The Messaging Layer Security (MLS) Extensions

2.1.1. Security

An extension is called safe if it does not modify the base MLS protocol or other MLS extensions beyond using components of the Safe Extension API. The Safe Extension API provides the following security guarantee: If an application uses MLS and only safe MLS extensions, then the security guarantees of the base MLS protocol and the security guarantees of safe extensions, each analyzed in isolation, still hold for the composed extended MLS protocol. In other words, the Safe Extension API protects applications from careless extension developers. As long as all used extensions are safe, it is not possible that a combination of extensions (the developers of which did not know about each other) impedes the security of the base MLS protocol or any used extension. No further analysis of the combination is necessary. This also means that any security vulnerabilities introduced by one extension do not spread to other extensions or the base MLS.¶

2.1.2. Core Struct Extensions

Every type of MLS extension can have data associated with it. The "MLS Extensions Types" registry historically represented extensibility of four core structs (GroupContext, GroupInfo, KeyPackage, and LeafNode) that have far reaching effects on the use of the protocol. The majority of MLS extensions registered at the time of this writing extend one or more of these core structs.¶

• GroupContext Extensions: Any data in a group context extension is agreed-upon by all members of the group in the same way as the rest of the group state. As part of the GroupContext, it is also sent encrypted to new joiners via Welcome messages and (depending on the architecture of the application) may be available to external joiners. Note that in some scenarios, the GroupContext may also be visible to components that implement the delivery service. While MLS extensions can define arbitrary GroupContext extensions, it is recommended to make use of ExtensionState extensions to store state in the group's GroupContext.¶

• GroupInfo Extensions: GroupInfo extensions are included in the GroupInfo struct and thus sent encrypted and authenticated by the signer of the GroupInfo to new joiners as part of Welcome messages. It can thus be used as a confidential and authenticated channel from the inviting group member to new joiners. Just like GroupContext extensions, they may also be visible to external joiners or even parts of the delivery service. Unlike GroupContext extensions, the GroupInfo struct is not part of the group state that all group members agree on.¶

• KeyPackage Extensions: KeyPackages (and the extensions they include) are pre-published by individual clients for asynchronous group joining. They are included in Add proposals and become part of the group state once the Add proposal is committed. They are, however, removed from the group state when the owner of the KeyPackage does the first commit with a path. As such, KeyPackage extensions can be used to communicate data to anyone who wants to invite the owner to a group, as well as the other members of the group the owner is added to. Note that KeyPackage extensions are visible to the server that provides the KeyPackages for download, as well as any part of the delivery service that can see the public group state.¶

• LeafNode Extensions: LeafNodes are a part of every KeyPackage and thus follow the same lifecycle. However, they are also part of any commit that includes an UpdatePath and clients generally have a leaf node in each group they are a member of. Leaf node extensions can thus be used to include member-specific data in a group state that can be updated by the owner at any time.¶

2.1.3. Common Data Structures

The Safe Extension API reuses the ExtensionType and the "MLS Extension Types" IANA registry used for these core structs (see Section 17.3 of [RFC9420]), even for safe extensions with no core struct changes. This is because many extensions modify a core struct, either primarily or to store state (related to the group or a client) associated with another aspect of that extension.¶

Most Safe Extension API components also use the following data structure, which provides domain separation by extension_type of various extension_data.¶

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

Where extension_type is set to the type of the extension to which the extension_data belongs.¶

When a label is required for an extension, the following data structure is used.¶

struct {
  opaque label;
  ExtensionContent extension_content;
} LabeledExtensionContent;

2.1.4. Negotiating Support for Safe Extensions

MLS defines a Capabilities struct for LeafNodes (in turn used in KeyPackages), which describes which extensions are supported by the associated node. However, that struct (defined in Section 7.2 of [RFC9420]) only has fields for a subset of the extensions possible in MLS, as reproduced below.¶

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

Therefore, all safe extensions MUST be represented by their extension_type in the extensions vector (originally intended for core struct extensions), regardless of their type.¶

