MLS Virtual Clients

4.1. 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 Section 6) 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.¶

4.2. 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.¶

5.1. 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 (Section 9), 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 only identifies 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 Section 12.4.3.2 of [RFC9420].¶

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 Section 6.5.2. 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.¶

opaque HPKEPrivateKey<V>;

struct {
  uint8 prefix_length;
  opaque prefix<V>;
  opaque node_secret<V>;
} PPRFNode;

struct {
  PPRFNode nodes<V>;
} PPRFState;

enum {
  reserved(0),
  key_package(1),
  leaf_node(2),
  application(3),
  (255)
} VirtualClientOperationType;

struct {
  opaque epoch_id<V>;
  VirtualClientOperationType operation_type;
  uint32 leaf_index;
  opaque random<V>;
  opaque operation_secret<V>;
} RetainedOperationSecret;

struct {
  uint32 node_index;
  opaque node_secret<V>;
} SecretTreeNodeState;

enum {
  reserved(0),
  application(1),
  handshake(2),
  (255)
} SecretTreeRatchetType;

struct {
  uint32 leaf_index;
  SecretTreeRatchetType ratchet_type;
  uint32 generation;
  opaque next_secret<V>;
} SecretTreeRatchetState;

struct {
  SecretTreeNodeState node_secrets<V>;
  SecretTreeRatchetState ratchet_states<V>;
} SecretTreeState;

struct {
  KeyPackageRef key_package_ref;
  opaque epoch_id<V>;
  uint32 leaf_index;
  opaque random<V>;
} KeyPackageDerivationInfo;

struct {
  opaque epoch_id<V>;
  PPRFState epoch_base_secret;
  opaque epoch_encryption_key<V>;
  opaque generation_id_secret<V>;
} EmulationEpochState;

enum {
  reserved(0),
  state_transfer(1),
  external_commit(2),
  (255)
} HigherLevelGroupStateType;

struct {
  opaque group_id<V>;
  HigherLevelGroupStateType state_type;
  select (HigherLevelGroupState.state_type) {
    case state_transfer:
      opaque init_secret<V>;
      opaque sender_data_secret<V>;
      opaque exporter_secret<V>;
      opaque external_secret<V>;
      opaque confirmation_key<V>;
      opaque membership_key<V>;
      opaque resumption_psk<V>;
      opaque epoch_authenticator<V>;
      SecretTreeState secret_tree;
      PPRFState safe_exporter_tree;
      HPKEPrivateKey direct_path_private_keys<V>;
    case external_commit:
      struct{};
  };
} HigherLevelGroupState;

struct {
  opaque signing_key_material<V>;
  KeyPackageDerivationInfo active_key_packages<V>;
  RetainedOperationSecret retained_operation_secrets<V>;
  EmulationEpochState past_emulation_epochs<V>;
  HigherLevelGroupState higher_level_groups<V>;
} NewEmulatorClientState;

5.2. 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:¶

1. Obtain a fresh credential that other emulation clients will accept.¶

2. Perform an external join to the emulation group.¶

3. 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 Section 6.2.¶

   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 Section 6.2). 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).¶

5.3. 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:¶

1. 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 Section 6.4.1).¶

2. 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 Section 6.2).¶

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.¶

6.1. 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.¶

6.2. 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 Section 4.4 of [I-D.ietf-mls-extensions], where virtual_clients_component_id is the component ID registered for the virtual_clients component in Section 9.¶

emulator_epoch_secret = SafeExportSecret(virtual_clients_component_id)

The emulator_epoch_secret is in turn used to derive four further secrets, after which it is deleted.¶

epoch_id =
  DeriveSecret(emulator_epoch_secret, "Epoch ID")
epoch_base_secret =
  DeriveSecret(emulator_epoch_secret, "Base Secret")
epoch_encryption_key =
  DeriveSecret(emulator_epoch_secret, "Encryption Key")
generation_id_secret =
  DeriveSecret(emulator_epoch_secret, "Generation ID Secret")

