openmls/group/mls_group/

staged_commit.rs

use core::fmt::Debug;

use openmls_traits::crypto::OpenMlsCrypto;
use openmls_traits::storage::StorageProvider as _;
use serde::{Deserialize, Serialize};
use tls_codec::Serialize as _;

use super::proposal_store::{
    QueuedAddProposal, QueuedPskProposal, QueuedRemoveProposal, QueuedUpdateProposal,
};

#[cfg(feature = "virtual-clients-draft")]
use super::Sender;
#[cfg(feature = "extensions-draft")]
use super::StagedCommitSafeExport;
use super::{
    super::errors::*, load_psks, Credential, ExportedSecret, Extension, GroupContext,
    GroupEpochSecrets, GroupId, JoinerSecret, KeySchedule, LeafNode, LibraryError, MessageSecrets,
    MlsGroup, MlsGroupState, OpenMlsProvider, PendingCommitState, Proposal, ProposalQueue,
    PskSecret, QueuedProposal, StagedCommitExport,
};
use crate::group::diff::PublicGroupDiff;
use crate::group::GroupEpoch;
use crate::messages::ConfirmationTag;
use crate::prelude::{Commit, LeafNodeIndex};
#[cfg(feature = "extensions-draft")]
use crate::{component::ComponentId, schedule::application_export_tree::ApplicationExportTree};

use crate::treesync::errors::TreeSyncFromNodesError;
use crate::treesync::RatchetTree;
use crate::{
    ciphersuite::{hash_ref::ProposalRef, Secret},
    framing::mls_auth_content::AuthenticatedContent,
    group::public_group::{
        diff::{apply_proposals::ApplyProposalsValues, StagedPublicGroupDiff},
        staged_commit::PublicStagedCommitState,
    },
    schedule::{
        CommitSecret, EpochAuthenticator, EpochSecretsResult, InitSecret, PreSharedKeyId,
        ResumptionPskSecret,
    },
    treesync::node::encryption_keys::EncryptionKeyPair,
};

#[cfg(feature = "extensions-draft")]
use super::proposal_store::{QueuedAppDataUpdateProposal, QueuedAppEphemeralProposal};
#[cfg(feature = "extensions-draft")]
use crate::prelude::processing::AppDataUpdates;

#[cfg(feature = "virtual-clients-draft")]
fn validate_vc_external_init_secret(
    is_sibling_resync: bool,
    has_external_init_proposal: bool,
    has_vc_external_init_secret: bool,
) -> Result<(), StageCommitError> {
    if is_sibling_resync && !has_vc_external_init_secret {
        return Err(
            crate::components::vc_derivation_info::VirtualClientsError::DerivationInfoMalformed
                .into(),
        );
    }
    if has_vc_external_init_secret && !has_external_init_proposal {
        return Err(
            crate::components::vc_derivation_info::VirtualClientsError::DerivationInfoMalformed
                .into(),
        );
    }
    Ok(())
}

/// Returns whether a commit's virtual-clients Safe AAD item carries a
/// `new_derivation_epoch` action, reading it off the commit's assembled
/// `authenticated_data`.
///
/// The wire bytes are authoritative: an application that staged the marker
/// itself is treated exactly like one that asked the commit builder for it, on
/// both the sending and the receiving side.
#[cfg(feature = "virtual-clients-draft")]
fn commit_marks_new_vc_derivation_epoch(
    authenticated_data: &[u8],
) -> Result<bool, StageCommitError> {
    use crate::components::vc_commit_data::VirtualClientCommitData;

    let (safe_aad, _prefix_len) =
        crate::framing::safe_aad::parse_authenticated_data_prefix(authenticated_data)
            .map_err(|e| StageCommitError::MalformedVcCommitData(e.to_string()))?;
    let commit_data = VirtualClientCommitData::from_safe_aad(&safe_aad)
        .map_err(|e| StageCommitError::MalformedVcCommitData(e.to_string()))?;
    Ok(commit_data.is_some_and(|commit_data| commit_data.creates_derivation_epoch()))
}