If the LeafNode supports any safe extension Credentials, the credentials vector will contain any non-safe credentials supported, plus the extension_credential defined in {extension-credential}.¶

If the LeafNode supports any safe extension Proposals, then proposals will contain any non-default non-safe extensions, and whichever safe extension proposal types defined in {mls-proposal-types} are relevant to the supported safe proposals.¶

Likewise, the required_capabilities GroupContext extension (defined in Section 11.1 of [RFC9420] and reproduced below) contains all mandatory to support non-default non-safe, and safe extensions in its extension_types vector. Its credential_types vector contains any mandatory non-safe credential types, plus extensions_credential if any safe credential is required. Its proposal_types vector contains any mandatory to support non-default non-safe Proposals, and the relevant safe proposal type or types corresponding to any required safe proposals.¶

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

2.1.5. Hybrid Public Key Encryption (HPKE)

This component of the Safe Extension API allows extensions to make use of all HPKE key pairs generated by MLS. An extension identified by an ExtensionType can use any HPKE key pair for any operation defined in [RFC9180], such as encryption, exporting keys and the PSK mode, as long as the info input to Setup<MODE>S and Setup<MODE>R is set to LabeledExtensionContent with extension_type set to ExtensionType. The extension_data can be set to an arbitrary Context specified by the extension designer (and can be empty if not needed). For example, an extension can use a key pair PublicKey, PrivateKey to encrypt data as follows:¶

SafeEncryptWithContext(ExtensionType, PublicKey, Context, Plaintext) =
    SealBase(PublicKey, LabeledExtensionContent, "", Plaintext)

SafeDecryptWithContext(ExtensionType, PrivateKey, Context, KEMOutput, Ciphertext) =
    OpenBase(KEMOutput, PrivateKey, LabeledExtensionContent, "", Ciphertext)

Where the fields of LabeledExtensionContent are set to¶

label = "MLS 1.0 ExtensionData"
extension_type = ExtensionType
extension_data = Context

For operations involving the secret key, ExtensionType MUST be set to the ExtensionType of the implemented extension, and not to the type of any other extension. In particular, this means that an extension cannot decrypt data meant for another extension, while extensions can encrypt data to other extensions.¶

In general, a ciphertext encrypted with a PublicKey can be decrypted by any entity who has the corresponding PrivateKey at a given point in time according to the MLS protocol (or extension). For convenience, the following list summarizes lifetimes of MLS key pairs.¶

• The key pair of a non-blank ratchet tree node. The PrivateKey of such a key pair is known to all members in the node’s subtree. In particular, a PrivateKey of a leaf node is known only to the member in that leaf. A member in the subtree stores the PrivateKey for a number of epochs, as long as the PublicKey does not change. The key pair of the root node SHOULD NOT be used, since the external key pair recalled below gives better security.¶

• The external_priv, external_pub key pair used for external initialization. The external_priv key is known to all group members in the current epoch. A member stores external_priv only for the current epoch. Using this key pair gives better security guarantees than using the key pair of the root of the ratchet tree and should always be preferred.¶

• The init_key in a KeyPackage and the corresponding secret key. The secret key is known only to the owner of the KeyPackage and is deleted immediately after it is used to join a group.¶

2.1.6. Signature Keys

MLS session states contain a number of signature keys including the ones in the LeafNode structs. Extensions can safely sign content and verify signatures using these keys via the SafeSignWithLabel and SafeVerifyWithLabel functions, respectively, much like how the basic MLS protocol uses SignWithLabel and VerifyWithLabel.¶

In more detail, an extension identified by ExtensionType should sign and verify using:¶

SafeSignWithLabel(ExtensionType, SignatureKey, Label, Content) =
    SignWithLabel(SignatureKey, "LabeledExtensionContent", LabeledExtensionContent)

SafeVerifyWithLabel(ExtensionType, VerificationKey, Label, Content, SignatureValue) =
    VerifyWithLabel(VerificationKey, "LabeledExtensionContent", LabeledExtensionContent, SignatureValue)

