MLS Virtual Clients

Introduction The MLS protocol facilitates communication between clients, where in an MLS group, each client is represented by the leaf to which it holds the private key material. In this document, we propose the notion of a virtual client that is jointly emulated by a group of emulator clients, where each emulator client holds the key material necessary to act as the virtual client. The use of a virtual client allows multiple distinct clients to be represented by a single leaf in an MLS group. This pattern of shared group membership provides a new way for applications to structure groups, can improve performance and help hide group metadata. The effect of the use of virtual clients depends largely on how it is applied (see ). We discuss technical challenges and propose a concrete scheme that allows a group of clients to emulate a virtual client that can participate in one or more MLS groups.

Terminology

Virtual Client: A client for which the secret key material is held by one or more emulator clients, each of which can act on behalf of the virtual client.
Emulator Client: A client that collaborates with other emulator clients in emulating a virtual client.
Emulation group: Group used by emulator clients to coordinate emulation of a virtual client.
Higher-level group: A group that is not an emulation group and that may contain one or more virtual clients.
Simple multi-client: A simple alternative to the concept of virtual clients, where entities that are represented by more than one client (e.g. a user with multiple devices) are implemented by including all of the entities' clients in all groups the entity is participating in.

TODO: Terminology is up for debate. We’ve sometimes called this “user trees”, but since there are other use cases, we should choose a more neutral name. For now, it’s virtual client emulation.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in () () when, and only when, they appear in all capitals, as shown here.

Applications Virtual clients generally allow multiple emulator clients to share membership in an MLS group, where the virtual client is represented as a single leaf. This is in contrast to the simple multi-client scenario as defined above. Depending on the application, the use of virtual clients can have different effects. However, in all cases, virtual client emulation introduces a small amount of overhead for the emulator clients and certain limitations when it comes to emulation group management (see ).

Virtual clients for performance If a group of emulator clients emulate a virtual client in more than one group, the overhead caused by the emulation process can be outweighed by two performance benefits. On the one hand, the use of virtual clients makes the higher-level groups (in which the virtual client is a member) smaller. Instead of one leaf for each emulator client, it only has a single leaf for the virtual client. As the complexity of most MLS operations depends on the number of group members, this increases performance for all members of that group. At the same time, the virtual client emulation process (see ) allows emulator clients to carry the benefit of a single operation in the emulation group to all higher-level groups in which the virtual client is a member.

Smaller trees As a general rule, groups where one or more sets of clients are replaced by virtual clients have fewer members, which leads to cheaper MLS operations where the cost depends on the group size, e.g., commits with a path, the download size of the group state for new members, etc. This increase in performance can offset performance penalties, for example, when using a PQ-secure cipher suite, or if the application requires high update frequencies.

Fewer blanks Blanks are typically created in the process of client removals. With virtual clients, the removal of an emulator client will not cause the leaf of the virtual client (or indeed any node in the virtual client's direct path) to be blanked, except if it is the last remaining emulator client. As a result, fluctuation in emulator clients does not necessarily lead to blanks in the group of the corresponding virtual clients, resulting in fewer overall blanks and better performance for all group members.

Emulation costs From a performance standpoint, using virtual clients only makes sense if the performance benefits from smaller trees and fewer blanks outweigh the performance overhead incurred by emulating the virtual client in the first place.

Metadata hiding Virtual clients can be used to hide the emulator clients from other members of higher-level groups. For example, removing group members of the emulation group will only be visible in the higher-level group as a regular group update. Similarly, when an emulator client wants to send a message in a higher-level group, recipients will see the virtual client as the sender and won't be able to discern which emulator client sent the message, or indeed the fact that the sender is a virtual client at all. Hiding emulator clients behind their virtual client(s) can, for example, hide the number of devices a human user has, or which device the user is sending messages from. As hiding of emulator clients by design obfuscates the membership in higher-level groups, it also means that other higher-level group members can't identify the actual senders and recipients of messages. From the point of view of other group members, the "end" of the end-to-end encryption and authentication provided by MLS ends with the virtual client. The relevance of this fact largely depends on the security goals of the application and the design of the authentication service. If the virtual client is used to hide the emulator clients, the delivery service and other higher-level group members also lose the ability to enforce policies to evict stale clients. For example, an emulator client could become stale (i.e. inactive), while another keeps sending updates. From the point of view of the higher-level group, the virtual client would remain active.

Client emulation To ensure that all emulator clients can act through the virtual client, they have to coordinate some of its actions. Each emulation group MUST emulate exactly one virtual client. An emulation group MAY emulate that virtual client in multiple higher-level groups, but the virtual client MUST NOT be represented by more than one leaf in any given higher-level group.

Delivery Service Client emulation requires that any message sent by an emulator client on behalf of a virtual client be delivered not just to the rest of the higher-level group to which the message is sent, but also to all other clients in the emulation group.

Generating Virtual Client Secrets Generally, secrets for virtual client operations are derived from the emulation group. To that end, emulator clients derive an emulator_epoch_secret with every new epoch of that group using the Safe Exporter API defined in , where virtual_clients_component_id is the component ID registered for the virtual_clients component in . The emulator_epoch_secret is in turn used to derive five further secrets, after which it is deleted. The epoch_base_secret keys a Virtual Client Operation Secret Tree, a tree of secrets with the same structure as the Secret Tree defined in Section 9 of , with the following differences:

Like the Secret Tree, the Virtual Client Operation Secret Tree has the same set of nodes and edges as the emulation group's ratchet tree at the corresponding epoch. The node_index fields of the serialized SecretTreeState are interpreted relative to this tree.
The root of the tree is the epoch_base_secret rather than a secret derived from the encryption_secret of an MLS group epoch.
Each leaf has one operation ratchet for each VirtualClientOperationType.

