encryption. Senders may use padding to reduce the ability of attackers outside the group to infer the size of the encrypted content. Note, however, that the transports used to carry MLS messages may have maximum message sizes, so padding schemes SHOULD avoid increasing message size beyond any such limits that exist in a given deployment scenario.¶

15.2. Restrictions

During each epoch, senders MUST NOT encrypt more data than permitted by the security bounds of the AEAD scheme used [I-D.irtf-cfrg-aead-limits].¶

Note that each change to the group through a handshake message will also set a new encryption_secret. Hence this change MUST be applied before encrypting any new application message. This is required both to ensure that any users removed from the group can no longer receive messages and to (potentially) recover confidentiality and authenticity for future messages despite a past state compromise.¶

15.3. Delayed and Reordered Application messages

Since each application message contains the group identifier, the epoch, and a generation counter, a client can receive messages out of order. When messages are received out of order, the client moves the sender ratchet forward to match the received generation counter. Any unused nonce and key pairs from the ratchet are potentially stored so that they can be used to decrypt the messages which were delayed or reordered.¶

Applications SHOULD define a policy on how long to keep unused nonce and key pairs for a sender, and the maximum number to keep. This is in addition to ensuring that these secrets are deleted according to the deletion schedule defined in Section 9.2. Applications SHOULD also define a policy limiting the maximum number of steps that clients will move the ratchet forward in response to a new message. Messages received with a generation counter that's too much higher than the last message received would then be rejected. This avoids causing a denial-of-service attack by requiring the recipient to perform an excessive number of key derivations. For example, a malicious group member could send a message with generation = 0xffffffff at the beginning of a new epoch, forcing recipients to perform billions of key derivations unless they apply limits of the type discussed above.¶

16.1. Transport Security

Because MLS messages are protected at the message level, the confidentiality and integrity of the group state do not depend on those messages being protected in transit. However, an attacker who can observe those messages in transit will be able to learn about the group state, including potentially the group membership (see Section 16.4.3 below). Such an attacker might also be able to mount denial-of-service attacks on the group or exclude new members by selectively removing messages in transit. In order to prevent this form of attack, it is RECOMMENDED that all MLS messages be carried over a secure transport such as TLS [RFC8446] or QUIC [RFC9000].¶

16.2. Confidentiality of the Group Secrets

Group secrets are partly derived from the output of a ratchet tree. Ratchet trees work by assigning each member of the group to a leaf in the tree and maintaining the following property: the private key of a node in the tree is known only to members of the group that are assigned a leaf in the node's subtree. This is called the ratchet tree invariant and it makes it possible to encrypt to all group members except one, with a number of ciphertexts that is logarithmic in the number of group members.¶

The ability to efficiently encrypt to all members except one allows members to be securely removed from a group. It also allows a member to rotate their keypair such that the old private key can no longer be used to decrypt new messages.¶

16.3. Confidentiality of Sender Data

The PrivateMessage framing encrypts "sender data" that identifies which group member sent an encrypted message, as described in Section 6.3.2. As with the QUIC header protection scheme [RFC9001], Section 5.4, this scheme is a variant of the HN1 construction analyzed in [NAN]. A sample of the ciphertext is combined with a sender_data_secret to derive a key and nonce that are used for AEAD encryption of the sender data.¶

pseudocode (key, nonce) = PRF(sender_data_secret, sample) encrypted_sender_data = AEAD.Seal(key, nonce, sender_data_aad, sender_data) ¶

The only differences between this construction and HN1 as described in [NAN] are (1) that it uses authenticated encryption instead of unauthenticated encryption and (2) that it protects information used to derive a nonce instead of the nonce itself.¶

Since the sender_data_secret is distinct from the content encryption key, it follows that the sender data encryption scheme achieves AE2 security as defined in [NAN], and therefore guarantees the confidentiality of the sender data.¶