Where the fields of LabeledExtensionContent are set to¶

label = Label
extension_type = ExtensionType
extension_data = Content

For signing operations, the ExtensionType MUST be set to the ExtensionType of the implemented extension, and not to the type of any other extension. In particular, this means that an extension cannot produce signatures in place of other extensions. However, extensions can verify signatures computed by other extensions. Note that domain separation is ensured by explicitly including the ExtensionType with every operation.¶

2.1.7. Exporting Secrets

An extension can use MLS as a group key agreement protocol by exporting symmetric keys. Such keys can be exported (i.e. derived from MLS key material) in two phases per epoch: Either at the start of the epoch, or during the epoch. Derivation at the start of the epoch has the added advantage that the source key material is deleted after use, allowing the derived key material to be deleted later even during the same MLS epoch to achieve forward secrecy. The following protocol secrets can be used to derive key from for use by extensions:¶

• epoch_secret at the beginning of an epoch¶

• extension_secret during an epoch¶

The extension_secret is an additional secret derived from the epoch_secret at the beginning of the epoch in the same way as the other secrets listed in Table 4 of [RFC9420] using the label "extension".¶

Any derivation performed by an extension either from the epoch_secret or the extension_secret has to use the following function:¶

DeriveExtensionSecret(Secret, Label) =
  ExpandWithLabel(Secret, "ExtensionExport " + ExtensionType + " " + Label)

Where ExpandWithLabel is defined in Section 8 of [RFC9420] and where ExtensionType MUST be set to the ExtensionType of the implemented extension.¶

2.1.8. Pre-Shared Keys (PSKs)

PSKs represent key material that is injected into the MLS key schedule when creating or processing a commit as defined in Section 8.4 of [RFC9420]. Its injection into the key schedule means that all group members have to agree on the value of the PSK.¶

While PSKs are typically cryptographic keys which due to their properties add to the overall security of the group, the PSK mechanism can also be used to ensure that all members of a group agree on arbitrary pieces of data represented as octet strings (without the necessity of sending the data itself over the wire). For example, an extension can use the PSK mechanism to enforce that all group members have access to and agree on a password or a shared file.¶

This is achieved by creating a new epoch via a PSK proposal. Transitioning to the new epoch requires using the information agreed upon.¶

To facilitate using PSKs in a safe way, this document defines a new PSKType for extensions. This provides domain separation between pre-shared keys used by the core MLS protocol and applications, and between those used by different extensions.¶

enum {
  reserved(0),
  external(1),
  resumption(2),
  extensions(3),
  (255)
} PSKType;

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

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

    case extensions:
      ExtensionType extension_type;
      opaque psk_id<V>;
  };
  opaque psk_nonce<V>;
} PreSharedKeyID;

2.1.9. Extension Designer Tools

The safe extension API allows extension designers to sign and encrypt payloads without the need to register their own IANA labels. Following the same pattern, this document also provides ways for extension designers to define their own wire formats, proposals, credentials, and for structured data in the Additional Authenticated Data.¶

case mls_extension_message:
    ExtensionContent extension_content;

case extension_proposal:
    ExtensionContent extension_content;
case extension_path_proposal:
    ExtensionContent extension_content;
case extension_external_proposal:
    ExtensionContent extension_content;

case extension_credential:
    ExtensionContent extension_content;

struct {
  ExtensionType extension_type;
  opaque aad_item_data<V>;
} SafeAADItem;

struct {
  SafeAADItem aad_items<V>;
} SafeAAD;

2.1.10. Extension state: anchoring, storage and agreement

The safe extension framework can help an MLS extension ensure that all group members agree on a piece of extension-specific state by using the ExtensionState GroupContext extension. The ownership of an ExtensionState extension in the context of the safe extension framework is determined by the extension_type field. The extension with a matching extension_type is called the owning extension.¶

enum {
  reserved(0),
  read(1),
  none(2),
 (255)
} Permissions;

enum {
  reserved(0),
  hash(1),
  data(2),
} HashOrData;

struct {
  HashOrData hash_or_data;
  select(hash_or_data) {
    case hash:
      HashReference state_hash;
    case data:
      opaque state<V>;
  }
} ExtensionPayload;

struct {
  extensionType extension_type;
  Permissions read;
  ExtensionPayload payload;
} ExtensionState;