Emulator clients use the same ratchet advancement, skipping, and deletion schedule for each operation ratchet as RFC 9420 uses for sender ratchets. An emulator client that learns of an operation with generation n of a ratchet whose current state is at generation m < n derives and retains the operation secrets for the skipped generations m, ..., n-1 in the same way RFC 9420 handles out-of-order PrivateMessages. Implementations SHOULD bound the number of skipped generations they retain and the time for which they retain them. The serialized state of the Virtual Client Operation Secret Tree is represented by SecretTreeState with SecretTreeRatchetState.ratchet_type set to operation and SecretTreeRatchetState.operation_type set to the corresponding VirtualClientOperationType. When a leaf secret in the Virtual Client Operation Secret Tree is expanded, the initial operation ratchet secret for each non-reserved VirtualClientOperationType defined in this document is derived as follows: In this derivation, operation_type is the TLS-encoded VirtualClientOperationType value. The leaf_secret MUST be deleted immediately after the initial operation ratchet secrets for all operation types have been derived. As a consequence, operation types defined by future documents cannot be added to already expanded leaves; they are only usable from emulation-group epochs onward in which all emulator clients derive them at expansion time. Emulator clients MUST retain the operation_secret_tree, the epoch_id, the epoch_encryption_key, the generation_id_secret, the reuse_guard_secret, and the number of leaf nodes the emulation group's ratchet tree had at the corresponding epoch (including blank leaves, see ) until no key material associated with that epoch is actively used anymore and no generation IDs or reuse guards need to be computed for that epoch. For each retained epoch in which an emulator client was a member, it MUST also retain its own leaf index at that epoch. The operation_secret_tree, epoch_id, and epoch_encryption_key are used for deriving and processing virtual-client key material, including DerivationInfos and state transferred to new clients (); the generation_id_secret is used for computing generation IDs (); and the reuse_guard_secret, leaf count, and leaf index are used for computing the reuse_guard (). When deriving a secret for a virtual client, e.g. for use in a KeyPackage upload or LeafNode update, the deriving client chooses a VirtualClientOperationType and uses the next unused generation of its own operation ratchet for that operation type. The operation_type is key_package when deriving per-KeyPackage seed secrets, either for a KeyPackageUpload batch or for the creator LeafNode of a newly created higher-level group (). A single key_package operation generation derives the batch operation secret for all KeyPackages in that upload, and individual KeyPackages are domain-separated with key_package_index as described in . The batch is closed by the upload: after a KeyPackageUpload has been sent, the same operation generation MUST NOT be used to derive additional KeyPackages. The operation_type is leaf_node when creating a LeafNode for an Update proposal, a Commit with an update path, or an external commit. Applications MAY use the operation type application to derive application-specific key material. Deriving an operation_secret consumes the corresponding operation-ratchet generation, even if the virtual-client operation does not result in a higher-level MLS message that is accepted into the higher-level group's transcript. Once a generation has been consumed, an emulator client MUST NOT use the same generation of the same operation-type ratchet for another virtual-client operation. A retry of a failed operation is another virtual-client operation: if, for example, a group creation or an external commit is rejected and attempted again, the retry MUST consume a fresh generation. If the operation fails, the emulator client MUST delete any retained secret material derived from the failed operation after the client determines that the operation will not be used. The operation context is a value bound into the derivation of the operation_secret. For key_package operations, operation_context is a zero-length octet string. For leaf_node operations, operation_context is the group_id of the higher-level group in which the LeafNode is used. For application operations, operation_context is application-defined and MUST be authenticated by the application together with the other derivation inputs. ; uint32 leaf_index; uint32 generation; VirtualClientOperationType operation_type; opaque operation_context; } OperationContext; operation_generation_secret = DeriveSecret(operation_ratchet_secret, "VC Operation Secret") next_operation_ratchet_secret = DeriveSecret(operation_ratchet_secret, "VC Operation Ratchet") operation_secret = ExpandWithLabel(operation_generation_secret, "vc operation", OperationContext, Kdf.Nh) ]]> operation_ratchet_secret is the ratchet secret for generation in the operation_type ratchet of leaf_index. After deriving operation_secret and next_operation_ratchet_secret, the emulator client MUST delete operation_ratchet_secret and operation_generation_secret and update the ratchet state for the selected operation_type to next_operation_ratchet_secret for generation + 1. The emulator client MUST retain enough secret material to reproduce all active key material derived from operation_secret. For key_package operations, the emulator client derives the per-KeyPackage seed secrets for every KeyPackage in the batch and MUST delete the batch operation_secret after all per-KeyPackage material needed for the batch has been derived. Retaining operation_secrets and per-KeyPackage seed secrets beyond their immediate use is only necessary if the application intends to onboard new emulator clients through provisioning state transfer (Variant A, ). If that is the case, the emulator client MUST retain them as RetainedOperationSecrets and RetainedKeyPackageMaterial until all key material derived from them is no longer active, and MUST delete them after that point. Otherwise, the emulator client MUST delete them as soon as it has derived the key material required for the operation. The derived key material that remains in use (such as the init_key private key of an outstanding KeyPackage) is retained independently of the secret it was derived from. Given an epoch_id, operation_type, generation, operation_context, and the leaf_index of the emulator client performing the virtual client operation, other emulator clients can derive the operation_secret, advance the same operation-type ratchet, and use the operation_secret to perform the same operation. For key_package operations, they also use key_package_index to derive the per-KeyPackage seed secret from the batch operation_secret. If the operation-type ratchet generation has already been deleted and the operation was not a key_package operation, for example because the emulator client joined after the operation took place, the emulator client uses the corresponding RetainedOperationSecret transferred in NewEmulatorClientState. For KeyPackage-derived material, the emulator client uses the corresponding RetainedKeyPackageMaterial, identified either by DerivationInfo or by an active_key_packages entry. Depending on the operation, the acting emulator client will have to derive one or more secrets from the operation_secret. This document defines four types of MLS-related secrets derived from virtual client operations.

signature_key_secret: Used to derive the signature key in a virtual client's leaf
init_key_secret: Used to derive the init_key HPKE key in a KeyPackage
encryption_key_secret: Used to derive the encryption_key HPKE key in the LeafNode of a virtual client
path_generation_secret: Used to generate path_secrets for the UpdatePath of a virtual client

For operations that do not use a per-KeyPackage seed secret, the MLS-related secrets are derived directly from the operation_secret: For key_package operations, the KeyPackageUpload derives one batch operation_secret. For each KeyPackageInfo in the upload, the creating emulator client derives a per-KeyPackage seed secret from that batch operation_secret using the KeyPackage's key_package_index as KDF context: The KeyPackage's init_key_secret, signature_key_secret, and encryption_key_secret are derived from key_package_seed_secret, not directly from the batch operation_secret: The source of the virtual client's signature key is application-defined. An application MAY use signature_key_secret to derive a fresh signing key per operation, or MAY provide signing-key material through other means, for example a per-group or global signing key issued by the Authentication Service, or a signing key derived from an emulation-group epoch secret. Whatever the choice, the signing_key_material field of NewEmulatorClientState (see ) carries the state a new emulator client needs; its contents are application-defined.