impl MlsGroup {
    /// Returns `true` when `received_tag` is the confirmation tag produced by
    /// our pending member commit, i.e. the incoming Commit is that pending
    /// commit this client got fanned out by the delivery service. Returns
    /// `false` when we hold no member pending commit or its tag differs.
    ///
    /// We compare confirmation tags rather than the full Commit contents: the
    /// signature has already authenticated the Commit as ours, and a matching
    /// confirmation tag binds the confirmed transcript hash of the new epoch.
    pub(crate) fn matches_pending_commit(&self, received_tag: &ConfirmationTag) -> bool {
        let MlsGroupState::PendingCommit(pending_commit_state) = &self.group_state else {
            return false;
        };
        let PendingCommitState::Member(staged_commit) = pending_commit_state.as_ref() else {
            return false;
        };
        let StagedCommitState::GroupMember(member_state) = &staged_commit.state else {
            return false;
        };
        member_state.staged_diff.confirmation_tag() == received_tag
    }

    fn derive_epoch_secrets(
        &self,
        provider: &impl OpenMlsProvider,
        apply_proposals_values: ApplyProposalsValues,
        epoch_secrets: &GroupEpochSecrets,
        commit_secret: CommitSecret,
        serialized_provisional_group_context: &[u8],
        #[cfg(feature = "virtual-clients-draft")] vc_external_init_secret: Option<
            &crate::components::vc_derivation_info::ExternalInitSecret,
        >,
    ) -> Result<EpochSecretsResult, StageCommitError> {
        // Check if we need to include the init secret from an external commit
        // we applied earlier or if we use the one from the previous epoch.
        let joiner_secret = if let Some(ref external_init_proposal) =
            apply_proposals_values.external_init_proposal_option
        {
            // A sibling emulator client processing the virtual client's
            // external commit uses the external init secret carried in the
            // commit's derivation info, since it may not hold the previous
            // epoch's `external_secret` (always absent without the feature).
            // Everyone else (ordinary external commits) decapsulates the
            // carried `kem_output` as usual.
            #[cfg(feature = "virtual-clients-draft")]
            let carried_init_secret = vc_external_init_secret
                .map(|carried| InitSecret::from(Secret::from_slice(carried.as_slice())));
            #[cfg(not(feature = "virtual-clients-draft"))]
            let carried_init_secret: Option<InitSecret> = None;

            let init_secret = match carried_init_secret {
                Some(init_secret) => init_secret,
                None => {
                    let external_priv = epoch_secrets
                        .external_secret()
                        .derive_external_keypair(provider.crypto(), self.ciphersuite())
                        .map_err(LibraryError::unexpected_crypto_error)?
                        .private;
                    InitSecret::from_kem_output(
                        provider.crypto(),
                        self.ciphersuite(),
                        self.version(),
                        &external_priv,
                        external_init_proposal.kem_output(),
                    )?
                }
            };
            JoinerSecret::new(
                provider.crypto(),
                self.ciphersuite(),
                commit_secret,
                &init_secret,
                serialized_provisional_group_context,
            )
            .map_err(LibraryError::unexpected_crypto_error)?
        } else {
            JoinerSecret::new(
                provider.crypto(),
                self.ciphersuite(),
                commit_secret,
                epoch_secrets.init_secret(),
                serialized_provisional_group_context,
            )
            .map_err(LibraryError::unexpected_crypto_error)?
        };

        // Prepare the PskSecret
        // Fails if PSKs are missing ([valn1205](https://validation.openmls.tech/#valn1205))
        let psk_secret = {
            let psks: Vec<(&PreSharedKeyId, Secret)> = load_psks(
                provider.storage(),
                &self.resumption_psk_store,
                &apply_proposals_values.presharedkeys,
            )?;

            PskSecret::new(provider.crypto(), self.ciphersuite(), psks)?
        };

        // Create key schedule
        let mut key_schedule = KeySchedule::init(
            self.ciphersuite(),
            provider.crypto(),
            &joiner_secret,
            psk_secret,
        )?;

        key_schedule
            .add_context(provider.crypto(), serialized_provisional_group_context)
            .map_err(|_| LibraryError::custom("Using the key schedule in the wrong state"))?;
        Ok(key_schedule
            .epoch_secrets(provider.crypto(), self.ciphersuite())
            .map_err(|_| LibraryError::custom("Using the key schedule in the wrong state"))?)
    }