The ExtensionState GroupContext extension contains data either directly (if hash_or_data = data) or inditectly via a hash (if hash_or_data = hash).¶

The owning extension can read and write the state stored in an ExtensionState extension using an extension-defined proposal (see Section 2.1.9.3). The semantics of the proposal determines how the state is changed.¶

The read variable determines the permissions that other MLS extensions have w.r.t. the data stored within. read allows other MLS extensions to read that data via their own proposals, while none marks the data as private to the owning MLS extension.¶

Other extensions may never write to the ExtensionState of the owning MLS extension.¶

3.1.1. Description

An AppAck proposal is used to acknowledge receipt of application messages. Though this information implies no change to the group, it is structured as a Proposal message so that it is included in the group's transcript by being included in Commit messages.¶

struct {
    uint32 sender;
    uint32 first_generation;
    uint32 last_generation;
} MessageRange;

struct {
    MessageRange received_ranges<V>;
} AppAck;

An AppAck proposal represents a set of messages received by the sender in the current epoch. Messages are represented by the sender and generation values in the MLSCiphertext for the message. Each MessageRange represents receipt of a span of messages whose generation values form a continuous range from first_generation to last_generation, inclusive.¶

AppAck proposals are sent as a guard against the Delivery Service dropping application messages. The sequential nature of the generation field provides a degree of loss detection, since gaps in the generation sequence indicate dropped messages. AppAck completes this story by addressing the scenario where the Delivery Service drops all messages after a certain point, so that a later generation is never observed. Obviously, there is a risk that AppAck messages could be suppressed as well, but their inclusion in the transcript means that if they are suppressed then the group cannot advance at all.¶

The schedule on which sending AppAck proposals are sent is up to the application, and determines which cases of loss/suppression are detected. For example:¶

• The application might have the committer include an AppAck proposal whenever a Commit is sent, so that other members could know when one of their messages did not reach the committer.¶

• The application could have a client send an AppAck whenever an application message is sent, covering all messages received since its last AppAck. This would provide a complete view of any losses experienced by active members.¶

• The application could simply have clients send AppAck proposals on a timer, so that all participants' state would be known.¶

An application using AppAck proposals to guard against loss/suppression of application messages also needs to ensure that AppAck messages and the Commits that reference them are not dropped. One way to do this is to always encrypt Proposal and Commit messages, to make it more difficult for the Delivery Service to recognize which messages contain AppAcks. The application can also have clients enforce an AppAck schedule, reporting loss if an AppAck is not received at the expected time.¶

3.2.1. Description

MLS application messages make sending encrypted messages to all group members easy and efficient. Sometimes application protocols mandate that messages are only sent to specific group members, either for privacy or for efficiency reasons.¶

Targeted messages are a way to achieve this without having to create a new group with the sender and the specific recipients – which might not be possible or desired. Instead, targeted messages define the format and encryption of a message that is sent from a member of an existing group to another member of that group.¶

The goal is to provide a one-shot messaging mechanism that provides confidentiality and authentication.¶

Targeted Messages makes use the Safe Extension API as defined in Section 2.1. reuse mechanisms from [mls-protocol], in particular [hpke].¶

3.2.2. Format

This extension uses the mls_extension_message WireFormat as defined in Section Section 2.1.9.2, where the content is a TargetedMessage.¶

struct {
  opaque group_id<V>;
  uint64 epoch;
  uint32 recipient_leaf_index;
  opaque authenticated_data<V>;
  opaque encrypted_sender_auth_data<V>;
  opaque hpke_ciphertext<V>;
} TargetedMessage;

enum {
  hpke_auth_psk(0),
  signature_hpke_psk(1),
} TargetedMessageAuthScheme;