Creating LeafNodes and UpdatePaths When creating a LeafNode, either for group creation, a Commit with an update path, an Update proposal, an external commit, or a KeyPackage, the creating emulator client MUST derive the necessary secrets from the current epoch of the emulation group as described in . For a LeafNode whose leaf_node_source is key_package, the creating emulator client MUST use a per-KeyPackage seed secret, derived from a batch key_package operation secret and the LeafNode's key_package_index. For the LeafNode of a KeyPackage, this MUST be the same seed secret used to derive the KeyPackage's init_key_secret. For the creator LeafNode of a newly created higher-level group, the seed secret is derived from a dedicated key_package operation as described in . For a LeafNode whose leaf_node_source is update or commit, the creating emulator client MUST use a leaf_node operation secret. Similarly, if an emulator client generates a Commit with an update path, it MUST use path_generation_secret as the path_secret for the first parent_node instead of generating it randomly. To signal to other emulator clients which epoch to use to derive the necessary secrets to recreate the key material, the emulator client includes a DerivationInfo struct as the virtual_clients component data in the LeafNode. ; opaque ciphertext; } DerivationInfo; struct { opaque init_secret; } ExternalInitSecret; struct { uint32 leaf_index; uint32 generation; select (LeafNode.leaf_node_source) { case key_package: uint32 key_package_index; case update: struct{}; case commit: optional external_init_secret; }; } DerivationInfoTBE; ]]> For a LeafNode in an external Commit, external_init_secret MUST contain the init_secret produced by external initialization as described in . For a LeafNode in any other Commit, the external_init_secret field MUST be absent. The ciphertext is the serialized DerivationInfoTBE encrypted with the AEAD scheme of the emulation group's ciphersuite, with the epoch_id as AAD. The LeafNode.leaf_node_source selector is the leaf_node_source of the LeafNode carrying the DerivationInfo. The AEAD key and nonce are derived from the epoch's epoch_encryption_key, using the serialized encryption_key field of the LeafNode carrying the component as context: Since every virtual-client operation produces a LeafNode with a fresh encryption_key, each distinct DerivationInfoTBE is encrypted under a distinct key-nonce pair. Re-encrypting the same DerivationInfoTBE for the same LeafNode yields an identical ciphertext, which is benign. This only holds because a consumed operation-ratchet generation is never reused: for external Commit LeafNodes, the DerivationInfoTBE contains the external_init_secret, which changes with every encapsulation. Deriving a LeafNode for a retried external Commit from the same generation would therefore encrypt a different DerivationInfoTBE under the same key and nonce and compromise the AEAD. Instead, the retry MUST consume a fresh generation as described in . The operation type used to derive the LeafNode's operation_secret is determined by the LeafNode's leaf_node_source (): key_package maps to key_package, while update and commit map to leaf_node. The operation_context is determined by the operation type as described in : zero-length for key_package operations, and the group_id of the higher-level group in which the LeafNode is used for leaf_node operations. The leaf_index and generation fields MUST be the values used with that operation type and operation context to derive the LeafNode's operation_secret. For KeyPackage LeafNodes, generation identifies the batch key_package operation generation and key_package_index identifies the individual KeyPackage within that batch. For leaf_node operations, the DerivationInfoTBE contains no key_package_index. For external Commit LeafNodes, the DerivationInfoTBE additionally carries the external init secret needed to process the Commit. When other emulator clients receive a LeafNode for the virtual client, they use the epoch_id to determine the epoch of the emulation group from which to derive the secrets necessary to re-create the key material of the LeafNode and potential UpdatePath. They decrypt the DerivationInfo and use the operation type determined from the LeafNode's leaf_node_source, together with the leaf_index and generation fields, as inputs to the operation secret derivation described in . For a KeyPackage LeafNode, they then use key_package_index to derive the per-KeyPackage seed secret from the batch key_package operation secret and use that seed to re-create the LeafNode key material. When processing an external Commit sent by the virtual client, an emulator client uses the external_init_secret from the DerivationInfoTBE as the external init secret for the new epoch. If external_init_secret is absent, the emulator client MUST reject the Commit. The external_init_secret is carried in the DerivationInfo because the other emulator clients cannot always derive it themselves. If the external Commit resynchronizes the virtual client's membership in a group it is already a member of, the other emulator clients hold the previous epoch's external_secret and could recover the init secret from the kem_output in the ExternalInit proposal (). If the virtual client externally joins a group it was not previously a member of, however, they hold no prior epoch state for that group and cannot. Carrying the init secret in the DerivationInfo covers both cases uniformly. The DerivationInfo on the active virtual-client LeafNode binds that virtual client's membership in the higher-level group to the emulation-group epoch identified by epoch_id. Protocol steps that require per-epoch emulation-group state for a higher-level group, such as computing the reuse_guard () or a generation ID (), MUST use the emulation-group epoch identified by the active virtual-client LeafNode.

