openmls/group/mls_group/

mod.rs

//! MLS Group
//!
//! This module contains [`MlsGroup`] and its submodules.
//!

use past_secrets::MessageSecretsStore;
use proposal_store::ProposalQueue;
use serde::{Deserialize, Serialize};
use tls_codec::Serialize as _;

#[cfg(test)]
use crate::treesync::node::leaf_node::TreePosition;

use super::proposal_store::{ProposalStore, QueuedProposal};
use crate::{
    binary_tree::array_representation::LeafNodeIndex,
    ciphersuite::{hash_ref::ProposalRef, signable::Signable},
    credentials::Credential,
    error::LibraryError,
    extensions::Extensions,
    framing::{mls_auth_content::AuthenticatedContent, *},
    group::{
        CreateGroupContextExtProposalError, DeletePastEpochSecretsError, Extension, ExtensionType,
        ExternalPubExtension, GroupContext, GroupEpoch, GroupId, MlsGroupJoinConfig,
        MlsGroupStateError, OutgoingWireFormatPolicy, PublicGroup, RatchetTreeExtension,
        RequiredCapabilitiesExtension, SetPastEpochDeletionPolicyError, StagedCommit,
    },
    key_packages::{InitKey, KeyPackageBundle},
    messages::{
        group_info::{GroupInfo, GroupInfoTBS, VerifiableGroupInfo},
        proposals::*,
        ConfirmationTag, GroupSecrets, Welcome,
    },
    schedule::{
        message_secrets::MessageSecrets,
        psk::{load_psks, store::ResumptionPskStore, PskSecret},
        GroupEpochSecrets, JoinerSecret, KeySchedule,
    },
    storage::{OpenMlsProvider, StorageProvider},
    treesync::{
        node::{encryption_keys::EncryptionKeyPair, leaf_node::LeafNode},
        RatchetTree, TreeSync,
    },
    versions::ProtocolVersion,
};
use openmls_traits::{
    crypto::OpenMlsCrypto, signatures::Signer, storage::StorageProvider as _, types::Ciphersuite,
};

#[cfg(feature = "extensions-draft")]
use crate::schedule::{application_export_tree::ApplicationExportTree, ApplicationExportSecret};

// Private
mod application;
mod exporting;
mod updates;

#[cfg(feature = "migration-import")]
pub(crate) mod migration_import;

#[cfg(feature = "virtual-clients-draft")]
pub use application::UnconfirmedMessage;
pub use proposal::Propose;

use config::*;

// Crate
pub(crate) mod builder;
pub(crate) mod commit_builder;
pub(crate) mod config;
pub(crate) mod creation;
pub(crate) mod errors;
pub(crate) mod membership;
pub(crate) mod past_secrets;
pub(crate) mod processing;
pub(crate) mod proposal;
pub(crate) mod proposal_store;
pub(crate) mod staged_commit;

#[cfg(feature = "extensions-draft")]
pub(crate) mod app_ephemeral;

#[cfg(feature = "targeted-messages-draft")]
mod targeted_messages;

// Tests
#[cfg(test)]
pub(crate) mod tests_and_kats;

#[derive(Debug)]
pub(crate) struct CreateCommitResult {
    pub(crate) commit: AuthenticatedContent,
    pub(crate) welcome_option: Option<Welcome>,
    pub(crate) staged_commit: StagedCommit,
    pub(crate) group_info: Option<GroupInfo>,
}

/// A member in the group is identified by this [`Member`] struct.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Member {
    /// The member's leaf index in the ratchet tree.
    pub index: LeafNodeIndex,
    /// The member's credential.
    pub credential: Credential,
    /// The member's public HPHKE encryption key.
    pub encryption_key: Vec<u8>,
    /// The member's public signature key.
    pub signature_key: Vec<u8>,
}

impl Member {
    /// Create new member.
    pub fn new(
        index: LeafNodeIndex,
        encryption_key: Vec<u8>,
        signature_key: Vec<u8>,
        credential: Credential,
    ) -> Self {
        Self {
            index,
            encryption_key,
            signature_key,
            credential,
        }
    }
}

/// Pending Commit state. Differentiates between Commits issued by group members
/// and External Commits.
#[derive(Debug, Serialize, Deserialize)]
#[cfg_attr(any(test, feature = "test-utils"), derive(Clone, PartialEq))]
pub enum PendingCommitState {
    /// Commit from a group member
    Member(StagedCommit),
    /// Commit from an external joiner
    External(StagedCommit),
}

impl PendingCommitState {
    /// Returns a reference to the [`StagedCommit`] contained in the
    /// [`PendingCommitState`] enum.
    pub(crate) fn staged_commit(&self) -> &StagedCommit {
        match self {
            PendingCommitState::Member(pc) => pc,
            PendingCommitState::External(pc) => pc,
        }
    }
}

impl From<PendingCommitState> for StagedCommit {
    fn from(pcs: PendingCommitState) -> Self {
        match pcs {
            PendingCommitState::Member(pc) => pc,
            PendingCommitState::External(pc) => pc,
        }
    }
}

/// [`MlsGroupState`] determines the state of an [`MlsGroup`]. The different
/// states and their transitions are as follows:
///
/// * [`MlsGroupState::Operational`]: This is the main state of the group, which
///   allows access to all of its functionality, (except merging pending commits,
///   see the [`MlsGroupState::PendingCommit`] for more information) and it's the
///   state the group starts in (except when created via
///   [`MlsGroup::external_commit_builder()`], see the functions documentation for
///   more information). From this `Operational`, the group state can either
///   transition to [`MlsGroupState::Inactive`], when it processes a commit that
///   removes this client from the group, or to [`MlsGroupState::PendingCommit`],
///   when this client creates a commit.
///
/// * [`MlsGroupState::Inactive`]: A group can enter this state from any other
///   state when it processes a commit that removes this client from the group.
///   This is a terminal state that the group can not exit from. If the clients
///   wants to re-join the group, it can either be added by a group member or it
///   can join via external commit.
///
/// * [`MlsGroupState::PendingCommit`]: This state is split into two possible
///   sub-states, one for each Commit type:
///   [`PendingCommitState::Member`] and [`PendingCommitState::External`]:
///
///   * If the client creates a commit for this group, the `PendingCommit` state
///     is entered with [`PendingCommitState::Member`] and with the [`StagedCommit`] as
///     additional state variable. In this state, it can perform the same
///     operations as in the [`MlsGroupState::Operational`], except that it cannot
///     create proposals or commits. However, it can merge or clear the stored
///     [`StagedCommit`], where both actions result in a transition to the
///     [`MlsGroupState::Operational`]. Additionally, if a commit from another
///     group member is processed, the own pending commit is also cleared and
///     either the `Inactive` state is entered (if this client was removed from
///     the group as part of the processed commit), or the `Operational` state is
///     entered.
///
///   * A group can enter the [`PendingCommitState::External`] sub-state only as
///     the initial state when the group is created via
///     [`MlsGroup::external_commit_builder()`]. In contrast to the
///     [`PendingCommitState::Member`] `PendingCommit` state, the only possible
///     functionality that can be used is the [`MlsGroup::merge_pending_commit()`]
///     function, which merges the pending external commit and transitions the
///     state to [`MlsGroupState::PendingCommit`]. For more information on the
///     external commit process, see [`MlsGroup::external_commit_builder()`] or
///     Section 11.2.1 of the MLS specification.
#[derive(Debug, Serialize, Deserialize)]
#[cfg_attr(any(test, feature = "test-utils"), derive(Clone, PartialEq))]
pub enum MlsGroupState {
    /// There is currently a pending Commit that hasn't been merged yet.
    PendingCommit(Box<PendingCommitState>),
    /// The group state is in an opertaional state, where new messages and Commits can be created.
    Operational,
    /// The group is inactive because the member has been removed.
    Inactive,
}