    /// Stages a commit message. The commit may have been sent by another group
    /// member or be our own Commit without an UpdatePath. This
    /// function does the following:
    ///  - Applies the proposals covered by the commit to the tree
    ///  - Applies the (optional) update path to the tree
    ///  - Decrypts and calculates the path secrets
    ///  - Initializes the key schedule for epoch rollover
    ///  - Verifies the confirmation tag
    ///
    /// Returns a [StagedCommit] that can be inspected and later merged into the
    /// group state with [MlsGroup::merge_commit()]. If the member was not
    /// removed from the group, the function also returns an
    /// [ApplicationExportSecret].
    ///
    /// This function does the following checks:
    ///  - ValSem101
    ///  - ValSem102
    ///  - ValSem104
    ///  - ValSem105
    ///  - ValSem106
    ///  - ValSem107
    ///  - ValSem108
    ///  - ValSem110
    ///  - ValSem111
    ///  - ValSem112
    ///  - ValSem113: All Proposals: The proposal type must be supported by all
    ///    members of the group
    ///  - ValSem200
    ///  - ValSem201
    ///  - ValSem202: Path must be the right length
    ///  - ValSem203: Path secrets must decrypt correctly
    ///  - ValSem204: Public keys from Path must be verified and match the
    ///    private keys from the direct path
    ///  - ValSem205
    ///  - ValSem240
    ///  - ValSem241
    ///  - ValSem242
    ///  - ValSem244
    pub(crate) fn stage_commit(
        &self,
        mls_content: &AuthenticatedContent,
        old_epoch_keypairs: Vec<EncryptionKeyPair>,
        leaf_node_keypairs: Vec<EncryptionKeyPair>,
        provider: &impl OpenMlsProvider,
        #[cfg(feature = "virtual-clients-draft")] vc_commit_material: Option<
            crate::components::vc_derivation_info::VcCommitMaterial,
        >,
    ) -> Result<StagedCommit, StageCommitError> {
        let (commit, proposal_queue, sender_index) = self
            .public_group
            .validate_commit(mls_content, provider.crypto())?;

        // Create the provisional public group state (including the tree and
        // group context) and apply proposals.
        let mut diff = self.public_group.empty_diff();

        #[cfg(not(feature = "extensions-draft"))]
        let apply_proposals_values =
            diff.apply_proposals(&proposal_queue, self.own_leaf_index())?;

        #[cfg(feature = "extensions-draft")]
        let apply_proposals_values = diff.apply_proposals_with_app_data_updates(
            &proposal_queue,
            self.own_leaf_index(),
            None,
        )?;
        self.stage_applied_proposal_values(
            apply_proposals_values,
            diff,
            commit,
            proposal_queue,
            sender_index,
            mls_content,
            old_epoch_keypairs,
            leaf_node_keypairs,
            provider,
            #[cfg(feature = "virtual-clients-draft")]
            vc_commit_material,
        )
    }

    #[cfg(feature = "extensions-draft")]
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn stage_commit_with_app_data_updates(
        &self,
        mls_content: &AuthenticatedContent,
        old_epoch_keypairs: Vec<EncryptionKeyPair>,
        leaf_node_keypairs: Vec<EncryptionKeyPair>,
        app_data_dict_updates: Option<AppDataUpdates>,
        provider: &impl OpenMlsProvider,
        #[cfg(feature = "virtual-clients-draft")] vc_commit_material: Option<
            crate::components::vc_derivation_info::VcCommitMaterial,
        >,
    ) -> Result<StagedCommit, StageCommitError> {
        let (commit, proposal_queue, sender_index) = self
            .public_group
            .validate_commit(mls_content, provider.crypto())?;

        // Create the provisional public group state (including the tree and
        // group context) and apply proposals.
        let mut diff = self.public_group.empty_diff();

        let apply_proposals_values = diff.apply_proposals_with_app_data_updates(
            &proposal_queue,
            self.own_leaf_index(),
            app_data_dict_updates,
        )?;

        self.stage_applied_proposal_values(
            apply_proposals_values,
            diff,
            commit,
            proposal_queue,
            sender_index,
            mls_content,
            old_epoch_keypairs,
            leaf_node_keypairs,
            provider,
            #[cfg(feature = "virtual-clients-draft")]
            vc_commit_material,
        )
    }

    #[allow(clippy::too_many_arguments)]
    fn stage_applied_proposal_values(
        &self,
        apply_proposals_values: ApplyProposalsValues,
        mut diff: PublicGroupDiff,
        commit: &Commit,
        proposal_queue: ProposalQueue,
        sender_index: LeafNodeIndex,
        mls_content: &AuthenticatedContent,
        old_epoch_keypairs: Vec<EncryptionKeyPair>,
        leaf_node_keypairs: Vec<EncryptionKeyPair>,
        provider: &impl OpenMlsProvider,
        #[cfg(feature = "virtual-clients-draft")] vc_commit_material: Option<
            crate::components::vc_derivation_info::VcCommitMaterial,
        >,
    ) -> Result<StagedCommit, StageCommitError> {
        let ciphersuite = self.ciphersuite();