Creating groups with the virtual client When an emulator client creates a higher-level group with the virtual client as the creator, it MUST create the initial LeafNode as described in . does not prescribe a leaf_node_source for the creator's LeafNode. The creator LeafNode of a virtual client MUST have leaf_node_source key_package. Since no KeyPackage is published for this LeafNode, the creating emulator client derives its key material from a dedicated key_package operation: it consumes a fresh key_package operation-ratchet generation and derives a single per-KeyPackage seed secret with key_package_index 0. No KeyPackageUpload is sent for this operation. The batch consists of only this derivation and is closed immediately, and the batch operation_secret is deleted after the seed secret has been derived. Like any other LeafNode with leaf_node_source key_package, the creator LeafNode carries a lifetime (), and members validating the ratchet tree, including members joining later, may reject a leaf whose lifetime has expired. The creating emulator client SHOULD choose the lifetime according to the application's LeafNode validity policy, and the virtual client's leaf SHOULD be updated before the lifetime expires. Instead of choosing the initial epoch secret of the new group randomly as described in , the creating emulator client MUST derive it from the creator LeafNode's per-KeyPackage seed secret: DeriveSecret is computed with the higher-level group's ciphersuite, so that epoch_secret has the size KDF.Nh required by that group's key schedule. This makes the initial epoch state of the higher-level group derivable by every emulator client from the DerivationInfo on the creator LeafNode alone, and it makes retried group creation attempts, each of which consumes a fresh generation (see ), independent of one another. The application MUST fan out the initial group creation material to all current emulator clients before delivering any other content from that higher-level group to those emulator clients. The initial group creation material consists of the GroupInfo for the newly created group, the creator LeafNode or a ratchet_tree extension that contains it, and any application-defined context needed to associate the material with the higher-level group. An emulator client MUST process the initial group creation material before processing any other content from that higher-level group. This requirement parallels the one for external join material in . When processing initial group creation material, an emulator client verifies the GroupInfo as described in and the ratchet tree as described in , decrypts the DerivationInfo in the creator LeafNode, reconstructs the LeafNode's key material as described in , re-derives epoch_secret from the same per-KeyPackage seed secret, and uses it to initialize the higher-level group's epoch 0 state. If the GroupInfo cannot be verified using the resulting epoch state, the emulator client MUST reject the initial group creation material.

Virtual client actions Emulator clients sometimes need to communicate directly to operate the virtual client. The acting emulator client can attach such information to a Commit to the emulation group using the SafeAAD mechanism described in . The SafeAAD component data of the virtual_clients component is a VirtualClientAction struct. Sending an action as part of a Commit serves two purposes: First, the agreement on message ordering facilitated by the DS prevents concurrent conflicting actions by two or more emulator clients. Second, the action is authenticated as part of the Commit. The key_package_upload action is one way applications MAY use to communicate a KeyPackageUpload message () to the other emulator clients.

Creating and uploading KeyPackages When creating KeyPackages to upload, the creating emulator client derives one key_package operation secret using (epoch_id, leaf_index, generation, operation_type = key_package, operation_context = zero-length), as described in . The generation is the single key_package operation-ratchet generation used for the whole upload batch. For each KeyPackage, the creating emulator client chooses a key_package_index and derives the init_key_secret, signature_key_secret, and encryption_key_secret from the per-KeyPackage seed secret derived using that index. The key_package_index values in a KeyPackageUpload MUST be unique. Senders SHOULD use consecutive values starting at zero. The KeyPackage's LeafNode MUST contain a DerivationInfo as described in whose encrypted DerivationInfoTBE contains the same leaf_index, generation, and key_package_index values used for the KeyPackage. The generation value MUST be the generation reported in the corresponding KeyPackageUpload message, and the key_package_index field, which is present for KeyPackage LeafNodes, MUST be the value reported for this KeyPackageRef in that upload. The KeyPackageUpload closes the batch. After sending the KeyPackageUpload, the creating emulator client MUST NOT use the same operation generation to derive additional KeyPackages. After deriving all per-KeyPackage material needed for the upload, the creating emulator client MUST delete the batch operation_secret. To make other emulator clients aware of the new KeyPackages and allow them to process any corresponding Welcome messages, the creating client MUST send them a KeyPackageUpload message covering all new KeyPackages before making the KeyPackages available to other parties. ; uint32 leaf_index; uint32 generation; KeyPackageInfo key_package_info; } KeyPackageUpload; ]]>

key_package_ref: The hash reference of the generated KeyPackage computed as described in .
key_package_index: A sender-chosen counter used as KDF context to domain-separate this KeyPackage from the other KeyPackages in the same KeyPackageUpload. It MUST be unique within the KeyPackageUpload.
epoch_id: The epoch ID of the emulation group epoch used to derive the batch key_package operation secret
leaf_index: The leaf index of the emulator client in the emulation group that created the KeyPackages
generation: The single key_package operation-ratchet generation used to derive the batch operation secret for this KeyPackageUpload
key_package_info: The information required to re-derive the init_key_secret for each individual KeyPackage

Any emulator client that receives a KeyPackageUpload message MUST verify that the key_package_index values are unique within the upload and MUST reject the upload otherwise. The recipient uses epoch_id, leaf_index, generation, operation_type key_package, and the zero-length operation_context to derive the batch operation_secret. For each KeyPackageInfo, the recipient uses key_package_index to derive the per-KeyPackage seed secret, then derives the KeyPackage's init_key and LeafNode key material from that seed. If the recipient receives a Welcome, it can then check which init_key to use based on the KeyPackageRef. To preserve outstanding KeyPackage semantics, recipients MUST process the KeyPackageUpload at receipt time and retain per-KeyPackage material sufficient to process any Welcome for each listed KeyPackageRef. After deriving the per-KeyPackage material needed for the upload, recipients MUST delete the batch operation_secret. How the creating client sends the message to the other emulator clients is up to the application, as long as every current emulator client receives it before the KeyPackages are made available and emulator clients added later receive equivalent state via active_key_packages in NewEmulatorClientState. One way to send the message is the key_package_upload action described in .

Externally joining groups with the virtual client An emulator client that uses an external commit to join a group with the virtual client relies on the higher-level group's Commit sequencing rules to determine whether that external commit is accepted. When creating the Commit to join the group externally, it MUST generate the LeafNode and path as described in . An external Commit sent on behalf of the virtual client reaches the other emulator clients like any other virtual-client message (see ). If the virtual client was already a member of the group, the other emulator clients hold the group's state and process the external Commit like any other handshake message. If the virtual client externally joins a group it was not previously a member of, however, the other emulator clients hold no state for that group and cannot process the Commit by itself. For such groups, the application MUST make the external join material available to all current emulator clients before delivering any other content from that higher-level group to them. The external join material consists of the GroupInfo the joining emulator client used to create the external Commit, the ratchet tree of the corresponding epoch (unless it is carried in the GroupInfo's ratchet_tree extension), and any application-defined context needed to associate the material with the higher-level group. An emulator client MUST process the external join material together with the external Commit before processing any other content from that higher-level group. When processing external join material, an emulator client verifies the GroupInfo as described in and the ratchet tree as described in before applying the external Commit.