Use of the same sender_data_secret and ciphertext sample more than once risks compromising sender data protection by reusing an AEAD (key, nonce) pair. For example, in many AEAD schemes, reusing a key and nonce reveals the exclusive OR of the two plaintexts. Assuming the ciphertext output of the AEAD algorithm is indistinguishable from random data (i.e., the AEAD is AE1-secure in the phrasing of [NAN]), the odds of two ciphertext samples being identical is roughly 2^-L/2, i.e., the birthday bound.¶

The AEAD algorithms for ciphersuites defined in this document all provide this property. The size of the sample depends on the ciphersuite's hash function, but in all cases, the probability of collision is no more than 2^-128. Any future ciphersuite MUST use an AE1-secure AEAD algorithm.¶

16.4. Confidentiality of Group Metadata

MLS does not provide confidentiality protection to some messages and fields within messages:¶

• KeyPackage messages¶
• GroupInfo messages¶
• The unencrypted portion of a Welcome message¶
• Any Proposal or Commit messages sent as PublicMessage messages¶
• The unencrypted header fields in PrivateMessage messages¶
• The lengths of encrypted Welcome and PrivateMessage messages¶

The only mechanism MLS provides for confidentially distributing a group's ratchet tree to new members is to send it in a Welcome message as a ratchet_tree extension. If an application distributes the tree in some other way, its security will depend on that application mechanism.¶

A party observing these fields might be able to infer certain properties of the group:¶

• Group ID¶
• Current epoch and frequency of epoch changes¶
• Frequency of messages within an epoch¶
• Group extensions¶
• Group membership¶

The amount of metadata exposed to parties outside the group, and thus the ability of these parties to infer the group's properties, depends on several aspects of the DS design, such as:¶

• How KeyPackages are distributed¶
• How the ratchet tree is distributed¶
• How prospective external joiners get a GroupInfo object for the group¶
• Whether Proposal and Commit messages are sent as PublicMessage or PrivateMessage¶

In the remainder of this section, we note the ways that the above properties of the group are reflected in unprotected group messages, as a guide to understanding how they might be exposed or protected in a given application.¶

16.5. Authentication

The first form of authentication we provide is that group members can verify a message originated from one of the members of the group. For encrypted messages, this is guaranteed because messages are encrypted with an AEAD under a key derived from the group secrets. For plaintext messages, this is guaranteed by the use of a membership_tag which constitutes a MAC over the message, under a key derived from the group secrets.¶

The second form of authentication is that group members can verify a message originated from a particular member of the group. This is guaranteed by a digital signature on each message from the sender's signature key.¶

The signature keys held by group members are critical to the security of MLS against active attacks. If a member's signature key is compromised, then an attacker can create LeafNodes and KeyPackages impersonating the member; depending on the application, this can then allow the attacker to join the group with the compromised member's identity. For example, if a group has enabled external parties to join via external commits, then an attacker that has compromised a member's signature key could use an external commit to insert themselves into the group -- even using a "resync"-style external commit to replace the compromised member in the group.¶

Applications can mitigate the risks of signature key compromise using pre-shared keys. If a group requires joiners to know a PSK in addition to authenticating with a credential, then in order to mount an impersonation attack, the attacker would need to compromise the relevant PSK as well as the victim's signature key. The cost of this mitigation is that the application needs some external arrangement that ensures that the legitimate members of the group have the required PSKs.¶

16.6. Forward Secrecy and Post-Compromise Security

Forward secrecy and post-compromise security are important security notions for long-lived MLS groups. Forward secrecy means that messages sent at a certain point in time are secure in the face of later compromise of a group member. Post-compromise security means that messages are secure even if a group member was compromised at some point in the past.¶

Post-compromise security is provided between epochs by members regularly updating their leaf key in the ratchet tree. Updating their leaf key prevents group secrets from continuing to be encrypted to public keys whose private keys had previously been compromised. Note that sending an Update proposal does not achieve PCS until another member includes it in a Commit. Members can achieve immediate PCS by sending their own Commit and populating the path field, as described in Section 12.4. To be clear, in all these cases, the PCS guarantees come into effect when the members of the group process the relevant Commit, not when the sender creates it.¶