        // Only an emulation group with Safe AAD framing acts on the marker, so
        // only there does a malformed item fail the commit.
        #[cfg(feature = "virtual-clients-draft")]
        let marks_new_vc_derivation_epoch =
            if self.is_emulation_group() && self.context().safe_aad_required() {
                commit_marks_new_vc_derivation_epoch(mls_content.authenticated_data())?
            } else {
                false
            };

        // Unbundle the sibling-VC commit material: the per-commit operation
        // secret recreates the path, the emulation `epoch_id` is recorded on
        // the staged commit, and the external init secret (external commits
        // only) feeds the key schedule.
        #[cfg(feature = "virtual-clients-draft")]
        let (vc_material, vc_derivation_epoch_id, vc_external_init_secret) =
            match vc_commit_material {
                Some(material) => (
                    Some(material.operation_secret),
                    Some(material.epoch_id),
                    material.external_init_secret,
                ),
                None => (None, None, None),
            };

        // A sibling-resync external commit is a VC external commit sent by a
        // sibling emulator client to onboard itself into this higher-level
        // group, inline-removing our existing leaf. The receiver-side gate in
        // `process_internal_authenticated_content[_with_app_data_updates]` (see
        // `is_sibling_vc_commit`) sets `vc_material = Some(_)` only for
        // own-leaf VC commits and sibling-resync external commits, so here the
        // two-condition check fully identifies the resync case:
        //
        //   - `vc_material` is `Some`, which after the upstream gate means the
        //     commit carries a VC derivation-info entry and we hold per-epoch
        //     state for the referenced `epoch_id`.
        //   - the sender is `NewMemberCommit`, since own-leaf VC commits arrive as
        //     `Sender::Member`, so this disambiguates the two sibling shapes.
        //
        // When this holds, we (a) skip the `self_removed` short-circuit so we
        // don't transition to `Inactive`, (b) derive the path from the
        // per-commit `OperationSecret` (we have no HPKE recipient on the path),
        // and (c) record the new leaf index on the staged commit so
        // `merge_commit` can update `own_leaf_index` before filtering owned
        // encryption keypairs.
        #[cfg(feature = "virtual-clients-draft")]
        let is_sibling_resync =
            vc_material.is_some() && matches!(mls_content.sender(), Sender::NewMemberCommit);
        #[cfg(not(feature = "virtual-clients-draft"))]
        let is_sibling_resync = false;

        // A sibling-resync external commit MUST carry the external init secret
        // in its derivation info (mls-virtual-clients draft): the
        // sibling uses it as the new epoch's external init secret. Reject the
        // commit if it is absent.
        #[cfg(feature = "virtual-clients-draft")]
        validate_vc_external_init_secret(
            is_sibling_resync,
            apply_proposals_values
                .external_init_proposal_option
                .is_some(),
            vc_external_init_secret.is_some(),
        )?;

        // Determine if Commit has a path
        let (commit_secret, new_keypairs, new_leaf_keypair_option, update_path_leaf_node) =
            if let Some(path) = commit.path.clone() {
                // Update the public group
                // ValSem202: Path must be the right length
                diff.apply_received_update_path(
                    provider.crypto(),
                    ciphersuite,
                    sender_index,
                    &path,
                )?;

                // Update group context
                diff.update_group_context(
                    provider.crypto(),
                    apply_proposals_values.extensions.clone(),
                )?;

                // Check if we were removed from the group. The sibling-resync
                // discriminator carves out the case where the `Remove` of our
                // leaf is the auto-Remove paired with a sibling emulator's
                // external commit. In that case our state survives on the
                // joiner's new leaf, so we must continue processing.
                if apply_proposals_values.self_removed && !is_sibling_resync {
                    // If so, we return here, because we can't decrypt the path
                    let staged_diff = diff.into_staged_diff(provider.crypto(), ciphersuite)?;
                    let staged_state = PublicStagedCommitState::new(
                        staged_diff,
                        commit.path.as_ref().map(|path| path.leaf_node().clone()),
                    );
                    let staged_commit = StagedCommit::new(
                        proposal_queue,
                        StagedCommitState::PublicState(Box::new(staged_state)),
                        #[cfg(feature = "virtual-clients-draft")]
                        None,
                    );
                    return Ok(staged_commit);
                }