Sending PrivateMessages Given that MLS generates the encryption keys and nonces for PrivateMessages sequentially, but multiple emulator clients may send messages through the virtual client simultaneously, this can create a situation where encryption keys and nonces are reused inappropriately. Critically, if two emulator clients encrypt a message with both the same key and nonce simultaneously, this could compromise the message's confidentiality and integrity. Emulator clients MUST prevent this by computing the reuse_guard, as described below instead of sampling it randomly.

Small-Space PRP A small-space pseudorandom permutation (PRP) is a cryptographic algorithm that works similar to a block cipher, while also being able to adhere to format constraints. In particular, it is able to perform a pseudorandom permutation over an arbitrary input and output space. This document uses the FF1 mode from instantiated with AES-128 as the underlying block cipher, with radix 2, numeral strings of length 32, and an empty tweak. A 32-bit unsigned integer is mapped to a numeral string by listing its bits from most significant to least significant; the output numeral string is mapped back to a 32-bit unsigned integer in the same way. key is a 16-byte AES-128 key; input and output are 32-bit unsigned integers. Where a 4-byte value such as the reuse_guard is required, the integer is encoded in network byte order (big-endian).

Reuse Guard MLS clients typically generate the bytes for the reuse_guard randomly. When sending a message with a virtual client, however, emulator clients choose a random value x such that x modulo N_e is equal to leaf_index_e, where:

e is the emulation-group epoch that produced the active virtual-client LeafNode in the higher-level group, identified by the epoch_id field of that LeafNode's DerivationInfo (see ).
N_e is the number of leaf nodes in the emulation group's ratchet tree at epoch e, including blank leaves (see ). Since the ratchet tree is always full, N_e is a power of two, every member's leaf index is strictly less than N_e, and no two emulator clients share the same residue modulo N_e.
leaf_index_e is the encrypting emulator client's leaf index in the emulation group at epoch e.

They then calculate: ExpandWithLabel is computed with the emulation group's ciphersuite's algorithms. reuse_guard_secret is derived as described in for emulation-group epoch e, identified by the virtual client's currently active LeafNode in the higher-level group. key_schedule_nonce is the nonce provided by the key schedule for encrypting this message. prp_key is computed in a way that it is unique to the key-nonce pair and computable by all emulator clients (but nobody else). reuse_guard is computed in a way that it appears random to outside observers (in particular, it does not leak which emulator client sent the message), but two emulator clients will never generate the same value. Recipient emulator clients can use the reuse_guard to recover the sender's leaf index in the emulation group. Since they share reuse_guard_secret and have access to key_schedule_nonce for the message, they can re-derive prp_key and invert the PRP to obtain x = SmallSpacePRP.Decrypt(prp_key, reuse_guard). The encrypting emulator client's leaf index in the emulation group at epoch e is then leaf_index_e = x mod N_e, where e and N_e are determined as described above. This allows recipients to attribute application messages to specific emulator clients without revealing the sender to outside observers.

Coordinating ratchet generations with the DS The method discussed above for computing reuse_guard prevents emulator clients from ever reusing the same key-nonce pair, as this would compromise the message. However, it does not prevent different emulator clients from attempting to encrypt messages with the same key but different nonces. While this doesn't create any security issues, it is a functionality issue due to the MLS deletion schedule. Other higher level group members (or indeed emulator clients) will delete the encryption key after using it to decrypt the first message they receive and will be unable to decrypt subsequent messages. The best solution depends on whether the Delivery Service is strongly or eventually consistent . Emulator clients communicating with a strongly-consistent DS SHOULD prevent this issue by coordinating the use of individual ratchet generations for encryption through the DS. Emulator clients MAY send a generation ID to the DS whenever they fan out a private message. The generation ID is derived as follows. ; uint64 epoch; uint32 generation; RatchetType ratchet_type; } PrivateMessageContext; generation_id = ExpandWithLabel(generation_id_secret, "generation id", PrivateMessageContext, Kdf.Nh) ]]>

group_id is the group_id of the higher-level group in which the PrivateMessage is sent
epoch is the epoch of that higher-level group at which the PrivateMessage is sent
generation is the generation of the ratchet used for encryption
ratchet_type is the type of ratchet used to encrypt the PrivateMessage
ExpandWithLabel as defined in
generation_id_secret is derived as specified in for the emulation-group epoch identified by the active virtual-client LeafNode in the higher-level group
Kdf.Nh is from the emulation group's ciphersuite

Attaching the generation ID to the PrivateMessage allows the DS to detect collisions between generations per higher-level group, per higher-level group epoch and per ratchet type. Alternatively, devices communicating with an eventually-consistent DS may need to simply retain messages and encryption keys for a short period of time after sending, in case it becomes necessary to decrypt another device's message and re-encrypt and re-send their original message with another encryption key.

Emulation group management Managing the emulation group is more elaborate than performing simple MLS operations within it. When adding a new emulator client, there are several pieces of cryptographic state that need to be synchronized before the new emulator client can start using the virtual client. The emulator client can either get this state from another emulator client, or if all other emulator clients are offline, the emulator client can use a series of external joins to onboard itself.

Adding an emulator client A joining emulator client is added to the emulation group by a provisioning emulator client via the usual MLS Add + Welcome. The GroupInfo in that Welcome carries the virtual_clients component (), whose component data is a NewEmulatorClientState struct that provides the state the joining emulator client needs to act as the virtual client going forward. The NewEmulatorClientState struct contains the complete secret state of the virtual client. Emulator clients MUST include it only in GroupInfo objects that are encrypted as part of a Welcome message for the emulation group. In particular, a GroupInfo object published to enable external commits into the emulation group MUST NOT contain a virtual_clients component. How the joining emulator client subsequently becomes an active participant in each higher-level group is application-defined. This document specifies two variants that the provisioning emulator client MAY use on a per-group basis:

Variant A (provisioning state transfer). The provisioning emulator client includes, for each higher-level group, the key-schedule outputs of the current epoch together with the HPKE private keys on the virtual client's direct path. The joining emulator obtains the RatchetTree and the GroupContext from an application-defined source.
Variant B (external commit). The provisioning emulator client need only convey (or otherwise let the joining emulator client learn) which higher-level groups the virtual client is in. The joining emulator client acquires each group's current GroupInfo (from the DS or some other application-defined source) and performs an external commit into the group, evicting the virtual client's prior membership with a "resync" Remove proposal as described in .