The epoch_base_secret keys a Virtual Client Secret Tree, a tree of secrets with the same structure as the Secret Tree defined in Section 9 of [RFC9420], with the following differences:¶

• A Virtual Client Secret Tree always has 2^256 leaves, so that the output of the emulation group's ciphersuite hash function can be used directly as a leaf index.¶

• The root of the tree is the epoch_base_secret rather than a secret derived from the encryption_secret of an MLS group epoch.¶

TODO: Re-evaluate the tree size. A smaller tree (e.g., 2^128 leaves with the leaf index derived by truncating pprf_input) is probably sufficient.¶

Secrets are derived from the Virtual Client Secret Tree as follows:¶

VirtualClientSecret(Input) = tree_node_[LeafIndex(Input)]_secret

Emulator clients MUST retain the (punctured) epoch_base_secret, the epoch_id, the epoch_encryption_key, the generation_id_secret, and the number of leaf nodes the emulation group's ratchet tree had at the corresponding epoch (including blank leaves, see Section 6.5.2) until no key material or generation IDs associated with that epoch are actively used anymore. For each retained epoch in which an emulator client was a member, it MUST also retain its own leaf index at that epoch. The epoch_base_secret, epoch_id, and epoch_encryption_key are used for deriving and processing virtual-client key material, including DerivationInfos and state transferred to new clients (Section 5.1); the generation_id_secret is used for computing generation IDs (Section 6.5.3); and the leaf count and leaf index are used for computing the reuse_guard (Section 6.5.2).¶

When deriving a secret for a virtual client, e.g. for use in a KeyPackage or LeafNode update, the deriving client chooses a VirtualClientOperationType, samples a random octet string random, and hashes it with its leaf index in the emulation group using the hash function of the emulation group's ciphersuite. The operation_type is key_package when creating a KeyPackage, including the KeyPackage's init_key and LeafNode key material. 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.¶

struct {
  uint32 leaf_index;
  VirtualClientOperationType operation_type;
  opaque random<V>;
} HashInput

pprf_input = Hash(HashInput)

The pprf_input is then used to derive an operation_secret.¶

operation_secret = VirtualClientSecret(pprf_input)

After deriving operation_secret, the emulator client MUST puncture pprf_input from the PPRF state for the corresponding epoch_base_secret. The emulator client MUST retain operation_secret as a RetainedOperationSecret until all key material derived from it is no longer active, and MUST delete operation_secret after that point.¶

Given an epoch_id, operation_type, random, and the leaf_index of the emulator client performing the virtual client operation, other emulator clients can derive the operation_secret, puncture the same pprf_input, and use the operation_secret to perform the same operation. If the PPRF input has already been punctured, for example because the emulator client joined after the operation took place, the emulator client uses the corresponding RetainedOperationSecret transferred in NewEmulatorClientState.¶

Depending on the operation, the acting emulator client will have to derive one or more secrets from the operation_secret.¶

There are five types of MLS-related secrets that can be derived from an operation_secret.¶

• 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¶

• reuse_guard_secret: Used to derive the PRP key for reuse_guard values when the virtual client sends a PrivateMessage (see Section 6.5.2)¶

signature_key_secret =
  DeriveSecret(operation_secret, "Signature Key")

encryption_key_secret =
  DeriveSecret(operation_secret, "Encryption Key")

init_key_secret =
  DeriveSecret(operation_secret, "Init Key")

path_generation_secret =
  DeriveSecret(operation_secret, "Path Generation")

reuse_guard_secret =
  DeriveSecret(operation_secret, "Reuse Guard")

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 Section 5.1) carries the state a new emulator client needs; its contents are application-defined.¶