Forward secrecy between epochs is provided by deleting private keys from past versions of the ratchet tree, as this prevents old group secrets from being re-derived. Forward secrecy within an epoch is provided by deleting message encryption keys once they've been used to encrypt or decrypt a message. Note that group secrets and message encryption keys are shared by the group, and thus there is a risk to forward secrecy as long as any member has not deleted these keys. This is a particular risk if a member is offline for a long period of time. Applications SHOULD have mechanisms for evicting group members which are offline for too long (i.e., have not changed their key within some period).¶

New groups are also at risk of using previously compromised keys (as with post-compromise security), if a member is added to a new group via an old KeyPackage whose corresponding private key has been compromised. This risk can be mitigated by having clients regularly generate new KeyPackages and upload them to the Delivery Service. This way, the key material used to add a member to a new group is more likely to be fresh and less likely to be compromised.¶

16.7. Uniqueness of Ratchet Tree Key Pairs

The encryption and signature keys stored in the encryption_key and signature_key fields of ratchet tree nodes MUST be distinct from one another. If two members' leaf nodes have the same signature key, for example, then the data origin authentication properties afforded by signatures within the group are degraded.¶

Uniqueness of keys in leaf nodes is assured by explicit checks on leaf nodes being added to the tree by Add or Update proposals, or in the path field of a Commit. Details can be found in Section 7.3, Section 12.2, and Section 12.4.2. Uniqueness of encryption keys in parent nodes is assured by checking that the keys in an UpdatePath are not found elsewhere in the tree (see Section 12.4.2.¶

16.8. KeyPackage Reuse

KeyPackages are intended to be used only once. That is, once a KeyPackage has been used to introduce the corresponding client to a group, it SHOULD be deleted from the KeyPackage publication system. Reuse of KeyPackages can lead to replay attacks.¶

An application MAY allow for reuse of a "last resort" KeyPackage in order to prevent denial-of-service attacks. Since a KeyPackage is needed to add a client to a new group, an attacker could prevent a client being added to new groups by exhausting all available KeyPackages. To prevent such a denial-of-service attack, the KeyPackage publication system SHOULD rate-limit KeyPackage requests, especially if not authenticated.¶

16.9. Delivery Service Compromise

MLS is designed to protect the confidentiality and integrity of the group data even in the face of a compromised DS. However, a compromised DS can still mount some attacks. While it cannot forge messages, it can selectively delay or remove them. This can in some cases be observed by detecting gaps in the per-sender generation counter, though it may not always be possible to distinguish an attack from message loss. In addition, the DS can permanently block messages to and from a group member. This will not always be detectable by other members. If an application uses the DS to resolve conflicts between simultaneous Commits (see Section 14), it is also possible for the DS to influence which Commit is applied, even to the point of preventing a member from ever having its Commits applied.¶

When put together, these abilities potentially allow a DS to collude with an attacker who has compromised a member's state to defeat PCS by suppressing the valid Update and Commit messages from the member that would lock out the attacker and update the member's leaf to a new, uncompromised state. Aside from the SenderData.generation value, MLS leaves loss detection up to the application.¶

16.10. Authentication Service Compromise

Authentication Service compromise is much more serious than compromise of the Delivery Service. A compromised AS can assert a binding for a signature key and identity pair of its choice, thus allowing impersonation of a given user. This ability is sufficient to allow the AS to join new groups as if it were that user. Depending on the application architecture, it may also be sufficient to allow the compromised AS to join the group as an existing user, for instance as if it were a new device associated with the same user. If the application uses a transparency mechanism such as CONIKS [CONIKS] or Key Transparency [KT], then it may be possible for end users to detect this kind of misbehavior by the AS. It is also possible to construct schemes in which the various clients owned by a user vouch for each other, e.g., by signing each others' keys.¶