Applications MAY mix the two variants across groups, for example using Variant A for groups where metadata hiding matters and Variant B elsewhere. Regardless of the variant chosen, a new emulator client MUST NOT send PrivateMessages in a higher-level group while the DerivationInfo of the active virtual-client LeafNode identifies an emulation-group epoch in which the new emulator client was not a member. The new emulator client has no leaf index at that epoch and therefore cannot compute the reuse_guard as described in . Before the new emulator client can send messages in such a higher-level group, the virtual-client LeafNode MUST be rotated (e.g., by a Commit with an update path or an Update proposal followed by a Commit) to a more recent emulation-group epoch in which the new emulator client is a member. Note that in higher-level groups where handshake messages are sent as PrivateMessages, the new emulator client cannot send this rotating Commit itself. In such groups, the rotation has to be performed by an existing emulator client, unless the application permits handshake messages framed as PublicMessages or external commits (Variant B), in which case the new emulator client can rotate the LeafNode itself.

Trade-offs

Observability. Variant A produces an ordinary Commit that looks like any other key rotation to the rest of the higher-level group. Variant B appears as the virtual client leaving and re-joining, which is visible to every member and may undermine metadata-hiding goals.
Forward secrecy. Variant A chains the new epoch from the previous epoch's init_secret. Variant B breaks the commit chain resulting in weaker forward secrecy at the point of onboarding.
Policy. Some applications may restrict external commits by policy leaving Variant A as the only option.
State transfer cost. Variant A requires O(log N) HPKE private keys and the current epoch's key-schedule state per higher-level group. Depending on group size and the group's ciphersuite, this may exceed the application's performance constraints. Variant B's overhead is constant per group.

State transfer protocol Applications MAY use the following structs to transfer group data to the newly added emulator client. ; struct { uint8 prefix_length; opaque prefix; opaque node_secret; } PPRFNode; struct { PPRFNode nodes; } PPRFState; enum { reserved(0), key_package(1), leaf_node(2), application(3), (255) } VirtualClientOperationType; struct { opaque epoch_id; VirtualClientOperationType operation_type; uint32 leaf_index; uint32 generation; opaque operation_context; opaque operation_secret; } RetainedOperationSecret; struct { uint32 node_index; opaque node_secret; } SecretTreeNodeState; enum { reserved(0), application(1), handshake(2), operation(3), (255) } SecretTreeRatchetType; struct { uint32 leaf_index; SecretTreeRatchetType ratchet_type; select (SecretTreeRatchetState.ratchet_type) { case application: struct{}; case handshake: struct{}; case operation: VirtualClientOperationType operation_type; }; uint32 generation; opaque next_secret; } SecretTreeRatchetState; struct { SecretTreeNodeState node_secrets; SecretTreeRatchetState ratchet_states; } SecretTreeState; struct { KeyPackageRef key_package_ref; opaque epoch_id; uint32 leaf_index; uint32 generation; uint32 key_package_index; } KeyPackageDerivationInfo; struct { opaque epoch_id; uint32 leaf_index; uint32 generation; uint32 key_package_index; opaque key_package_seed_secret; } RetainedKeyPackageMaterial; struct { opaque epoch_id; SecretTreeState operation_secret_tree; opaque epoch_encryption_key; opaque generation_id_secret; opaque reuse_guard_secret; } EmulationEpochState; enum { reserved(0), state_transfer(1), external_commit(2), (255) } HigherLevelGroupStateType; struct { opaque group_id; HigherLevelGroupStateType state_type; select (HigherLevelGroupState.state_type) { case state_transfer: opaque init_secret; opaque sender_data_secret; opaque exporter_secret; opaque external_secret; opaque confirmation_key; opaque membership_key; opaque resumption_psk; opaque epoch_authenticator; SecretTreeState secret_tree; PPRFState safe_exporter_tree; HPKEPrivateKey direct_path_private_keys; case external_commit: struct{}; }; } HigherLevelGroupState; struct { opaque signing_key_material; KeyPackageDerivationInfo active_key_packages; RetainedKeyPackageMaterial retained_key_package_material; RetainedOperationSecret retained_operation_secrets; EmulationEpochState past_emulation_epochs; HigherLevelGroupState higher_level_groups; } NewEmulatorClientState; ]]>