/// A `MlsGroup` represents an MLS group with a high-level API. The API exposes
/// high level functions to manage a group by adding/removing members, get the
/// current member list, etc.
///
/// The API is modeled such that it can serve as a direct interface to the
/// Delivery Service. Functions that modify the public state of the group will
/// return a `Vec<MLSMessageOut>` that can be sent to the Delivery Service
/// directly. Conversely, incoming messages from the Delivery Service can be fed
/// into [process_message()](`MlsGroup::process_message()`).
///
/// An `MlsGroup` has an internal queue of pending proposals that builds up as
/// new messages are processed. When creating proposals, those messages are not
/// automatically appended to this queue, instead they have to be processed
/// again through [process_message()](`MlsGroup::process_message()`). This
/// allows the Delivery Service to reject them (e.g. if they reference the wrong
/// epoch).
///
/// If incoming messages or applied operations are semantically or syntactically
/// incorrect, an error event will be returned with a corresponding error
/// message and the state of the group will remain unchanged.
///
/// An `MlsGroup` has an internal state variable determining if it is active or
/// inactive, as well as if it has a pending commit. See [`MlsGroupState`] for
/// more information.
#[derive(Debug)]
#[cfg_attr(feature = "migration-import", derive(serde::Deserialize))]
#[cfg_attr(
    all(feature = "migration-import", feature = "test-utils"),
    derive(serde::Serialize)
)]
#[cfg_attr(feature = "test-utils", derive(Clone, PartialEq))]
pub struct MlsGroup {
    /// The group configuration. See [`MlsGroupJoinConfig`] for more information.
    mls_group_config: MlsGroupJoinConfig,
    /// The public state of the group.
    public_group: PublicGroup,
    /// Epoch-specific secrets of the group.
    group_epoch_secrets: GroupEpochSecrets,
    /// The own leaf index in the ratchet tree.
    own_leaf_index: LeafNodeIndex,
    /// A [`MessageSecretsStore`] that stores message secrets.
    /// By default this store has the length of 1, i.e. only the [`MessageSecrets`]
    /// of the current epoch is kept.
    /// If more secrets from past epochs should be kept in order to be
    /// able to decrypt application messages from previous epochs, the size of
    /// the store must be increased through [`max_past_epochs()`].
    message_secrets_store: MessageSecretsStore,
    // Resumption psk store. This is where the resumption psks are kept in a rollover list.
    resumption_psk_store: ResumptionPskStore,
    // Own [`LeafNode`]s that were created for update proposals and that
    // are needed in case an update proposal is committed by another group
    // member. The vector is emptied after every epoch change.
    own_leaf_nodes: Vec<LeafNode>,
    // Additional authenticated data (AAD) for the next outgoing message. This
    // is ephemeral and will be reset by every API call that successfully
    // returns an [`MlsMessageOut`].
    aad: Vec<u8>,
    // Safe AAD items to attach to the next outgoing message. Ephemeral, reset
    // alongside `aad`. Only consulted when the group's GroupContext requires
    // Safe AAD framing.
    #[cfg(feature = "extensions-draft")]
    // Migration bridge:
    // absent from older serializations, so default it to empty on import — it is
    // ephemeral, so a freshly migrated group has nothing staged anyway.
    #[cfg_attr(
        feature = "migration-import",
        serde(default = "crate::framing::SafeAad::empty")
    )]
    safe_aad: SafeAad,
    // A variable that indicates the state of the group. See [`MlsGroupState`]
    // for more information.
    group_state: MlsGroupState,
    /// The state of the Application Exporter. See the MLS Extensions Draft 08
    /// for more information. This is `None` if an old OpenMLS group state was
    /// loaded and has not yet merged a commit.
    #[cfg(feature = "extensions-draft")]
    // Migration bridge (see the note on the struct): absent when migrating in a
    // group from a version that did not have `extensions-draft`, so default it to
    // `None` on import — it initializes on the next merged commit.
    #[cfg_attr(feature = "migration-import", serde(default))]
    application_export_tree: Option<ApplicationExportTree>,
    /// Whether this group is an emulation group of a virtual client. Not
    /// persisted on its own: [`MlsGroup::load`] recovers it from the presence of
    /// the group's derivation-epoch registration record.
    #[cfg(feature = "virtual-clients-draft")]
    // Migration bridge (see the note on the struct): a group migrated in from a
    // version without `virtual-clients-draft` is never an emulation group.
    #[cfg_attr(feature = "migration-import", serde(default))]
    emulation_group: bool,
}

impl MlsGroup {
    // === Configuration ===

    /// Returns the configuration.
    pub fn configuration(&self) -> &MlsGroupJoinConfig {
        &self.mls_group_config
    }

    /// Sets the configuration.
    pub fn set_configuration<Storage: StorageProvider>(
        &mut self,
        storage: &Storage,
        mls_group_config: &MlsGroupJoinConfig,
    ) -> Result<(), Storage::Error> {
        let policy_changed = self.mls_group_config.past_epoch_deletion_policy()
            != mls_group_config.past_epoch_deletion_policy();

        self.mls_group_config = mls_group_config.clone();
        storage.write_mls_join_config(self.group_id(), mls_group_config)?;

        if policy_changed {
            // Resize the store to adhere to the new policy.
            self.resize_message_secrets_store(mls_group_config.past_epoch_deletion_policy());
            storage.write_message_secrets(self.group_id(), &self.message_secrets_store)?;
        }

        Ok(())
    }

    /// Sets the additional authenticated data (AAD) for the next outgoing
    /// message. This is ephemeral and will be reset by every API call that
    /// successfully returns an [`MlsMessageOut`].
    pub fn set_aad(&mut self, aad: Vec<u8>) {
        self.aad = aad;
    }

    /// Returns the additional authenticated data (AAD) for the next outgoing
    /// message.
    pub fn aad(&self) -> &[u8] {
        &self.aad
    }

    /// Stage Safe AAD items for the next outgoing message. Items must be
    /// sorted by [`ComponentId`] in strictly-increasing order and contain no
    /// duplicates; otherwise the call fails and the previously staged items
    /// are left untouched.
    ///
    /// Ephemeral, like [`Self::set_aad`]: cleared whenever an outgoing message
    /// is produced.
    ///
    /// [`ComponentId`]: crate::component::ComponentId
    #[cfg(feature = "extensions-draft")]
    pub fn set_safe_aad(&mut self, items: Vec<SafeAadItem>) -> Result<(), SafeAadError> {
        self.safe_aad = SafeAad::from_items(items)?;
        Ok(())
    }

    /// Returns the currently staged Safe AAD items for the next outgoing
    /// message.
    #[cfg(feature = "extensions-draft")]
    pub fn safe_aad_items(&self) -> &[SafeAadItem] {
        self.safe_aad.items()
    }