16.11. Additional Policy Enforcement

The DS and AS may also apply additional policies to MLS operations to obtain additional security properties. For example, MLS enables any participant to add or remove members of a group; a DS could enforce a policy that only certain members are allowed to perform these operations. MLS authenticates all members of a group; a DS could help ensure that only clients with certain types of credentials are admitted. MLS provides no inherent protection against denial of service; a DS could also enforce rate limits in order to mitigate these risks.¶

16.12. Group Fragmentation by Malicious Insiders

It is possible for a malicious member of a group to "fragment" the group by crafting an invalid UpdatePath. Recall that an UpdatePath encrypts a sequence of path secrets to different subtrees of the group's ratchet trees. These path secrets should be derived in a sequence as described in Section 7.4, but the UpdatePath syntax allows the sender to encrypt arbitrary, unrelated secrets. The syntax also does not guarantee that the encrypted path secret for a given node corresponds to the public key provided for that node.¶

Both of these types of corruption will cause processing of a Commit to fail for some members of the group. If the public key for a node does not match the path secret, then the members that decrypt that path secret will reject the commit based on this mismatch. If the path secret sequence is incorrect at some point, then members that can decrypt nodes before that point will compute a different public key for the mismatched node than the one in the UpdatePath, which also causes the Commit to fail. Applications SHOULD provide mechanisms for failed commits to be reported, so that group members who were not able to recognize the error themselves can reinitialize the group if necessary.¶

Even with such an error reporting mechanism in place, however, it is still possible for members to get locked out of the group by a malformed commit. Since malformed Commits can only be recognized by certain members of the group, in an asynchronous application, it may be the case that all members that could detect a fault in a Commit are offline. In such a case, the Commit will be accepted by the group, and the resulting state possibly used as the basis for further Commits. When the affected members come back online, they will reject the first commit, and thus be unable to catch up with the group. These members will either need to add themselves back with an external Commit, or reinitialize the group from scratch.¶

Applications can address this risk by requiring certain members of the group to acknowledge successful processing of a Commit before the group regards the Commit as accepted. The minimum set of acknowledgements necessary to verify that a Commit is well-formed comprises an acknowledgement from one member per node in the UpdatePath, that is, one member from each subtree rooted in the copath node corresponding to the node in the UpdatePath. MLS does not provide a built-in mechanism for such acknowledgements, but they can be added at the application layer.¶

17.1. MLS Ciphersuites

A ciphersuite is a combination of a protocol version and the set of cryptographic algorithms that should be used.¶

Ciphersuite names follow the naming convention:¶

CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;

Where VALUE is represented as a sixteen-bit integer:¶

uint16 CipherSuite;

The columns in the registry are as follows:¶

• Value: The numeric value of the ciphersuite¶
• Name: The name of the ciphersuite¶

[[ RFC EDITOR: This section should be the same as the corresponding text in draft-ietf-tls-rfc8447bis. Please align the two documents if they have diverged in the approval process. ]]¶

• Recommended: Whether support for this ciphersuite is recommended by the IETF. Valid values are "Y", "N", and "D", as described below. The default value of the "Recommended" column is "N". Setting the Recommended item to "Y" or "D", or changing an item whose current value is "Y" or "D", requires Standards Action [RFC8126].¶

  • Y: Indicates that the IETF has consensus that the item is RECOMMENDED. This only means that the associated mechanism is fit for the purpose for which it was defined. Careful reading of the documentation for the mechanism is necessary to understand the applicability of that mechanism. The IETF could recommend mechanisms that have limited applicability, but will provide applicability statements that describe any limitations of the mechanism or necessary constraints on its use.¶
  • N: Indicates that the item has not been evaluated by the IETF and that the IETF has made no statement about the suitability of the associated mechanism. This does not necessarily mean that the mechanism is flawed, only that no consensus exists. The IETF might have consensus to leave an item marked as "N" on the basis of it having limited applicability or usage constraints.¶
  • D: Indicates that the item is discouraged and SHOULD NOT or MUST NOT be used. This marking could be used to identify mechanisms that might result in problems if they are used, such as a weak cryptographic algorithm or a mechanism that might cause interoperability problems in deployment.¶
• Reference: The document where this ciphersuite is defined¶