                // When processing a commit sent by a sibling virtual client
                // (either our own leaf, or a sibling-resync external commit
                // onto a new leaf), we have no HPKE recipient on the path.
                // Re-derive the path from the per-commit `OperationSecret`
                // the receiver derived from the per-epoch operation secret
                // tree, and verify the resulting public keys against the
                // commit. The non-VC `decrypt_path` is the fallback for
                // everyone else.
                #[cfg(feature = "virtual-clients-draft")]
                let vc_path: Option<(Vec<EncryptionKeyPair>, CommitSecret)> = if sender_index
                    == self.own_leaf_index()
                    || is_sibling_resync
                {
                    let operation_secret = vc_material.ok_or(
                            crate::components::vc_derivation_info::VirtualClientsError::MissingOperationTree,
                        )?;
                    Some(self.recreate_path_for_own_commit(
                        &diff,
                        &path,
                        ciphersuite,
                        self.group_id(),
                        provider.crypto(),
                        sender_index,
                        operation_secret,
                    )?)
                } else {
                    None
                };
                #[cfg(not(feature = "virtual-clients-draft"))]
                let vc_path: Option<(Vec<EncryptionKeyPair>, CommitSecret)> = None;

                // ValSem203: Path secrets must decrypt correctly
                // ValSem204: Public keys from Path must be verified and match the private keys from the direct path
                let (new_keypairs, commit_secret) = if let Some(pair) = vc_path {
                    pair
                } else {
                    let decryption_keypairs: Vec<&EncryptionKeyPair> = old_epoch_keypairs
                        .iter()
                        .chain(leaf_node_keypairs.iter())
                        .collect();
                    diff.decrypt_path(
                        provider.crypto(),
                        &decryption_keypairs,
                        self.own_leaf_index(),
                        sender_index,
                        path.nodes(),
                        &apply_proposals_values.exclusion_list(),
                    )?
                };

                // Check if one of our update proposals was applied. If so, we
                // need to store that keypair separately, because after merging
                // it needs to be removed from the key store separately and in
                // addition to the removal of the keypairs of the previous
                // epoch.
                let new_leaf_keypair_option = if is_sibling_resync {
                    None
                } else if let Some(leaf) = diff.leaf(self.own_leaf_index()) {
                    leaf_node_keypairs.into_iter().find_map(|keypair| {
                        if leaf.encryption_key() == keypair.public_key() {
                            Some(keypair)
                        } else {
                            None
                        }
                    })
                } else {
                    // We should have an own leaf at this point.
                    debug_assert!(false);
                    None
                };

                // Return the leaf node in the update path so the credential can be validated.
                // Since the diff has already been updated, this should be the same as the leaf
                // at the sender index.
                let update_path_leaf_node = Some(path.leaf_node().clone());
                debug_assert_eq!(diff.leaf(sender_index), path.leaf_node().into());

                (
                    commit_secret,
                    new_keypairs,
                    new_leaf_keypair_option,
                    update_path_leaf_node,
                )
            } else {
                if apply_proposals_values.path_required {
                    // ValSem201
                    return Err(StageCommitError::RequiredPathNotFound);
                }

                // Even if there is no path, we have to update the group context.
                diff.update_group_context(
                    provider.crypto(),
                    apply_proposals_values.extensions.clone(),
                )?;

                (CommitSecret::zero_secret(ciphersuite), vec![], None, None)
            };

        // Update the confirmed transcript hash before we compute the confirmation tag.
        diff.update_confirmed_transcript_hash(provider.crypto(), mls_content)?;

        let received_confirmation_tag = mls_content
            .confirmation_tag()
            .ok_or(StageCommitError::ConfirmationTagMissing)?;

        let serialized_provisional_group_context = diff
            .group_context()
            .tls_serialize_detached()
            .map_err(LibraryError::missing_bound_check)?;

        #[cfg(feature = "virtual-clients-draft")]
        let provisional_own_leaf_index = if is_sibling_resync {
            sender_index
        } else {
            self.own_leaf_index()
        };
        #[cfg(not(feature = "virtual-clients-draft"))]
        let provisional_own_leaf_index = self.own_leaf_index();

        let EpochSecretsResult {
            epoch_secrets,
            #[cfg(feature = "extensions-draft")]
            application_exporter,
        } = self.derive_epoch_secrets(
            provider,
            apply_proposals_values,
            self.group_epoch_secrets(),
            commit_secret,
            &serialized_provisional_group_context,
            #[cfg(feature = "virtual-clients-draft")]
            vc_external_init_secret.as_ref(),
        )?;
        let (provisional_group_secrets, provisional_message_secrets) = epoch_secrets.split_secrets(
            serialized_provisional_group_context,
            diff.tree_size(),
            provisional_own_leaf_index,
        );