    // === Advanced functions ===

    /// Returns the group's ciphersuite.
    pub fn ciphersuite(&self) -> Ciphersuite {
        self.public_group.ciphersuite()
    }

    /// Get confirmation tag.
    pub fn confirmation_tag(&self) -> &ConfirmationTag {
        self.public_group.confirmation_tag()
    }

    /// Returns whether the own client is still a member of the group or if it
    /// was already evicted
    pub fn is_active(&self) -> bool {
        !matches!(self.group_state, MlsGroupState::Inactive)
    }

    /// Returns own credential. If the group is inactive, it returns a
    /// `UseAfterEviction` error.
    pub fn credential(&self) -> Result<&Credential, MlsGroupStateError> {
        if !self.is_active() {
            return Err(MlsGroupStateError::UseAfterEviction);
        }
        self.public_group
            .leaf(self.own_leaf_index())
            .map(|node| node.credential())
            .ok_or_else(|| LibraryError::custom("Own leaf node missing").into())
    }

    /// Returns the leaf index of the client in the tree owning this group.
    pub fn own_leaf_index(&self) -> LeafNodeIndex {
        self.own_leaf_index
    }

    /// Returns the leaf node of the client in the tree owning this group.
    pub fn own_leaf_node(&self) -> Option<&LeafNode> {
        self.public_group().leaf(self.own_leaf_index())
    }

    /// Returns the group ID.
    pub fn group_id(&self) -> &GroupId {
        self.public_group.group_id()
    }

    /// Returns the epoch.
    pub fn epoch(&self) -> GroupEpoch {
        self.public_group.group_context().epoch()
    }

    /// Returns an `Iterator` over pending proposals.
    pub fn pending_proposals(&self) -> impl Iterator<Item = &QueuedProposal> {
        self.proposal_store().proposals()
    }

    /// Returns the current tree state of the group, in the form of a [`TreeSync`].
    pub fn treesync(&self) -> &TreeSync {
        self.public_group.treesync()
    }

    /// Returns a reference to the [`StagedCommit`] of the most recently created
    /// commit. If there was no commit created in this epoch, either because
    /// this commit or another commit was merged, it returns `None`.
    pub fn pending_commit(&self) -> Option<&StagedCommit> {
        match self.group_state {
            MlsGroupState::PendingCommit(ref pending_commit_state) => {
                Some(pending_commit_state.staged_commit())
            }
            MlsGroupState::Operational => None,
            MlsGroupState::Inactive => None,
        }
    }

    /// Sets the `group_state` to [`MlsGroupState::Operational`], thus clearing
    /// any potentially pending commits.
    ///
    /// Note that this has no effect if the group was created through an external commit and
    /// the resulting external commit has not been merged yet. For more
    /// information, see [`MlsGroup::external_commit_builder()`].
    ///
    /// Use with caution! This function should only be used if it is clear that
    /// the pending commit will not be used in the group. In particular, if a
    /// pending commit is later accepted by the group, this client will lack the
    /// key material to encrypt or decrypt group messages.
    pub fn clear_pending_commit<Storage: StorageProvider>(
        &mut self,
        storage: &Storage,
    ) -> Result<(), Storage::Error> {
        match self.group_state {
            MlsGroupState::PendingCommit(ref pending_commit_state) => {
                if let PendingCommitState::Member(_) = **pending_commit_state {
                    self.group_state = MlsGroupState::Operational;
                    storage.write_group_state(self.group_id(), &self.group_state)
                } else {
                    Ok(())
                }
            }
            MlsGroupState::Operational | MlsGroupState::Inactive => Ok(()),
        }
    }

    /// Clear the pending proposals, if the proposal store is not empty.
    ///
    /// Warning: Once the pending proposals are cleared it will be impossible to process
    /// a Commit message that references those proposals. Only use this
    /// function as a last resort, e.g. when a call to
    /// `MlsGroup::commit_to_pending_proposals` fails.
    pub fn clear_pending_proposals<Storage: StorageProvider>(
        &mut self,
        storage: &Storage,
    ) -> Result<(), Storage::Error> {
        // If the proposal store is not empty...
        if !self.proposal_store().is_empty() {
            // Empty the proposal store
            self.proposal_store_mut().empty();

            // Clear proposals in storage
            storage.clear_proposal_queue::<GroupId, ProposalRef>(self.group_id())?;
        }

        Ok(())
    }

    /// Get a reference to the group context [`Extensions`] of this [`MlsGroup`].
    pub fn extensions(&self) -> &Extensions<GroupContext> {
        self.public_group().group_context().extensions()
    }

    /// Returns the index of the sender of a staged, external commit.
    pub fn ext_commit_sender_index(
        &self,
        commit: &StagedCommit,
    ) -> Result<LeafNodeIndex, LibraryError> {
        self.public_group().ext_commit_sender_index(commit)
    }

    // === Storage Methods ===

    /// Loads the state of the group with given id from persisted state.
    pub fn load<Storage: crate::storage::StorageProvider>(
        storage: &Storage,
        group_id: &GroupId,
    ) -> Result<Option<MlsGroup>, Storage::Error> {
        let public_group = PublicGroup::load(storage, group_id)?;
        let group_epoch_secrets = storage.group_epoch_secrets(group_id)?;
        let own_leaf_index = storage.own_leaf_index(group_id)?;
        let message_secrets_store = storage.message_secrets(group_id)?;
        let resumption_psk_store = storage.resumption_psk_store(group_id)?;
        let mls_group_config = storage.mls_group_join_config(group_id)?;
        let own_leaf_nodes = storage.own_leaf_nodes(group_id)?;
        let group_state = storage.group_state(group_id)?;
        #[cfg(feature = "extensions-draft")]
        let application_export_tree = storage.application_export_tree(group_id)?;
        // A group has a derivation-epoch registration record for exactly as long
        // as it is an emulation group. The record is written by the initial
        // registration at creation or Welcome join and removed by `delete`.
        #[cfg(feature = "virtual-clients-draft")]
        let emulation_group =
            crate::components::vc_derivation_info::newest_vc_derivation_epoch(storage, group_id)?
                .is_some();

        let build = || -> Option<Self> {
            Some(Self {
                public_group: public_group?,
                group_epoch_secrets: group_epoch_secrets?,
                own_leaf_index: own_leaf_index?,
                message_secrets_store: message_secrets_store?,
                resumption_psk_store: resumption_psk_store?,
                mls_group_config: mls_group_config?,
                own_leaf_nodes,
                aad: vec![],
                #[cfg(feature = "extensions-draft")]
                safe_aad: SafeAad::empty(),
                group_state: group_state?,
                #[cfg(feature = "extensions-draft")]
                application_export_tree,
                #[cfg(feature = "virtual-clients-draft")]
                emulation_group,
            })
        };