Initial contents:¶

All of these ciphersuites use HMAC [RFC2104] as their MAC function, with different hashes per ciphersuite. The mapping of ciphersuites to HPKE primitives, HMAC hash functions, and TLS signature schemes is as follows [RFC9180] [RFC8446]:¶

The hash used for the MLS transcript hash is the one referenced in the ciphersuite name. In the ciphersuites defined above, "SHA256", "SHA384", and "SHA512" refer to the SHA-256, SHA-384, and SHA-512 functions defined in [SHS].¶

In addition to the general requirements of Section 13.1, future ciphersuites MUST meet the requirements of Section 16.3.¶

It is advisable to keep the number of ciphersuites low to increase the chances clients can interoperate in a federated environment, therefore the ciphersuites only include modern, yet well-established algorithms. Depending on their requirements, clients can choose between two security levels (roughly 128-bit and 256-bit). Within the security levels clients can choose between faster X25519/X448 curves and FIPS 140-2 compliant curves for Diffie-Hellman key negotiations. Clients may also choose ChaCha20Poly1305 or AES-GCM, e.g., for performance reasons. Since ChaCha20Poly1305 is not listed by FIPS 140-2 it is not paired with FIPS 140-2 compliant curves. The security level of symmetric encryption algorithms and hash functions is paired with the security level of the curves.¶

The mandatory-to-implement ciphersuite for MLS 1.0 is MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 which uses Curve25519 for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and Ed25519 for signatures. MLS clients MUST implement this ciphersuite.¶

New ciphersuite values are assigned by IANA as described in Section 17.¶

17.2. MLS Wire Formats

This registry lists identifiers for the types of messages that can be sent in MLS. The wire format field is two bytes wide, so the valid wire format values are in the range 0x0000 to 0xFFFF.¶

Template:¶

• Value: The numeric value of the wire format¶
• Name: The name of the wire format¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this wire format is defined¶

Initial contents:¶

17.3. MLS Extension Types

This registry lists identifiers for extensions to the MLS protocol. The extension type field is two bytes wide, so valid extension type values are in the range 0x0000 to 0xFFFF.¶

Template:¶

• Value: The numeric value of the extension type¶
• Name: The name of the extension type¶

• Message(s): The messages in which the extension may appear, drawn from the following list:¶

  • KP: KeyPackage objects¶
  • LN: LeafNode objects¶
  • GC: GroupContext objects¶
  • GI: GroupInfo objects¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this extension is defined¶

Initial contents:¶

17.4. MLS Proposal Types

This registry lists identifiers for types of proposals that can be made for changes to an MLS group. The extension type field is two bytes wide, so valid extension type values are in the range 0x0000 to 0xFFFF.¶

Template:¶

• Value: The numeric value of the proposal type¶
• Name: The name of the proposal type¶
• Recommended: Same as in Section 17.1¶
• External: Whether a proposal of this type may be sent by an external sender (see Section 12.1.8).¶
• Path Required: Whether a Commit covering a proposal of this type is required to have its path field populated (see Section 12.4).¶
• Reference: The document where this extension is defined¶

Initial contents:¶

17.5. MLS Credential Types

This registry lists identifiers for types of credentials that can be used for authentication in the MLS protocol. The credential type field is two bytes wide, so valid credential type values are in the range 0x0000 to 0xFFFF.¶

Template:¶

• Value: The numeric value of the credential type¶
• Name: The name of the credential type¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this credential is defined¶

Initial contents:¶

17.6. MLS Signature Labels

The SignWithLabel function defined in Section 5.1.2 avoids the risk of confusion between signatures in different contexts. Each context is assigned a distinct label that is incorporated into the signature. This registry records the labels defined in this document, and allows additional labels to be registered in case extensions add other types of signature using the same signature keys used elsewhere in MLS.¶

Template:¶

• Label: The string to be used as the Label parameter to SignWithLabel¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this credential is defined¶