struct {
  uint32 sender_leaf_index;
  TargetedMessageAuthScheme authentication_scheme;
  select (authentication_scheme) {
    case HPKEAuthPsk:
    case SignatureHPKEPsk:
      opaque signature<V>;
  }
  opaque kem_output<V>;
} TargetedMessageSenderAuthData;

struct {
  opaque group_id<V>;
  uint64 epoch;
  uint32 recipient_leaf_index;
  opaque authenticated_data<V>;
  TargetedMessageSenderAuthData sender_auth_data;
} TargetedMessageTBM;

struct {
  opaque group_id<V>;
  uint64 epoch;
  uint32 recipient_leaf_index;
  opaque authenticated_data<V>;
  uint32 sender_leaf_index;
  TargetedMessageAuthScheme authentication_scheme;
  opaque kem_output<V>;
  opaque hpke_ciphertext<V>;
} TargetedMessageTBS;

struct {
  opaque group_id<V>;
  uint64 epoch;
  opaque label<V> = "MLS 1.0 targeted message psk";
} PSKId;

Note that TargetedMessageTBS is only used with the TargetedMessageAuthScheme.SignatureHPKEPsk authentication mode.¶

3.2.3. Encryption

Targeted messages uses HPKE to encrypt the message content between two leaves.¶

sender_auth_data_secret
  = DeriveExtensionSecret(extension_secret, "targeted message sender auth data")

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

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

struct {
  opaque group_id<V>;
  uint64 epoch;
  uint32 recipient_leaf_index;
} SenderAuthDataAAD;

3.2.4. Authentication

For ciphersuites that support it, HPKE mode_auth_psk is used for authentication. For other ciphersuites, HPKE mode_psk is used along with a signature. The authentication scheme is indicated by the authentication_scheme field in TargetedMessageContent. See Section 3.2.5 for more information.¶

For the PSK part of the authentication, clients export a dedicated secret:¶

targeted_message_psk
  = DeriveExtensionSecret(extension_secret, "targeted message psk")

The functions SealAuth and OpenAuth defined in [hpke] are used as described in Section 2.1.5 with an empty context. Other functions are defined in [mls-protocol].¶

label = "MLS 1.0 ExtensionData"
extension_type = ExtensionType
extension_data = group_context

(kem_output, hpke_ciphertext) = SealAuthPSK(receiver_node_public_key,
                                            hpke_context,
                                            targeted_message_tbm,
                                            message,
                                            targeted_message_psk,
                                            psk_id,
                                            sender_node_private_key)

message = OpenAuthPSK(kem_output,
                      receiver_node_private_key,
                      hpke_context,
                      targeted_message_tbm,
                      hpke_ciphertext,
                      targeted_message_psk,
                      psk_id,
                      sender_node_public_key)

(kem_output, hpke_ciphertext) = SealPSK(receiver_node_public_key,
                                        hpke_context,
                                        targeted_message_tbm,
                                        message,
                                        targeted_message_psk,
                                        epoch)

signature = SafeSignWithLabel(extension_type, ., "TargetedMessageTBS", targeted_message_tbs)

message = OpenPSK(kem_output,
                  receiver_node_private_key,
                  hpke_context,
                  targeted_message_tbm,
                  hpke_ciphertext,
                  targeted_message_psk,
                  epoch)

SafeVerifyWithLabel.verify(extension_type,
                        sender_leaf_node.signature_key,
                        "TargetedMessageTBS",
                        targeted_message_tbs,
                        signature)

3.2.5. Guidance on authentication schemes

If the group’s ciphersuite does not support HPKE mode_auth_psk, implementations MUST choose TargetedMessageAuthScheme.SignatureHPKEPsk.¶

If the group’s ciphersuite does support HPKE mode_auth_psk, implementations CAN choose TargetedMessageAuthScheme.HPKEAuthPsk if better efficiency and/or repudiability is desired. Implementations SHOULD consult [hpke-security-considerations] beforehand.¶