HPKEPrivateKey contains the serialized HPKE KEM private key for the ciphersuite used by the higher-level group.
PPRFState serializes a punctured binary-tree PRF as the set of retained subtree roots. The root of an unpunctured tree is represented by a PPRFNode with prefix_length 0 and a zero-length prefix. For other nodes, prefix contains the first prefix_length bits of the node's path, packed from most-significant bit to least-significant bit in each octet. Any unused low-order bits in the final octet MUST be zero. The entries in nodes MUST be prefix-free and sorted lexicographically by prefix.
RetainedOperationSecret carries an operation_secret whose operation ratchet generation has been deleted, but whose derived key material is still live. Entries are identified by (epoch_id, operation_type, leaf_index, generation, operation_context).
RetainedKeyPackageMaterial carries a key_package_seed_secret derived from a batch key_package operation secret whose operation-ratchet generation has been deleted. Entries are identified by (epoch_id, leaf_index, generation, key_package_index).
SecretTreeState serializes the retained state of an RFC 9420 Secret Tree. Each SecretTreeNodeState contains an unexpanded tree node secret and its RFC 9420 tree node index. Each SecretTreeRatchetState contains the current state of a derived per-leaf hash ratchet; next_secret is the ratchet secret that will be used to derive the secret for generation. The serialized state MUST contain exactly the retained secrets needed to continue the Secret Tree deletion schedule from the sender's current state and MUST NOT contain secrets that have already been deleted. The application and handshake ratchet types are used for higher-level MLS Secret Trees. The operation ratchet type is used for virtual-client operation secrets and includes an operation_type field that identifies the per-leaf operation-type ratchet.
signing_key_material is an application-defined blob that conveys whatever signing-key state the joining emulator client needs. For configurations where signing keys are derived from emulation-group secrets, it MAY be zero-length. See .
active_key_packages lists every KeyPackage the virtual client has outstanding. Each entry carries the KeyPackageRef together with the identifiers needed to find the corresponding per-KeyPackage material. When a Welcome arrives encrypted to one of these KeyPackages, the joining emulator client identifies the entry by KeyPackageRef, then finds the RetainedKeyPackageMaterial matching (epoch_id, leaf_index, generation, key_package_index).
retained_key_package_material contains per-KeyPackage seed secrets whose derived key material is still live, for example because the corresponding KeyPackage is outstanding or because the current LeafNode of a higher-level group was created from that seed secret. This includes the seed secret of the creator LeafNode of a higher-level group created by the virtual client (), for which no KeyPackage exists.
retained_operation_secrets contains retained operation_secret values whose derived key material is still live. This includes operation secrets for the current LeafNode of each higher-level group and operation secrets for sent but uncommitted LeafNodes or UpdatePaths. It MUST NOT include a key_package operation secret after that secret has been used to derive a KeyPackageUpload batch; KeyPackage-derived material is represented by retained_key_package_material. retained_operation_secrets is needed for the state_transfer onboarding variant because the joining emulator client needs to reconstruct still-live virtual-client artifacts whose operation-type ratchet generations have already been deleted. The external_commit onboarding variant does not require these retained operation secrets or retained KeyPackage material if it replaces every still-live virtual-client artifact derived from an earlier emulation-group epoch, including outstanding KeyPackages and active LeafNodes in higher-level groups.
past_emulation_epochs carries, for every emulation-group epoch still referenced by an active LeafNode, outstanding KeyPackage, or retained operation secret or retained KeyPackage material and not equal to the current epoch, the retained operation_secret_tree state, the epoch_encryption_key, the generation_id_secret, and the reuse_guard_secret. State for the current emulation-group epoch is not included here because the joining emulator client derives it from the emulation group's Welcome.
higher_level_groups contains one entry per active higher-level group. Each entry carries the group_id and a state_type identifying which variant applies.

For state_transfer entries, the provisioning emulator client transfers the current epoch's key-schedule outputs and the retained per-epoch state:

init_secret chains the joining emulator client's subsequent Commit into the next epoch (init_secret[n-1] combines with the new commit_secret to produce joiner_secret[n]).
sender_data_secret lets the joining emulator client decrypt sender data on incoming PrivateMessages.
exporter_secret lets the joining emulator client produce application-level exports from this epoch.
external_secret lets other parties perform further external commits from this epoch.
confirmation_key lets the joining emulator client verify the confirmation_tag on any incoming Commit.
membership_key lets the joining emulator client produce a valid membership_tag on its own Commit sent as PublicMessage.
resumption_psk lets the joining emulator client participate as an origin for resumption PSKs in this epoch, if the application uses them.
epoch_authenticator authenticates the current group state.
secret_tree is the current state of the group's Secret Tree, from which the joining emulator client derives per-member handshake- and application-ratchet keys needed to decrypt in-flight PrivateMessages.
safe_exporter_tree is the current punctured state of the Safe Exporter API's Exporter Tree if the higher-level group uses that API. If the group does not use the Safe Exporter API, nodes is zero-length. When used for a Safe Exporter Tree, PPRFState paths are 16-bit paths indexed by ComponentID.
direct_path_private_keys is the list of HPKE private keys for each non-blank node in the virtual client's filtered direct path. The first entry is the private key for the parent of the virtual client's leaf, if that node is non-blank. Subsequent entries correspond to the next non-blank nodes on the direct path toward the root, in order. The virtual client's leaf private key itself is derivable from the retained operation secret or retained per-KeyPackage material identified by the DerivationInfo on the current LeafNode and is therefore not transferred explicitly.

For external_commit entries, no additional per-group fields are included. The joining emulator client external-commits into the group — fetching the current GroupInfo from the DS or another application-defined source — and includes a Remove proposal for the virtual client's prior leaf to evict the old membership.

Joining externally Without another online emulator client to bootstrap from, a new emulator can join the emulation group externally. A prerequisite for this external join is that the new client has the ability to learn which groups the virtual client is in and to externally join those groups. If those prerequisites are met, the new client needs to follow these steps:

Obtain a fresh credential that other emulation clients will accept.
Perform an external join to the emulation group.
Replace the virtual client's active key material by performing the following steps. Before each step, generate, derive or otherwise obtain the necessary credential and private signature key material. Ensure that other emulation clients can obtain the private key material. Details on credential and signature key generation and distribution are left to the application. The application MAY derive signature key material based on the new emulation group epoch as described in .
1. Replace all active KeyPackages with new KeyPackages, generated from the new emulation group epoch.
2. Perform an external join to all of the groups that the virtual client is a member of, using LeafNodes generated from the new emulation group epoch (see ). Welcome messages which were unprocessed by the offline devices are discarded, and these groups are joined externally instead (potentially being queued for user approval first).

Removing emulator clients Since all emulator clients hold all key material of the virtual client, removing the emulator client entails its removal from the emulation group, as well as rotating or revoking any other virtual-client specific key material. To effectively remove an emulator client, the emulator client committing the proposal that removes the target client MUST take the following steps in order:

Effect the deletion of outstanding KeyPackages of the virtual client from the DS. The set of outstanding KeyPackages can be determined from the KeyPackageUpload messages previously sent to the emulation group (see ).
Commit a Remove proposal for the emulation client to be removed in the emulation group, advancing it to a new epoch from which new virtual client secrets will be derived (see ).

Next, the application MUST provide new credentials and authentication key material for the virtual client that can be used to replace existing credentials and authentication key material in the next steps. The application MAY use the signature_key_secret derived from any of the operation_secrets during the next steps to derive group-specific key material. The application SHOULD take steps to revoke any valid long lived credentials associated with the virtual client that the removed emulator client had access to. Using the new emulation-group epoch, any combination of remaining emulator clients MUST do the following steps (in any order):

Effect an update of the key material in every higher-level group in which the virtual client is a member, using the new credential and authentication key material provided by the application. This MAY be done by creating a commit with an update path or sending an update proposal.
Upload a fresh set of KeyPackages derived from the new emulation-group epoch to replace those deleted in step 1.