        // Verify confirmation tag
        // ValSem205
        let own_confirmation_tag = provisional_message_secrets
            .confirmation_key()
            .tag(
                provider.crypto(),
                self.ciphersuite(),
                diff.group_context().confirmed_transcript_hash(),
            )
            .map_err(LibraryError::unexpected_crypto_error)?;
        if &own_confirmation_tag != received_confirmation_tag {
            log::error!("Confirmation tag mismatch");
            log_crypto!(trace, "  Got:      {:x?}", received_confirmation_tag);
            log_crypto!(trace, "  Expected: {:x?}", own_confirmation_tag);
            // TODO: We have tests expecting this error.
            //       They need to be rewritten.
            // debug_assert!(false, "Confirmation tag mismatch");

            // in some tests we need to be able to proceed despite the tag being wrong,
            // e.g. to test whether a later validation check is performed correctly.
            if !crate::skip_validation::is_disabled::confirmation_tag() {
                return Err(StageCommitError::ConfirmationTagMismatch);
            }
        }

        diff.update_interim_transcript_hash(ciphersuite, provider.crypto(), own_confirmation_tag)?;

        let staged_diff = diff.into_staged_diff(provider.crypto(), ciphersuite)?;
        #[cfg(feature = "extensions-draft")]
        let application_export_tree = ApplicationExportTree::new(application_exporter);
        #[cfg(feature = "virtual-clients-draft")]
        let new_own_leaf_index = is_sibling_resync.then_some(provisional_own_leaf_index);
        let staged_commit_state =
            StagedCommitState::GroupMember(Box::new(MemberStagedCommitState::new(
                provisional_group_secrets,
                provisional_message_secrets,
                staged_diff,
                new_keypairs,
                new_leaf_keypair_option,
                update_path_leaf_node,
                #[cfg(feature = "extensions-draft")]
                application_export_tree,
                #[cfg(feature = "virtual-clients-draft")]
                new_own_leaf_index,
            )));
        #[cfg_attr(not(feature = "virtual-clients-draft"), allow(unused_mut))]
        let mut staged_commit = StagedCommit::new(
            proposal_queue,
            staged_commit_state,
            #[cfg(feature = "virtual-clients-draft")]
            vc_derivation_epoch_id,
        );
        #[cfg(feature = "virtual-clients-draft")]
        {
            staged_commit.marks_new_vc_derivation_epoch = marks_new_vc_derivation_epoch;
        }

        Ok(staged_commit)
    }

    /// Re-derive the path of a commit sent by a sibling virtual client
    /// (either through our own higher-level leaf, or onto a new leaf via a
    /// sibling-resync external commit) from the per-commit `OperationSecret`
    /// resolved by the caller (in `process_message`, via PPRF evaluation), and
    /// verify that the public keys derived from the path secret match the
    /// leaf node in the path. Returns the derived parent-node keypairs and
    /// commit secret, ready to be slotted in where `decrypt_path` would
    /// normally produce them.
    ///
    /// `sender_index` is the leaf the path originates from. For an own-leaf
    /// VC commit this equals `self.own_leaf_index()`. For a sibling-resync
    /// external commit it is the joiner's new leaf (the `leftmost_free_index`
    /// the external-commit builder chose), which is where the path actually
    /// starts.
    ///
    /// Signature key changes are not verified here: not every signature
    /// scheme has an interoperable seed-to-keypair construction, so the
    /// application is responsible for supplying any rotated signature key
    /// pair to the storage provider out-of-band. The new public key on the
    /// path leaf is already authenticated by the commit's standard
    /// path-validation against the previous signature key.
    #[cfg(feature = "virtual-clients-draft")]
    #[expect(clippy::too_many_arguments)]
    fn recreate_path_for_own_commit(
        &self,
        diff: &PublicGroupDiff,
        path: &crate::treesync::treekem::UpdatePath,
        group_ciphersuite: openmls_traits::types::Ciphersuite,
        group_id: &crate::prelude::GroupId,
        crypto: &impl OpenMlsCrypto,
        sender_index: LeafNodeIndex,
        operation_secret: crate::components::vc_derivation_info::OperationSecret,
    ) -> Result<(Vec<EncryptionKeyPair>, CommitSecret), StageCommitError> {
        use crate::components::vc_derivation_info::VirtualClientsError;

        let target_operation_secret =
            operation_secret.derive_target_operation_secret(crypto, group_ciphersuite, group_id)?;

        let path_secret = target_operation_secret
            .derive_path_generation_secret(crypto, group_ciphersuite)?
            .into();
        let (encryption_key_pairs, commit_secret) =
            diff.recreate_path_from_path_secret(crypto, path_secret, sender_index, path.nodes())?;