3.3.1. Description

This section describes two extensions to MLS. The first allows MLS clients to advertise their support for specific formats inside MLS application_data. These are expressed using the extensive IANA Media Types registry (formerly called MIME Types). The accepted_media_types LeafNode extension lists the formats a client supports inside application_data. The second, the required_media_types GroupContext extension specifies which media types need to be supported by all members of a particular MLS group. These allow clients to confirm that all members of a group can communicate. Note that when the membership of a group changes, or when the policy of the group changes, it is responsibility of the committer to insure that the membership and policies are compatible.¶

Finally, this document defines a minimal framing format so MLS clients can signal which media type is being sent when multiple formats are permitted in the same group. As clients are upgraded to support new formats they can use these extensions to detect when all members support a new or more efficient encoding, or select the relevant format or formats to send.¶

Note that the usage of IANA media types in general does not imply the usage of MIME Headers [RFC2045] for framing. Vendor-specific media subtypes starting with vnd. can be registered with IANA without standards action as described in [RFC6838]. Implementations which wish to send multiple formats in a single application message, may be interested in the multipart/alternative media type defined in [RFC2046] or may use or define another type with similar semantics (for example using TLS Presentation Language syntax [RFC8446]).¶

3.3.2. Syntax

MediaType is a TLS encoding of a single IANA media type (including top-level type and subtype) and any of its parameters. Even if the parameter_value would have required formatting as a quoted-string in a text encoding, only the contents inside the quoted-string are included in parameter_value. MediaTypeList is an ordered list of MediaType objects.¶

struct {
    opaque parameter_name<V>;
    /* Note: parameter_value never includes the quotation marks of an
     * RFC 2045 quoted-string */
    opaque parameter_value<V>;
} Parameter;

struct {
    /* media_type is an IANA top-level media type, a "/" character,
     * and the IANA media subtype */
    opaque media_type<V>;

    /* a list of zero or more parameters defined for the subtype */
    Parameter parameters<V>;
} MediaType;

struct {
    MediaType media_types<V>;
} MediaTypeList;

MediaTypeList accepted_media_types;
MediaTypeList required_media_types;

Example IANA media types with optional parameters:¶

  image/png
  text/plain ;charset="UTF-8"
  application/json
  application/vnd.example.msgbus+cbor

For the example media type for text/plain, the media_type field would be text/plain, parameters would contain a single Parameter with a parameter_name of charset and a parameter_value of UTF-8.¶

3.3.3. Expected Behavior

An MLS client which implements this section SHOULD include the accepted_media_types extension in its LeafNodes, listing all the media types it can receive. As usual, the client also includes accepted_media_types in its capabilities field in its LeafNodes (including LeafNodes inside its KeyPackages).¶

When creating a new MLS group for an application using this specification, the group MAY include a required_media_type extension in the GroupContext Extensions. As usual, the client also includes required_media_types in its capabilities field in its LeafNodes (including LeafNodes inside its KeyPackages). When used in a group, the client MUST include the required_media_types and accepted_media_types extensions in the list of extensions in RequiredCapabilities.¶

MLS clients SHOULD NOT add an MLS client to an MLS group with required_media_types unless the MLS client advertises it can support all of the required MediaTypes. As an exception, a client could be preconfigured to know that certain clients support the requried types. Likewise, an MLS client is already forbidden from issuing or committing a GroupContextExtensions Proposal which introduces required extensions which are not supported by all members in the resulting epoch.¶

3.3.4. Framing of application_data

When an MLS group contains the required_media_types GroupContext extension, the application_data sent in that group is interpreted as ApplicationFraming as defined below:¶

  struct {
      MediaType media_type;
      opaque<V> application_content;
  } ApplicationFraming;

The media_type MAY be zero length, in which case, the media type of the application_content is interpreted as the first MediaType specified in required_media_types.¶

3.4.1. Extension Description

This document specifies a new MLS Proposal type called SelfRemove. Its syntax is described using the TLS Presentation Language [@!RFC8446] below (its content is an empty struct). It is allowed in External Commits and requires an UpdatePath. SelfRemove proposals are only allowed in a Commit by reference. SelfRemove cannot be sent as an external proposal.¶

struct {} SelfRemove;

struct {
    ProposalType msg_type;
    select (Proposal.msg_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;
        case self_remove:              SelfRemove;
    };
} Proposal;