        Ok(build())
    }

    /// Remove the persisted state of this group from storage. Note that
    /// signature key material is not managed by OpenMLS and has to be removed
    /// from the storage provider separately (if desired).
    pub fn delete<Storage: crate::storage::StorageProvider>(
        &mut self,
        storage: &Storage,
    ) -> Result<(), Storage::Error> {
        PublicGroup::delete(storage, self.group_id())?;
        storage.delete_own_leaf_index(self.group_id())?;
        storage.delete_group_epoch_secrets(self.group_id())?;
        storage.delete_message_secrets(self.group_id())?;
        storage.delete_all_resumption_psk_secrets(self.group_id())?;
        storage.delete_group_config(self.group_id())?;
        storage.delete_own_leaf_nodes(self.group_id())?;
        storage.delete_group_state(self.group_id())?;
        storage.clear_proposal_queue::<GroupId, ProposalRef>(self.group_id())?;

        #[cfg(feature = "extensions-draft")]
        storage.delete_application_export_tree::<_, ApplicationExportTree>(self.group_id())?;

        // Drop this group's derivation-epoch bindings and its registration
        // record. `VcDerivationEpochState` and the operation secret tree are
        // keyed on the derivation epoch and may still be referenced by other
        // higher-level groups, so they're not deleted here.
        #[cfg(feature = "virtual-clients-draft")]
        {
            storage.delete_vc_emulation_bindings(self.group_id())?;
            storage.delete_registered_vc_derivation_epoch(self.group_id())?;
        }

        self.proposal_store_mut().empty();
        storage.delete_encryption_epoch_key_pairs(
            self.group_id(),
            &self.epoch(),
            self.own_leaf_index().u32(),
        )?;

        Ok(())
    }

    // === Extensions ===

    /// Exports the Ratchet Tree.
    pub fn export_ratchet_tree(&self) -> RatchetTree {
        self.public_group().export_ratchet_tree()
    }
}

/// Error resolving the [`VcDerivationEpochState`] bound to a group at a given
/// epoch via [`MlsGroup::vc_derivation_state_at_epoch`]. Callers map it to
/// their own error type.
///
/// [`VcDerivationEpochState`]: crate::components::vc_derivation_info::VcDerivationEpochState
#[cfg(feature = "virtual-clients-draft")]
#[derive(thiserror::Error, Debug, PartialEq, Clone)]
pub(crate) enum VcDerivationStateError<StorageError> {
    /// Reading the binding or the derivation-epoch state from storage failed.
    #[error("Error reading the binding or derivation-epoch state from storage: {0}")]
    Storage(StorageError),
    /// The group is bound to a derivation epoch, but its state is missing.
    #[error("The group is bound to a derivation epoch, but its state is missing.")]
    MissingDerivationEpochState,
}

// Crate-public functions
impl MlsGroup {
    /// Get the required capabilities extension of this group.
    pub(crate) fn required_capabilities(&self) -> Option<&RequiredCapabilitiesExtension> {
        self.public_group.required_capabilities()
    }

    /// Get a reference to the group epoch secrets from the group
    pub(crate) fn group_epoch_secrets(&self) -> &GroupEpochSecrets {
        &self.group_epoch_secrets
    }

    /// Get a reference to the message secrets from a group
    pub(crate) fn message_secrets(&self) -> &MessageSecrets {
        self.message_secrets_store.message_secrets()
    }

    /// Sets the size of the [`MessageSecretsStore`], i.e. the number of past
    /// epochs to keep.
    /// This allows application messages from previous epochs to be decrypted.
    pub(crate) fn resize_message_secrets_store(&mut self, policy: &PastEpochDeletionPolicy) {
        self.message_secrets_store.resize(policy);
    }

    /// Get the past epoch secret deletion policy for the group.
    pub fn past_epoch_deletion_policy(&self) -> &PastEpochDeletionPolicy {
        self.mls_group_config.past_epoch_deletion_policy()
    }

    /// Set the past epoch secret deletion policy for the group.
    pub fn set_past_epoch_deletion_policy<Provider: OpenMlsProvider>(
        &mut self,
        provider: &Provider,
        policy: PastEpochDeletionPolicy,
    ) -> Result<(), SetPastEpochDeletionPolicyError<Provider::StorageError>> {
        // resize the store
        self.resize_message_secrets_store(&policy);

        // set the policy on the join config
        self.mls_group_config.past_epoch_deletion_policy = policy;

        // persist the join config
        provider
            .storage()
            .write_mls_join_config(self.group_id(), &self.mls_group_config)?;

        // update the message secrets store in storage
        provider
            .storage()
            .write_message_secrets(self.group_id(), &self.message_secrets_store)?;

        Ok(())
    }

    /// Get the message secrets. Either from the secrets store or from the group.
    pub(crate) fn message_secrets_for_epoch_mut(
        &mut self,
        epoch: GroupEpoch,
    ) -> Result<&mut MessageSecrets, SecretTreeError> {
        if epoch < self.context().epoch() {
            self.message_secrets_store
                .secrets_for_epoch_mut(epoch)
                .ok_or(SecretTreeError::TooDistantInThePast)
        } else {
            Ok(self.message_secrets_store.message_secrets_mut())
        }
    }

    /// Get the message secrets. Either from the secrets store or from the group.
    pub(crate) fn message_secrets_for_epoch(
        &self,
        epoch: GroupEpoch,
    ) -> Result<&MessageSecrets, SecretTreeError> {
        if epoch < self.context().epoch() {
            self.message_secrets_store
                .secrets_for_epoch(epoch)
                .ok_or(SecretTreeError::TooDistantInThePast)
        } else {
            Ok(self.message_secrets_store.message_secrets())
        }
    }

    /// Get the message secrets and leaves for the given epoch. Either from the
    /// secrets store or from the group.
    ///
    /// Note that the leaves vector is empty for message secrets of the current
    /// epoch. The caller can use treesync in this case.
    pub(crate) fn message_secrets_and_leaves(
        &self,
        epoch: GroupEpoch,
    ) -> Result<(&MessageSecrets, &[Member]), SecretTreeError> {
        if epoch < self.context().epoch() {
            self.message_secrets_store
                .secrets_and_leaves_for_epoch(epoch)
                .ok_or(SecretTreeError::TooDistantInThePast)
        } else {
            // No need for leaves here. The tree of the current epoch is
            // available to the caller.
            Ok((self.message_secrets_store.message_secrets(), &[]))
        }
    }

    /// Create a new group context extension proposal
    pub(crate) fn create_group_context_ext_proposal<Provider: OpenMlsProvider>(
        &self,
        framing_parameters: FramingParameters,
        extensions: Extensions<GroupContext>,
        signer: &impl Signer,
    ) -> Result<AuthenticatedContent, CreateGroupContextExtProposalError<Provider::StorageError>>
    {
        // Ensure that the group supports all the extensions that are wanted.
        let required_extension = extensions
            .iter()
            .find(|extension| extension.extension_type() == ExtensionType::RequiredCapabilities);
        if let Some(required_extension) = required_extension {
            let required_capabilities = required_extension.as_required_capabilities_extension()?;
            // Ensure we support all the capabilities.
            self.own_leaf_node()
                .ok_or_else(|| LibraryError::custom("Tree has no own leaf."))?
                .capabilities()
                .supports_required_capabilities(required_capabilities)?;

            // Ensure that all other leaf nodes support all the required
            // extensions as well.
            self.public_group()
                .check_extension_support(required_capabilities.extension_types())?;
        }
        let proposal = GroupContextExtensionProposal::new(extensions);
        let proposal = Proposal::GroupContextExtensions(Box::new(proposal));
        AuthenticatedContent::member_proposal(
            framing_parameters,
            self.own_leaf_index(),
            proposal,
            self.context(),
            signer,
        )
        .map_err(|e| e.into())
    }