        // Verify that the leaf encryption key in the path matches the one
        // derived from the operation secret.
        let leaf_keypair = target_operation_secret
            .derive_encryption_key_secret(crypto, group_ciphersuite)?
            .generate_encryption_key_pair(crypto, group_ciphersuite)?;
        drop(target_operation_secret);
        if leaf_keypair.public_key() != path.leaf_node().encryption_key() {
            return Err(VirtualClientsError::EncryptionKeyMismatch.into());
        }

        // Mirror the sender's `apply_own_update_path`: the leaf keypair is
        // prepended to the parent keypairs so the merge step has private
        // keys for all of the new epoch's owned encryption keys.
        let mut keypairs = Vec::with_capacity(1 + encryption_key_pairs.len());
        keypairs.push(leaf_keypair);
        keypairs.extend(encryption_key_pairs);
        Ok((keypairs, commit_secret))
    }

    /// Returns whether merging `staged_commit` has to register the epoch it
    /// moves this group into as a virtual-clients derivation epoch.
    ///
    /// That is the case when all of the following hold:
    ///
    /// - this group is an emulation group, since no other group derives
    ///   virtual-client secrets from its exporter,
    /// - the merge installs member state, since a self-removal or a
    ///   public-state merge leaves no exporter to puncture, and
    /// - the commit changes membership or carries a `new_derivation_epoch`
    ///   action in its virtual-clients Safe AAD item.
    #[cfg(feature = "virtual-clients-draft")]
    pub(crate) fn commit_creates_vc_derivation_epoch(&self, staged_commit: &StagedCommit) -> bool {
        self.is_emulation_group()
            && matches!(staged_commit.state, StagedCommitState::GroupMember(_))
            && (staged_commit.marks_new_vc_derivation_epoch || staged_commit.changes_membership())
    }

    /// Merges a [StagedCommit] into the group state and optionally return a [`SecretTree`]
    /// from the previous epoch. The secret tree is returned if the Commit does not contain a self removal.
    ///
    /// Beyond a `LibraryError`, this fails on a storage error, and on an
    /// emulation group it also fails if registering the derivation epoch the
    /// commit creates fails. The group is already advanced in memory by then,
    /// and a storage transaction does not roll that back, so a caller that sees
    /// an error has to discard this group and load it again.
    pub(crate) fn merge_commit<Provider: OpenMlsProvider>(
        &mut self,
        provider: &Provider,
        staged_commit: StagedCommit,
    ) -> Result<(), MergeCommitError<Provider::StorageError>> {
        // Get all keypairs from the old epoch, so we can later store the ones
        // that are still relevant in the new epoch.
        let old_epoch_keypairs = self
            .read_epoch_keypairs(provider.storage())
            .map_err(MergeCommitError::StorageError)?;

        #[cfg(feature = "virtual-clients-draft")]
        let creates_vc_derivation_epoch = self.commit_creates_vc_derivation_epoch(&staged_commit);

        match staged_commit.state {
            StagedCommitState::PublicState(staged_state) => {
                self.public_group
                    .merge_diff(staged_state.into_staged_diff());
                self.store(provider.storage())
                    .map_err(MergeCommitError::StorageError)?;
                Ok(())
            }
            StagedCommitState::GroupMember(state) => {
                // Save the past epoch
                let past_epoch = self.context().epoch();
                // Get all the full leaves
                let leaves = self.public_group().members().collect();
                // Merge the staged commit into the group state and store the secret tree from the
                // previous epoch in the message secrets store.
                self.group_epoch_secrets = state.group_epoch_secrets;

                // Replace the previous message secrets with the new ones and return the previous message secrets
                let old_message_secrets = self
                    .message_secrets_store
                    .replace_current_message_secrets(state.message_secrets);
                self.message_secrets_store.add_past_epoch_tree(
                    past_epoch,
                    old_message_secrets,
                    leaves,
                );

                self.public_group.merge_diff(state.staged_diff);