The description of behavior below only applies if all the members of a group support this extension in their capabilities; such a group is a "self-remove-capable group".¶

An MLS client which supports this extension can send a SelfRemove Proposal whenever it would like to remove itself from a self-remove-capable group. Because the point of a SelfRemove Proposal is to be available to external joiners (which are not yet members), these proposals MUST be sent in an MLS PublicMessage.¶

Whenever a member receives a SelfRemove Proposal, it includes it along with any other pending Propsals when sending a Commit. It already MUST send a Commit of pending Proposals before sending new application messages.¶

When a member receives a Commit referencing one or more SelfRemove Proposals, it treats the proposal like a Remove Proposal, except the leaf node to remove is determined by looking in the Sender leaf_index of the original Proposal. The member is able to verify that the Sender was a member.¶

Whenever a new joiner is about to join a self-remove-capable group with an External Commit, the new joiner MUST fetch any pending SelfRemove Proposals along with the GroupInfo object, and include the SelfRemove Proposals in its External Commit by reference. (An ExternalCommit can contain zero or more SelfRemove proposals). The new joiner MUST validate the SelfRemove Proposal before including it by reference, except that it skips the validation of the membership_tag because a non-member cannot verify membership.¶

During validation, SelfRemove proposals are processed after Update proposals and before Remove proposals. If there is a pending SelfRemove proposal for a specific leaf node and a pending Remove proposal for the same leaf node, the Remove proposal is invalid. A client MUST NOT issue more than one SelfRemove proposal per epoch.¶

The MLS Delivery Service (DS) needs to validate SelfRemove Proposals it receives (except that it cannot validate the membership_tag). If the DS provides a GroupInfo object to an external joiner, the DS SHOULD attach any SelfRemove proposals known to the DS to the GroupInfo object.¶

As with Remove proposals, clients need to be able to receive a Commit message which removes them from the group via a SelfRemove. If the DS does not forward a Commit to a removed client, it needs to inform the removed client out-of-band.¶

3.5.1. Description

Section 10 of [RFC9420] details that clients are required to pre-publish KeyPackages s.t. other clients can add them to groups asynchronously. It also states that they should not be re-used:¶

• KeyPackages are intended to be used only once and SHOULD NOT be reused except in the case of a "last resort" KeyPackage (see Section 16.8). Clients MAY generate and publish multiple KeyPackages to support multiple cipher suites.¶

Section 16.8 of [RFC9420] then introduces the notion of last-resort KeyPackages as follows:¶

• An application MAY allow for reuse of a "last resort" KeyPackage in order to prevent denial-of-service attacks.¶

However, [RFC9420] does not specify how to distinguish regular KeyPackages from last-resort ones. The last_resort_key_package KeyPackage extension defined in this section fills this gap and allows clients to specifically mark KeyPackages as KeyPackages of last resort that MAY be used more than once in scenarios where all other KeyPackages have already been used.¶

The extension allows clients that pre-publish KeyPackages to signal to the Delivery Service which KeyPackage(s) are meant to be used as last resort KeyPackages.¶

An additional benefit of using an extension rather than communicating the information out-of-band is that the extension is still present in Add proposals. Clients processing such Add proposals can authenticate that a KeyPackage is a last-resort KeyPackage and MAY make policy decisions based on that information.¶

3.5.2. Format

The purpose of the extension is simply to mark a given KeyPackage, which means it carries no additional data.¶

As a result, a LastResort Extension contains the ExtensionType with an empty extension_data field.¶

4.1.1. MLS Extension Message

• Value: 0x0006¶

• Name: mls_extension_message¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.1. targeted_messages_capability MLS Extension

The targeted_messages_capability MLS Extension Type is used in the capabilities field of LeafNodes to indicate the support for the Targeted Messages Extension. The extension does not carry any payload.¶