    /// Load the [`VcDerivationEpochState`] this group is bound to at `epoch`,
    /// if any. Returns `None` when the group has no virtual-clients binding for
    /// that epoch. The binding is resolved at the epoch a message was sent in,
    /// so a delayed message from a past epoch deprotects with the state that
    /// was bound then, not the latest one.
    ///
    /// [`VcDerivationEpochState`]: crate::components::vc_derivation_info::VcDerivationEpochState
    #[cfg(feature = "virtual-clients-draft")]
    pub(crate) fn vc_derivation_state_at_epoch<Storage: StorageProvider>(
        &self,
        storage: &Storage,
        epoch: GroupEpoch,
    ) -> Result<
        Option<crate::components::vc_derivation_info::VcDerivationEpochState>,
        VcDerivationStateError<Storage::Error>,
    > {
        let bindings: Option<crate::components::vc_derivation_info::VcEmulationBindings> = storage
            .vc_emulation_bindings(self.group_id())
            .map_err(VcDerivationStateError::Storage)?;
        let Some(epoch_id) = bindings.and_then(|bindings| bindings.get(epoch).cloned()) else {
            return Ok(None);
        };
        let state = storage
            .vc_derivation_epoch_state(&epoch_id)
            .map_err(VcDerivationStateError::Storage)?
            .ok_or_else(|| {
                log::error!("vc: group is bound to derivation epoch, but state is missing");
                VcDerivationStateError::MissingDerivationEpochState
            })?;
        Ok(Some(state))
    }

    /// Returns whether this group is an emulation group of a virtual client.
    ///
    /// The flag is set when the application creates the group as an emulation
    /// group or joins one, and it is restored from storage when the group is
    /// loaded. See [`MlsGroupCreateConfigBuilder::emulation_group`].
    ///
    /// [`MlsGroupCreateConfigBuilder::emulation_group`]: crate::group::MlsGroupCreateConfigBuilder::emulation_group
    #[cfg(feature = "virtual-clients-draft")]
    pub fn is_emulation_group(&self) -> bool {
        self.emulation_group
    }

    /// Returns the [`EpochId`] of the newest derivation epoch of this emulation
    /// group, or `None` if none was registered yet.
    ///
    /// All virtual-client operations resolve to this derivation epoch. It is
    /// sourced from the newest group epoch that was a derivation epoch, which
    /// may be older than the group's current epoch: only commits that change
    /// membership or that carry a `new_derivation_epoch` action create one.
    ///
    /// The sender-side operation entry points take the emulation group and
    /// resolve the epoch themselves, so this getter is for inspection only.
    ///
    /// Returns `None` for groups that are not emulation groups.
    ///
    /// [`EpochId`]: crate::components::vc_derivation_info::EpochId
    #[cfg(feature = "virtual-clients-draft")]
    pub fn newest_vc_derivation_epoch<Storage: StorageProvider>(
        &self,
        storage: &Storage,
    ) -> Result<Option<crate::components::vc_derivation_info::EpochId>, Storage::Error> {
        crate::components::vc_derivation_info::newest_vc_derivation_epoch(storage, self.group_id())
    }

    // Encrypt an AuthenticatedContent into an PrivateMessage
    pub(crate) fn encrypt<Provider: OpenMlsProvider>(
        &mut self,
        public_message: AuthenticatedContent,
        provider: &Provider,
    ) -> Result<EncryptionOutput, MessageEncryptionError<Provider::StorageError>> {
        let padding_size = self.configuration().padding_size();

        // If this group is bound to a derivation epoch at its current epoch,
        // load the state so the framing layer can derive a deterministic
        // reuse guard.
        #[cfg(feature = "virtual-clients-draft")]
        let derivation_state = self
            .vc_derivation_state_at_epoch(provider.storage(), self.epoch())
            .map_err(|e| match e {
                VcDerivationStateError::Storage(e) => MessageEncryptionError::StorageError(e),
                VcDerivationStateError::MissingDerivationEpochState => {
                    MessageEncryptionError::VirtualClientsError(
                        crate::components::vc_derivation_info::VirtualClientsError::MissingDerivationEpochState,
                    )
                }
            })?;
        #[cfg(feature = "virtual-clients-draft")]
        let emulator_ctx: Option<crate::framing::EmulatorReuseGuardCtx<'_>> = derivation_state
            .as_ref()
            .map(|state| state.reuse_guard_inputs());

        let msg = PrivateMessage::try_from_authenticated_content(
            provider.crypto(),
            provider.rand(),
            &public_message,
            self.ciphersuite(),
            self.message_secrets_store.message_secrets_mut(),
            padding_size,
            #[cfg(feature = "virtual-clients-draft")]
            emulator_ctx.as_ref(),
        )?;

        // When the group is bound to a derivation epoch, derive the generation
        // ID the application hands to the DS to detect generation collisions
        // between siblings. Application content draws it from the application
        // ratchet, proposals and commits from the handshake ratchet.
        #[cfg(feature = "virtual-clients-draft")]
        let msg = {
            use crate::components::vc_derivation_info::RatchetType;
            let mut msg = msg;
            if let Some(state) = &derivation_state {
                let ratchet_type = match public_message.content().content_type() {
                    ContentType::Application => RatchetType::Application,
                    ContentType::Proposal | ContentType::Commit => RatchetType::Handshake,
                };
                let generation_id = state
                    .derive_generation_id(
                        provider.crypto(),
                        self.group_id(),
                        self.epoch(),
                        msg.generation,
                        ratchet_type,
                    )
                    .map_err(MessageEncryptionError::VirtualClientsError)?;
                msg.generation_id = Some(generation_id);
            }
            msg
        };

        provider
            .storage()
            .write_message_secrets(self.group_id(), &self.message_secrets_store)
            .map_err(MessageEncryptionError::StorageError)?;

        Ok(msg)
    }

    /// Outgoing wire format derived from the group's configured policy.
    pub(crate) fn outgoing_wire_format(&self) -> WireFormat {
        self.mls_group_config.wire_format_policy().outgoing().into()
    }

    /// Owned `authenticated_data` bytes for the next outgoing message, taking
    /// the GroupContext's Safe AAD requirement into account.
    ///
    /// Callers borrow the returned buffer into a [`FramingParameters`] for the
    /// duration of message construction.
    pub(crate) fn outgoing_authenticated_data(&self) -> Result<Vec<u8>, LibraryError> {
        #[cfg(feature = "extensions-draft")]
        {
            self.assembled_authenticated_data()
        }
        #[cfg(not(feature = "extensions-draft"))]
        {
            Ok(self.aad.clone())
        }
    }

    /// Build the bytes that go into `authenticated_data` for the next outgoing
    /// message. When the GroupContext requires Safe AAD framing, the result is
    /// the TLS serialization of the staged [`SafeAad`] followed by the bytes of
    /// `self.aad`. Otherwise, the result is `self.aad` unchanged.
    #[cfg(feature = "extensions-draft")]
    pub(crate) fn assembled_authenticated_data(&self) -> Result<Vec<u8>, LibraryError> {
        if !self.context().safe_aad_required() {
            return Ok(self.aad.clone());
        }
        crate::framing::safe_aad::assemble_authenticated_data(&self.safe_aad, &self.aad)
            .map_err(|_| LibraryError::custom("SafeAad serialization failed"))
    }