Initial contents:¶

17.7. MLS Public Key Encryption Labels

The EncryptWithLabel function defined in Section 5.1.3 avoids the risk of confusion between ciphertexts produced for different purposes in different contexts. Each context is assigned a distinct label that is incorporated into the signature. This registry records the labels defined in this document, and allows additional labels to be registered in case extensions add other types of public-key encryption using the same HPKE keys used elsewhere in MLS.¶

Template:¶

• Label: The string to be used as the Label parameter to EncryptWithLabel¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this credential is defined¶

Initial contents:¶

17.8. MLS Exporter Labels

The exporter function defined in Section 8.5 allows applications to derive key material from the MLS key schedule. Like the TLS exporter [RFC8446], the MLS exporter uses a label to distinguish between different applications' use of the exporter. This registry allows applications to register their usage to avoid collisions.¶

Template:¶

• Label: The string to be used as the Label parameter to MLS-Exporter¶
• Recommended: Same as in Section 17.1¶
• Reference: The document where this credential is defined¶

The registry has no initial contents, since it is intended to be used by applications, not the core protocol. The table below is intended only to show the column layout of the registry.¶

17.9. MLS Designated Expert Pool

Specification Required [RFC8126] registry requests are registered after a three-week review period on the MLS DEs' mailing list: mls-reg-review@ietf.org, on the advice of one or more of the MLS DEs. However, to allow for the allocation of values prior to publication, the MLS DEs may approve registration once they are satisfied that such a specification will be published.¶

Registration requests sent to the MLS DEs mailing list for review SHOULD use an appropriate subject (e.g., "Request to register value in MLS Bar registry").¶

Within the review period, the MLS DEs will either approve or deny the registration request, communicating this decision to the MLS DEs mailing list and IANA. Denials SHOULD include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention for resolution using the iesg@ietf.org mailing list.¶

Criteria that SHOULD be applied by the MLS DEs includes determining whether the proposed registration duplicates existing functionality, whether it is likely to be of general applicability or useful only for a single application, and whether the registration description is clear. For example, the MLS DEs will apply the ciphersuite-related advisory found in Section 17.1.¶

IANA MUST only accept registry updates from the MLS DEs and SHOULD direct all requests for registration to the MLS DEs' mailing list.¶

It is suggested that multiple MLS DEs be appointed who are able to represent the perspectives of different applications using this specification, in order to enable broadly informed review of registration decisions. In cases where a registration decision could be perceived as creating a conflict of interest for a particular MLS DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.¶

17.10. The "message/mls" MIME Type

This document registers the "message/mls" MIME media type in order to allow other protocols (e.g., HTTP [RFC9113]) to convey MLS messages.¶

Type name:

message¶

Subtype name:

mls¶

Required parameters:

none¶

Optional parameters:

version

¶

version:

The MLS protocol version expressed as a string <major>.<minor>. If omitted the version is "1.0", which corresponds to MLS ProtocolVersion mls10. If for some reason the version number in the MIME type parameter differs from the ProtocolVersion embedded in the protocol, the protocol takes precedence.¶

Encoding considerations:

MLS messages are represented using the TLS presentation language [RFC8446]. Therefore MLS messages need to be treated as binary data.¶

Security considerations:

MLS is an encrypted messaging layer designed to be transmitted over arbitrary lower layer protocols. The security considerations in this document (RFC XXXX) also apply.¶

Interoperability considerations:

N/A¶

Published specification:

RFC XXXX¶

Applications that use this media type:

MLS-based messaging applications¶

Fragment identifier considerations:

N/A¶

Additional information:¶

• Deprecated alias names for this type: N/A¶
• Magic number(s): N/A¶
• File extension(s): N/A¶
• Macintosh file type code(s): N/A¶

Person & email address to contact for further information:

IETF MLS Working Group mls@ietf.org¶

Intended usage:

COMMON¶

Restrictions on usage:

N/A¶

Author:

IETF MLS Working Group¶

Change controller:

IESG¶

Provisional registration? (standards tree only):

No¶