• Value: 0x0006¶

• Name: targeted_messages_capability¶

• Message(s): LN: This extension may appear in LeafNode objects¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.2. targeted_messages MLS Extension

The targeted_messages MLS Extension Type is used inside GroupContext objects. It indicates that the group supports the Targeted Messages Extension.¶

• Value: 0x0007¶

• Name: targeted_messages¶

• Message(s): GC: This extension may appear in GroupContext objects¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.3. accepted_media_types MLS Extension

The accepted_media_types MLS Extension Type is used inside LeafNode objects. It contains a MediaTypeList representing all the media types supported by the MLS client referred to by the LeafNode.¶

• Value: 0x0008¶

• Name: accepted_media_types¶

• Message(s): LN: This extension may appear in LeafNode objects¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.4. required_media_types MLS Extension

The required_media_types MLS Extension Type is used inside GroupContext objects. It contains a MediaTypeList representing the media types which are mandatory for all MLS members of the group to support.¶

• Value: 0x0009¶

• Name: required_media_types¶

• Message(s): GC: This extension may appear in GroupContext objects¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.5. last_resort_key_package MLS Extension

The last_resort_key_package MLS Extension Type is used inside KeyPackage objects. It marks the KeyPackage for usage in last resort scenarios and contains no additional data.¶

• Value: 0x000A¶

• Name: last_resort_key_package¶

• Message(s): KP: This extension may appear in KeyPackage objects¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.6. extension_aad MLS Extension

The extension_aad MLS Extension Type is used to signal support for SafeAAD in LeafNode capabilities, and in GroupContext required_capabilities. It contains no additional data.¶

• Value: 0x000B¶

• Name: extension_aad¶

• Message(s): LN,GC: This extension may appear in LeafNode and GroupContext objects.¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.7. safe_extensions MLS Extension

The safe_extensions MLS Extension Type is used to signal support for the Safe Extensions Framework in LeafNode capabilities, and in GroupContext required_capabilities. It contains no additional data.¶

• Value: 0x000C¶

• Name: safe_extensions¶

• Message(s): LN,GC: This extension may appear in LeafNode and GroupContext objects.¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.2.8. core_struct_extensions MLS Extension

The core_struct_extensions MLS Extension Type is used to signal support for one or more Core Struct Extensions using the Safe Extensions Framework. It appears in LeafNode capabilities, and in GroupContext required_capabilities. It contains no additional data.¶

• Value: 0x000D¶

• Name: core_struct_extensions¶

• Message(s): LN,GC: This extension may appear in LeafNode and GroupContext objects.¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.3.1. Extension Proposal

• Value: 0x0008¶

• Name: extension_proposal¶

• Recommended: Y¶

• Path Required: N¶

• External Sender: N¶

• Reference: RFC XXXX¶

4.3.2. Extension Path Proposal

• Value: 0x0009¶

• Name: extension_path_proposal¶

• Recommended: Y¶

• Path Required: Y¶

• External Sender: N¶

• Reference: RFC XXXX¶

4.3.3. Extension External Proposal

• Value: 0x000a¶

• Name: extension_external_proposal¶

• Recommended: Y¶

• Path Required: N¶

• External Sender: Y¶

• Reference: RFC XXXX¶

4.3.4. AppAck Proposal

• Value: 0x000b¶

• Name: app_ack¶

• Recommended: Y¶

• Path Required: Y¶

• Reference: RFC XXXX¶

4.3.5. SelfRemove Proposal

The self_remove MLS Proposal Type is used for a member to remove itself from a group more efficiently than using a remove proposal type, as the self_remove type is permitted in External Commits.¶

• Value: 0x000c¶

• Name: self_remove¶

• Recommended: Y¶

• External: N¶

• Path Required: Y¶

4.4.1. Extension Credential

• Value: 0x0003¶

• Name: extension_credential¶

• Recommended: Y¶

• Reference: RFC XXXX¶

4.5.1. Labeled Extension Content

• Label: "LabeledExtensionContent"¶

• Recommended: Y¶

• Reference: RFC XXXX¶