    /// Delete all past epoch secrets.
    ///
    /// For more information on the arguments to this method, see [`PastEpochDeletion`].
    pub fn delete_past_epoch_secrets<Provider: OpenMlsProvider>(
        &mut self,
        provider: &Provider,
        policy: PastEpochDeletion,
    ) -> Result<(), DeletePastEpochSecretsError<Provider::StorageError>> {
        // delete past epoch secrets in memory
        self.message_secrets_store.delete_past_epoch_secrets(policy);
        // update the message secrets store in storage
        provider
            .storage()
            .write_message_secrets(self.group_id(), &self.message_secrets_store)?;

        Ok(())
    }

    /// Returns a reference to the proposal store.
    pub fn proposal_store(&self) -> &ProposalStore {
        self.public_group.proposal_store()
    }

    /// Returns a mutable reference to the proposal store.
    pub(crate) fn proposal_store_mut(&mut self) -> &mut ProposalStore {
        self.public_group.proposal_store_mut()
    }

    /// Get the group context
    pub(crate) fn context(&self) -> &GroupContext {
        self.public_group.group_context()
    }

    /// Get the MLS version used in this group.
    pub(crate) fn version(&self) -> ProtocolVersion {
        self.public_group.version()
    }

    /// Resets the AAD, including any staged Safe AAD items.
    #[inline]
    pub(crate) fn reset_aad(&mut self) {
        self.aad.clear();
        #[cfg(feature = "extensions-draft")]
        {
            self.safe_aad = SafeAad::empty();
        }
    }

    /// Returns a reference to the public group.
    pub fn public_group(&self) -> &PublicGroup {
        &self.public_group
    }
}

/// Bookkeeping a virtual client needs to confirm a handshake message
/// (proposal or commit) that was framed as a PrivateMessage.
///
/// Pass `epoch` and `generation` to [`MlsGroup::confirm_handshake_message`]
/// once the DS has accepted the message, to delete the retained handshake
/// secret. `generation_id` is present when the group is bound to a derivation
/// epoch and is attached to the fanned-out message so a strongly-consistent DS
/// can detect generation collisions between siblings; it is `None` otherwise.
///
/// [`MlsGroup::confirm_handshake_message`]: crate::group::MlsGroup::confirm_handshake_message
#[cfg(feature = "virtual-clients-draft")]
#[derive(Debug, Clone)]
pub struct HandshakeConfirmationData {
    /// The epoch the message was encrypted in, which is the epoch before a
    /// commit is merged.
    pub epoch: GroupEpoch,
    /// The handshake-ratchet generation used for encryption.
    pub generation: u32,
    /// The [`GenerationId`] to attach to the fanned-out message, present when
    /// the group is bound to a derivation epoch and `None` otherwise.
    ///
    /// [`GenerationId`]: crate::components::vc_derivation_info::GenerationId
    pub generation_id: Option<crate::components::vc_derivation_info::GenerationId>,
}

/// Result of framing an [`AuthenticatedContent`] handshake message into an
/// [`MlsMessageOut`]. Mirrors the cfg-gated field pattern of
/// [`EncryptionOutput`]: with the `virtual-clients-draft` feature it also
/// carries the [`HandshakeConfirmationData`] for a ciphertext-framed message
/// (`None` when the message was framed as a plaintext PublicMessage).
///
/// [`EncryptionOutput`]: crate::framing::EncryptionOutput
pub(crate) struct HandshakeFramingOutput {
    pub(crate) message: MlsMessageOut,
    #[cfg(feature = "virtual-clients-draft")]
    pub(crate) confirmation: Option<HandshakeConfirmationData>,
}

// Private methods of MlsGroup
impl MlsGroup {
    /// Store the given [`EncryptionKeyPair`]s in the `provider`'s key store
    /// indexed by this group's [`GroupId`] and [`GroupEpoch`].
    ///
    /// Returns an error if access to the key store fails.
    pub(super) fn store_epoch_keypairs<Storage: StorageProvider>(
        &self,
        store: &Storage,
        keypair_references: &[EncryptionKeyPair],
    ) -> Result<(), Storage::Error> {
        store.write_encryption_epoch_key_pairs(
            self.group_id(),
            &self.context().epoch(),
            self.own_leaf_index().u32(),
            keypair_references,
        )
    }

    /// Read the [`EncryptionKeyPair`]s of this group and its current
    /// [`GroupEpoch`] from the `provider`'s storage.
    ///
    /// Returns an error if the lookup in the [`StorageProvider`] fails.
    pub(super) fn read_epoch_keypairs<Storage: StorageProvider>(
        &self,
        store: &Storage,
    ) -> Result<Vec<EncryptionKeyPair>, Storage::Error> {
        store.encryption_epoch_key_pairs(
            self.group_id(),
            &self.context().epoch(),
            self.own_leaf_index().u32(),
        )
    }

    /// Delete the [`EncryptionKeyPair`]s from the previous [`GroupEpoch`] from
    /// the `provider`'s key store.
    ///
    /// Returns an error if access to the key store fails.
    #[cfg(not(feature = "virtual-clients-draft"))]
    pub(super) fn delete_previous_epoch_keypairs<Storage: StorageProvider>(
        &self,
        store: &Storage,
    ) -> Result<(), Storage::Error> {
        store.delete_encryption_epoch_key_pairs(
            self.group_id(),
            &GroupEpoch::from(self.context().epoch().as_u64() - 1),
            self.own_leaf_index().u32(),
        )
    }

    #[cfg(feature = "virtual-clients-draft")]
    pub(super) fn delete_previous_epoch_keypairs<Storage: StorageProvider>(
        &self,
        store: &Storage,
        previous_own_leaf_index: LeafNodeIndex,
    ) -> Result<(), Storage::Error> {
        // In the sibling-resync flow, `merge_commit` installs the joiner's
        // leaf as our own leaf before it filters and stores the new epoch
        // keypairs. Previous-epoch keypairs are still stored under the leaf
        // index from that previous epoch, so the caller must pass that index
        // explicitly instead of having this helper read `self.own_leaf_index()`.
        store.delete_encryption_epoch_key_pairs(
            self.group_id(),
            &GroupEpoch::from(self.context().epoch().as_u64() - 1),
            previous_own_leaf_index.u32(),
        )
    }

    /// Stores the state of this group. Only to be called from constructors to
    /// store the initial state of the group.
    pub(super) fn store<Storage: crate::storage::StorageProvider>(
        &self,
        storage: &Storage,
    ) -> Result<(), Storage::Error> {
        self.public_group.store(storage)?;
        storage.write_group_epoch_secrets(self.group_id(), &self.group_epoch_secrets)?;
        storage.write_own_leaf_index(self.group_id(), &self.own_leaf_index)?;
        storage.write_message_secrets(self.group_id(), &self.message_secrets_store)?;
        storage.write_resumption_psk_store(self.group_id(), &self.resumption_psk_store)?;
        storage.write_mls_join_config(self.group_id(), &self.mls_group_config)?;
        storage.write_group_state(self.group_id(), &self.group_state)?;
        #[cfg(feature = "extensions-draft")]
        if let Some(application_export_tree) = &self.application_export_tree {
            storage.write_application_export_tree(self.group_id(), application_export_tree)?;
        }