6.3. Creating LeafNodes and UpdatePaths

When creating a LeafNode, either for 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 Section Section 6.2. For a LeafNode in a KeyPackage, the creating emulator client MUST use the same key_package operation secret used to derive the KeyPackage's init_key_secret. For other LeafNodes, 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.¶

struct {
  opaque epoch_id<V>;
  opaque ciphertext<V>;
} DerivationInfo

struct {
  uint32 leaf_index;
  opaque random<V>;
} DerivationInfoTBE

The ciphertext is the serialized DerivationInfoTBE encrypted with the AEAD scheme of the emulation group's ciphersuite, with the epoch_id as AAD. 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:¶

derivation_info_key = ExpandWithLabel(epoch_encryption_key, "key",
                                      encryption_key, AEAD.Nk)
derivation_info_nonce = ExpandWithLabel(epoch_encryption_key, "nonce",
                                        encryption_key, AEAD.Nn)

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.¶

The operation type used to derive the LeafNode's operation_secret is determined by the LeafNode's leaf_node_source (Section 7.2 of [RFC9420]): key_package maps to key_package, while update and commit map to leaf_node. The leaf_index and random fields MUST be the values used with that operation type to derive the LeafNode's operation_secret.¶

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 random fields, as inputs to the operation secret derivation described in Section 6.2.¶

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 (Section 6.5.2) or a generation ID (Section 6.5.3), MUST use the emulation-group epoch identified by the active virtual-client LeafNode.¶

6.4. Virtual client actions

There are two occasions where emulator clients need to communicate directly to operate the virtual client. In both cases, the acting emulator client sends a Commit to the emulation group before taking an action with the virtual client.¶

The 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 acting emulator client can attach additional information to the Commit using the SafeAAD mechanism described in Section 4.9 of [I-D.ietf-mls-extensions]. The SafeAAD component data of the virtual_clients component is a VirtualClientAction struct.¶

enum {
  reserved(0),
  key_package_upload(1),
  external_join(2),
  (255)
} ActionType;

struct {
  ActionType action_type;
  select (VirtualClientAction.action_type) {
    case key_package_upload:
      KeyPackageUpload key_package_upload;
    case external_join:
      ExternalJoin external_join;
  };
} VirtualClientAction;

struct {
  KeyPackageRef key_package_ref;
  opaque random<V>;
} KeyPackageInfo

struct {
  opaque epoch_id<V>;
  KeyPackageInfo key_package_info<V>;
} KeyPackageUpload

struct {
  opaque group_id<V>;
} ExternalJoin

6.5. 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.¶

output = SmallSpacePRP.Encrypt(key, input)
input = SmallSpacePRP.Decrypt(key, output)

prp_key = ExpandWithLabel(reuse_guard_secret, "reuse guard",
                          key_schedule_nonce, 16)
reuse_guard = SmallSpacePRP.Encrypt(prp_key, x)

enum {
  reserved(0),
  application(1),
  handshake(2),
  (255)
} RatchetType

struct {
  opaque group_id<V>;
  uint64 epoch;
  uint32 generation;
  RatchetType ratchet_type;
} PrivateMessageContext

generation_id = ExpandWithLabel(generation_id_secret, "generation id",
                      PrivateMessageContext, Kdf.Nh)

7.1. 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.¶

7.2. 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 Section 9.2 of [RFC9420]. 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 and past emulation-epoch state (Section 5.1) 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.¶

7.3. 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.¶

7.4. Removing emulator clients

The removal procedure in Section 5.3 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.¶

7.5. 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 Section 5.1. 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.¶

7.6. Reuse guard

The guarantee that no two emulator clients select colliding nonces (Section 6.5.2) relies on all emulator clients agreeing on N_e, on their leaf indices at epoch e, and on the exact PRP instantiation described in Section 6.5.1. 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.¶

9.1. 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 (Section 5.1).¶

• In LeafNode objects of a virtual client, the component data is a DerivationInfo struct communicating how to derive the LeafNode's key material (Section 6.3).¶

• 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 (Section 6.4).¶

• The component ID is additionally used with the Safe Exporter API to export the per-epoch emulator_epoch_secret from the emulation group (Section 6.2).¶

The requested registration is:¶

• Value: 0x0006 (suggested)¶

• Name: virtual_clients¶

• Where: GI, LN, AD, ES¶

• Recommended: Y¶