A corollary of this removal procedure is that in most scenarios another emulator client is required to be online and perform the necessary updates. The DS must also support deletion of previously uploaded KeyPackages. This is in contrast to the simple multi-client setup, where an external sender can effectively remove individual clients and there are no additional functional requirements for the DS.

Security Considerations

Trust between emulator clients All emulator clients hold the complete secret state of the virtual client. Compromise of a single emulator client is therefore equivalent to compromise of the virtual client itself: the attacker learns the current epoch secrets of the emulation group and of every higher-level group the virtual client is a member of, and can impersonate the virtual client in all of them. Emulator clients have to trust each other fully; the emulation group does not provide any isolation between them.

Forward secrecy Forward secrecy for messages in higher-level groups depends on every emulator client deleting consumed key material according to the deletion schedule of . Since every emulator client processes every message sent to a higher-level group, the effective forward secrecy of such a group is bounded by the emulator client that is slowest to advance its ratchets and delete its keys. The same applies to the retained state defined in this document: RetainedOperationSecrets, RetainedKeyPackageMaterial, and past emulation-epoch state () extend the window during which a compromise reveals previously transmitted data, and emulator clients SHOULD delete them as soon as they are no longer needed. Applications that do not onboard emulator clients through state transfer need not retain RetainedOperationSecrets or RetainedKeyPackageMaterial at all (see ) and can instead retain only the derived key material still in use, reducing this window. Some of this state is reachable through the virtual client's LeafNodes. The DerivationInfo of an external Commit LeafNode embeds the init_secret of the epoch created by that Commit, encrypted under the emulation-group epoch's epoch_encryption_key. The LeafNode persists in the higher-level group's ratchet tree until the virtual client's leaf is next updated, and emulator clients retain the epoch_encryption_key for as long as the LeafNode is active. Similarly, the initial epoch secret of a higher-level group created by the virtual client remains derivable from the retained per-KeyPackage seed secret of the creator LeafNode (). Updating the virtual client's LeafNode soon after an external commit or a group creation allows emulator clients to delete this state and limits the exposure.

Post-compromise security Some of the performance benefits of this scheme depend on the fact that one can update once in the emulation group and "re-use" the new randomness for updates in multiple higher-level groups. At that point, clients only really recover when they update the emulation group, i.e. re-using somewhat old randomness of the emulation group won't provide real PCS in higher-level groups.

Removing emulator clients The removal procedure in is not atomic. Between the Commit that removes an emulator client from the emulation group and the completion of the key rotations in all higher-level groups, the removed client still holds valid key material for those groups and can read traffic in them and impersonate the virtual client. Applications SHOULD minimize this window and SHOULD revoke any long-lived credentials the removed client had access to, since a removed client that retains a valid virtual-client signing key and credential can otherwise re-join a higher-level group via an external commit. The procedure also depends on the DS actually deleting outstanding KeyPackages; a KeyPackage that escapes deletion allows the removed client to process Welcome messages addressed to the virtual client.

Handling of NewEmulatorClientState The NewEmulatorClientState struct contains the complete secret state of the virtual client and MUST only appear in GroupInfo objects encrypted within Welcome messages, as specified in . Publishing a GroupInfo containing the virtual_clients component, for example to enable external commits, discloses the secrets of all higher-level groups the virtual client is a member of.

Reuse guard The guarantee that no two emulator clients select colliding nonces () relies on all emulator clients agreeing on N_e, on their leaf indices at epoch e, and on the exact PRP instantiation described in . The PRP key is 128 bits regardless of the ciphersuites in use; this bounds the sender-attribution and unlinkability properties of the reuse guard but does not affect the confidentiality of MLS messages. Since reuse_guard_secret is shared across all higher-level groups for a given emulation-group epoch, two messages in different higher-level groups that happen to use the same key_schedule_nonce will use the same prp_key. This does not endanger nonce uniqueness, because the two messages are encrypted under independent keys.

Privacy Considerations Using a virtual client masks the emulation group's membership and activity from other members of higher-level groups: additions, removals, and updates of emulator clients appear in higher-level groups as ordinary LeafNode updates of the virtual client, and PrivateMessages sent by different emulator clients are indistinguishable to other higher-level group members. Whether the identity of the emulator clients is hidden additionally depends on the design of the Authentication Service and on the credentials the application provisions for the virtual client. The following residual metadata remains observable:

The presence of a DerivationInfo in a LeafNode reveals to anyone with access to the ratchet tree (including a DS that distributes GroupInfo or ratchet trees) that the leaf belongs to a virtual client. Applications that want to hide which leaves are virtual would need non-virtual clients to include indistinguishable components.
The epoch_id in a DerivationInfo is identical across all LeafNodes and KeyPackages that the virtual client produces from the same emulation-group epoch. Parties with access to the ratchet trees of multiple higher-level groups can use it to link the virtual client's leaves across those groups, and can observe how often the emulation group commits.
Generation IDs () are only attached by virtual-client senders and therefore identify virtual-client traffic to the DS, in addition to revealing how many distinct ratchet generations are in use.
An external commit by a virtual client (Variant B of , or ) is visible to all members of the higher-level group as the virtual client leaving and re-joining.
The DerivationInfo ciphertext of a LeafNode with leaf_node_source commit is longer when it carries an external_init_secret. Parties with access to the ratchet tree, including members who join after the external commit, can therefore tell that the virtual client's current leaf was created by an external commit rather than a regular Commit, for as long as that leaf remains in the tree.

IANA Considerations This document requests the addition of a new value under the heading "Messaging Layer Security" in the "MLS Component Types" registry.

virtual_clients The component implementing the virtual client emulation defined in this document. The component data carried in each location is determined by the object in which the component appears:

In GroupInfo objects of the emulation group, the component data is a NewEmulatorClientState struct carrying the state a newly added emulator client needs to act as the virtual client ().
In LeafNode objects of a virtual client, the component data is a DerivationInfo struct communicating how to derive the LeafNode's key material ().
In SafeAAD objects of Commits in the emulation group, the component data is a VirtualClientAction struct communicating which virtual client action is taken in conjunction with the Commit ().
The component ID is additionally used with the Safe Exporter API to export the per-epoch emulator_epoch_secret from the emulation group ().

The requested registration is:

Value: 0x0006 (suggested)
Name: virtual_clients
Where: GI, LN, AD, ES
Recommended: Y