        Ok(())
    }

    /// Converts PublicMessage to MlsMessage. Depending on whether handshake
    /// message should be encrypted, PublicMessage messages are encrypted to
    /// PrivateMessage first.
    fn content_to_mls_message(
        &mut self,
        mls_auth_content: AuthenticatedContent,
        provider: &impl OpenMlsProvider,
    ) -> Result<HandshakeFramingOutput, LibraryError> {
        let output = match self.configuration().wire_format_policy().outgoing() {
            OutgoingWireFormatPolicy::AlwaysPlaintext => {
                let mut plaintext: PublicMessage = mls_auth_content.into();
                // Set the membership tag only if the sender type is `Member`.
                if plaintext.sender().is_member() {
                    plaintext.set_membership_tag(
                        provider.crypto(),
                        self.ciphersuite(),
                        self.message_secrets().membership_key(),
                        self.message_secrets().serialized_context(),
                    )?;
                }
                HandshakeFramingOutput {
                    message: plaintext.into(),
                    #[cfg(feature = "virtual-clients-draft")]
                    confirmation: None,
                }
            }
            OutgoingWireFormatPolicy::AlwaysCiphertext => {
                // A ciphertext-framed handshake message ties its confirmation
                // to the epoch it was encrypted in, which is the current epoch
                // at framing time, before a commit is merged.
                #[cfg(feature = "virtual-clients-draft")]
                let epoch = self.epoch();
                let encryption_output = self
                    .encrypt(mls_auth_content, provider)
                    // We can be sure the encryption will work because the plaintext was created by us
                    .map_err(|_| LibraryError::custom("Malformed plaintext"))?;
                let message = MlsMessageOut::from_private_message(
                    encryption_output.private_message,
                    self.version(),
                );
                HandshakeFramingOutput {
                    message,
                    #[cfg(feature = "virtual-clients-draft")]
                    confirmation: Some(HandshakeConfirmationData {
                        epoch,
                        generation: encryption_output.generation,
                        generation_id: encryption_output.generation_id,
                    }),
                }
            }
        };
        Ok(output)
    }

    /// Check if the group is operational. Throws an error if the group is
    /// inactive or if there is a pending commit.
    fn is_operational(&self) -> Result<(), MlsGroupStateError> {
        match self.group_state {
            MlsGroupState::PendingCommit(_) => Err(MlsGroupStateError::PendingCommit),
            MlsGroupState::Inactive => Err(MlsGroupStateError::UseAfterEviction),
            MlsGroupState::Operational => Ok(()),
        }
    }
}

// Methods used in tests
impl MlsGroup {
    #[cfg(any(feature = "test-utils", test))]
    pub fn export_group_context(&self) -> &GroupContext {
        self.context()
    }

    #[cfg(any(feature = "test-utils", test))]
    pub fn tree_hash(&self) -> &[u8] {
        self.public_group().group_context().tree_hash()
    }

    #[cfg(any(feature = "test-utils", test))]
    pub(crate) fn message_secrets_test_mut(&mut self) -> &mut MessageSecrets {
        self.message_secrets_store.message_secrets_mut()
    }

    #[cfg(any(feature = "test-utils", test))]
    pub fn print_ratchet_tree(&self, message: &str) {
        println!("{}: {}", message, self.public_group().export_ratchet_tree());
    }

    #[cfg(any(feature = "test-utils", test))]
    pub(crate) fn context_mut(&mut self) -> &mut GroupContext {
        self.public_group.context_mut()
    }

    #[cfg(test)]
    pub(crate) fn set_own_leaf_index(&mut self, own_leaf_index: LeafNodeIndex) {
        self.own_leaf_index = own_leaf_index;
    }

    #[cfg(test)]
    pub(crate) fn own_tree_position(&self) -> TreePosition {
        TreePosition::new(self.group_id().clone(), self.own_leaf_index())
    }

    #[cfg(test)]
    pub(crate) fn message_secrets_store(&self) -> &MessageSecretsStore {
        &self.message_secrets_store
    }

    #[cfg(test)]
    pub(crate) fn resumption_psk_store(&self) -> &ResumptionPskStore {
        &self.resumption_psk_store
    }

    #[cfg(test)]
    pub(crate) fn set_group_context(&mut self, group_context: GroupContext) {
        self.public_group.set_group_context(group_context)
    }

    #[cfg(any(test, feature = "test-utils"))]
    pub fn ensure_persistence(&self, storage: &impl StorageProvider) -> Result<(), LibraryError> {
        let loaded = MlsGroup::load(storage, self.group_id())
            .map_err(|_| LibraryError::custom("Failed to load group from storage"))?;
        let other = loaded.ok_or_else(|| LibraryError::custom("Group not found in storage"))?;

        if self != &other {
            let mut diagnostics = Vec::new();

            if self.mls_group_config != other.mls_group_config {
                diagnostics.push(format!(
                    "mls_group_config:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.mls_group_config, other.mls_group_config
                ));
            }
            if self.public_group != other.public_group {
                diagnostics.push(format!(
                    "public_group:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.public_group, other.public_group
                ));
            }
            if self.group_epoch_secrets != other.group_epoch_secrets {
                diagnostics.push(format!(
                    "group_epoch_secrets:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.group_epoch_secrets, other.group_epoch_secrets
                ));
            }
            if self.own_leaf_index != other.own_leaf_index {
                diagnostics.push(format!(
                    "own_leaf_index:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.own_leaf_index, other.own_leaf_index
                ));
            }
            if self.message_secrets_store != other.message_secrets_store {
                diagnostics.push(format!(
                    "message_secrets_store:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.message_secrets_store, other.message_secrets_store
                ));
            }
            if self.resumption_psk_store != other.resumption_psk_store {
                diagnostics.push(format!(
                    "resumption_psk_store:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.resumption_psk_store, other.resumption_psk_store
                ));
            }
            if self.own_leaf_nodes != other.own_leaf_nodes {
                diagnostics.push(format!(
                    "own_leaf_nodes:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.own_leaf_nodes, other.own_leaf_nodes
                ));
            }
            if self.aad != other.aad {
                diagnostics.push(format!(
                    "aad:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.aad, other.aad
                ));
            }
            if self.group_state != other.group_state {
                diagnostics.push(format!(
                    "group_state:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.group_state, other.group_state
                ));
            }
            #[cfg(feature = "extensions-draft")]
            if self.application_export_tree != other.application_export_tree {
                diagnostics.push(format!(
                    "application_export_tree:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.application_export_tree, other.application_export_tree
                ));
            }
            #[cfg(feature = "virtual-clients-draft")]
            if self.emulation_group != other.emulation_group {
                diagnostics.push(format!(
                    "emulation_group:\n  Current: {:?}\n  Loaded:  {:?}",
                    self.emulation_group, other.emulation_group
                ));
            }

            log::error!(
                "Loaded group does not match current group! Differing fields ({}):\n\n{}",
                diagnostics.len(),
                diagnostics.join("\n\n")
            );

            return Err(LibraryError::custom(
                "Loaded group does not match current group",
            ));
        }

        Ok(())
    }
}

/// A [`StagedWelcome`] can be inspected and then turned into a [`MlsGroup`].
/// This allows checking who authored the Welcome message.
#[derive(Debug)]
pub struct StagedWelcome {
    // The group configuration. See [`MlsGroupJoinConfig`] for more information.
    mls_group_config: MlsGroupJoinConfig,
    public_group: PublicGroup,
    group_epoch_secrets: GroupEpochSecrets,
    own_leaf_index: LeafNodeIndex,

    /// A [`MessageSecretsStore`] that stores message secrets.
    /// By default this store has the length of 1, i.e. only the [`MessageSecrets`]
    /// of the current epoch is kept.
    /// If more secrets from past epochs should be kept in order to be
    /// able to decrypt application messages from previous epochs, the size of
    /// the store must be increased through [`max_past_epochs()`].
    message_secrets_store: MessageSecretsStore,

    /// A secret that is not stored as part of the [`MlsGroup`] after the group is created.
    /// It can be used by the application to derive forward secure secrets.
    #[cfg(feature = "extensions-draft")]
    application_export_secret: ApplicationExportSecret,

    /// Resumption psk store. This is where the resumption psks are kept in a rollover list.
    resumption_psk_store: ResumptionPskStore,

    /// The [`VerifiableGroupInfo`] from the [`Welcome`] message.
    verifiable_group_info: VerifiableGroupInfo,

    /// The key material used to join via this welcome.
    key_material: WelcomeKeyMaterial,

    /// If we got a path secret, these are the derived path keys.
    path_keypairs: Option<Vec<EncryptionKeyPair>>,

    /// Whether to join the group as an emulation group of a virtual client. Set
    /// by [`Self::emulation_group`].
    #[cfg(feature = "virtual-clients-draft")]
    emulation_group: bool,
}

/// A `Welcome` message that has been processed but not staged yet.
///
/// This may be used in order to retrieve information from the `Welcome` about
/// the ratchet tree and PSKs.
///
/// Use `into_staged_welcome` to stage it into a [`StagedWelcome`].
pub struct ProcessedWelcome {
    // The group configuration. See [`MlsGroupJoinConfig`] for more information.
    mls_group_config: MlsGroupJoinConfig,

    // The following is the state after parsing the Welcome message, before actually
    // building the group.
    ciphersuite: Ciphersuite,
    group_secrets: GroupSecrets,
    epoch_secrets: crate::schedule::EpochSecretsResult,
    verifiable_group_info: crate::messages::group_info::VerifiableGroupInfo,
    resumption_psk_store: crate::schedule::psk::store::ResumptionPskStore,
    key_material: WelcomeKeyMaterial,
}

/// The key material a client uses to process a [`Welcome`] message.
#[derive(Debug)]
pub struct WelcomeKeyMaterial {
    inner: WelcomeKeyMaterialInner,
}

/// The inner data of a [`WelcomeKeyMaterial`].
///
/// A regular member holds a local [`KeyPackageBundle`]. A sibling emulator
/// joining a higher-level group as a virtual client has no local bundle: it
/// derives the init and leaf-encryption keys from the operation secret tree of
/// the derivation epoch the KeyPackage belongs to.
///
/// [`Welcome`]: crate::messages::Welcome
#[derive(Debug)]
pub(crate) enum WelcomeKeyMaterialInner {
    /// A locally stored [`KeyPackageBundle`]. Boxed to keep the enum small,
    /// since the virtual-client variant is much smaller.
    KeyPackage(Box<KeyPackageBundle>),
    /// Virtual-client material derived from a derivation epoch's operation
    /// secret tree.
    #[cfg(feature = "virtual-clients-draft")]
    VirtualClient(crate::components::vc_derivation_info::VcWelcomeMaterial),
}

impl WelcomeKeyMaterial {
    /// Create a new [`WelcomeKeyMaterial`] from a [`KeyPackageBundle`].
    pub(crate) fn with_key_package_bundle(key_package: KeyPackageBundle) -> Self {
        Self {
            inner: WelcomeKeyMaterialInner::KeyPackage(Box::new(key_package)),
        }
    }

    /// Create a new [`WelcomeKeyMaterial`] from a [`VcWelcomeMaterial`].
    ///
    /// [`VcWelcomeMaterial`]: crate::components::vc_derivation_info::VcWelcomeMaterial
    #[cfg(feature = "virtual-clients-draft")]
    pub(crate) fn with_vc_welcome_material(
        material: crate::components::vc_derivation_info::VcWelcomeMaterial,
    ) -> Self {
        Self {
            inner: WelcomeKeyMaterialInner::VirtualClient(material),
        }
    }

    pub(crate) fn inner(&self) -> &WelcomeKeyMaterialInner {
        &self.inner
    }

    /// The [`KeyPackageRef`] addressed by the welcome's encrypted group
    /// secrets. The bundle computes it from its KeyPackage, the virtual-client
    /// material carries the ref it was matched on.
    ///
    /// [`KeyPackageRef`]: crate::ciphersuite::hash_ref::KeyPackageRef
    pub fn key_package_ref(
        &self,
        crypto: &impl OpenMlsCrypto,
    ) -> Result<crate::ciphersuite::hash_ref::KeyPackageRef, LibraryError> {
        match &self.inner {
            WelcomeKeyMaterialInner::KeyPackage(bundle) => bundle.key_package().hash_ref(crypto),
            #[cfg(feature = "virtual-clients-draft")]
            WelcomeKeyMaterialInner::VirtualClient(material) => {
                Ok(material.key_package_ref.clone())
            }
        }
    }

    /// The init private key used to decrypt the encrypted group secrets.
    pub fn init_private_key(&self) -> &crate::ciphersuite::HpkePrivateKey {
        match &self.inner {
            WelcomeKeyMaterialInner::KeyPackage(bundle) => bundle.init_private_key(),
            #[cfg(feature = "virtual-clients-draft")]
            WelcomeKeyMaterialInner::VirtualClient(material) => &material.init_private_key,
        }
    }

    /// The public init key the encrypted group secrets are encrypted to.
    pub fn hpke_init_key(&self) -> &InitKey {
        match &self.inner {
            WelcomeKeyMaterialInner::KeyPackage(bundle) => bundle.key_package().hpke_init_key(),
            #[cfg(feature = "virtual-clients-draft")]
            WelcomeKeyMaterialInner::VirtualClient(material) => &material.init_key,
        }
    }

    /// The local [`KeyPackageBundle`] on the regular path, or `None` on the
    /// virtual-client path. Checks that only apply when there is a local
    /// KeyPackage to compare against branch on this.
    pub fn key_package_bundle(&self) -> Option<&KeyPackageBundle> {
        match &self.inner {
            WelcomeKeyMaterialInner::KeyPackage(bundle) => Some(bundle),
            #[cfg(feature = "virtual-clients-draft")]
            WelcomeKeyMaterialInner::VirtualClient(_) => None,
        }
    }

    /// The joiner's leaf encryption keypair.
    fn encryption_key_pair(&self) -> EncryptionKeyPair {
        match &self.inner {
            WelcomeKeyMaterialInner::KeyPackage(bundle) => bundle.encryption_key_pair(),
            #[cfg(feature = "virtual-clients-draft")]
            WelcomeKeyMaterialInner::VirtualClient(material) => material.encryption_keypair.clone(),
        }
    }
}