                #[cfg(feature = "virtual-clients-draft")]
                let previous_own_leaf_index = self.own_leaf_index;

                // Sibling-resync external commit: install the joiner's new
                // leaf as our own before filtering keypairs. The call to
                // `owned_encryption_keys(self.own_leaf_index())` below relies
                // on this value.
                #[cfg(feature = "virtual-clients-draft")]
                if let Some(new_idx) = state.new_own_leaf_index {
                    self.own_leaf_index = new_idx;
                }

                // Replace the previous exporter tree with the new one. This
                // happens after the public group is merged, so that a
                // virtual-clients registration sees the new epoch's tree size
                // and own leaf index.
                #[cfg(feature = "extensions-draft")]
                {
                    // The application exporter is only None if the group was
                    // stored using an older version of OpenMLS that did not
                    // support the application exporter. Registration then fails,
                    // which it has to: merging without registering would
                    // silently keep the old derivation epoch active.
                    #[cfg_attr(not(feature = "virtual-clients-draft"), allow(unused_mut))]
                    let mut application_export_tree = state.application_export_tree;

                    // The registration punctures the new epoch's exporter, so it
                    // has to run before the tree is persisted.
                    #[cfg(feature = "virtual-clients-draft")]
                    if creates_vc_derivation_epoch {
                        crate::components::vc_derivation_info::register_vc_derivation_epoch(
                            provider.crypto(),
                            provider.storage(),
                            application_export_tree.as_mut(),
                            crate::components::vc_derivation_info::VcDerivationEpochParams::for_public_group(
                                self.public_group(),
                                self.own_leaf_index(),
                                self.mls_group_config
                                    .vc_derivation_epoch_retention_policy()
                                    .clone(),
                            ),
                        )?;
                    }

                    if let Some(application_export_tree) = application_export_tree {
                        // Overwrite the existing exporter tree in the storage.
                        use openmls_traits::storage::StorageProvider as _;
                        provider
                            .storage()
                            .write_application_export_tree(
                                self.group_id(),
                                &application_export_tree,
                            )
                            .map_err(MergeCommitError::StorageError)?;

                        self.application_export_tree = Some(application_export_tree);
                    }
                }

                let leaf_keypair = if let Some(keypair) = &state.new_leaf_keypair_option {
                    vec![keypair.clone()]
                } else {
                    vec![]
                };

                // Figure out which keys we need in the new epoch.
                let new_owned_encryption_keys = self
                    .public_group()
                    .owned_encryption_keys(self.own_leaf_index());
                // From the old and new keys, keep the ones that are still relevant in the new epoch.
                let epoch_keypairs: Vec<EncryptionKeyPair> = old_epoch_keypairs
                    .into_iter()
                    .chain(state.new_keypairs)
                    .chain(leaf_keypair)
                    .filter(|keypair| new_owned_encryption_keys.contains(keypair.public_key()))
                    .collect();

                // We should have private keys for all owned encryption keys.
                debug_assert_eq!(new_owned_encryption_keys.len(), epoch_keypairs.len());
                if new_owned_encryption_keys.len() != epoch_keypairs.len() {
                    return Err(LibraryError::custom(
                        "We should have all the private key material we need.",
                    )
                    .into());
                }

                // Store the updated group state
                let storage = provider.storage();
                let group_id = self.group_id();

                self.public_group
                    .store(storage)
                    .map_err(MergeCommitError::StorageError)?;
                storage
                    .write_own_leaf_index(group_id, &self.own_leaf_index)
                    .map_err(MergeCommitError::StorageError)?;
                storage
                    .write_group_epoch_secrets(group_id, &self.group_epoch_secrets)
                    .map_err(MergeCommitError::StorageError)?;
                storage
                    .write_message_secrets(group_id, &self.message_secrets_store)
                    .map_err(MergeCommitError::StorageError)?;

                // Store the relevant keys under the new epoch
                self.store_epoch_keypairs(storage, epoch_keypairs.as_slice())
                    .map_err(MergeCommitError::StorageError)?;

                // Delete the old keys.
                self.delete_previous_epoch_keypairs(
                    storage,
                    #[cfg(feature = "virtual-clients-draft")]
                    previous_own_leaf_index,
                )
                .map_err(MergeCommitError::StorageError)?;
                if let Some(keypair) = state.new_leaf_keypair_option {
                    keypair
                        .delete(storage)
                        .map_err(MergeCommitError::StorageError)?;
                }

                // Empty the proposal store
                storage
                    .clear_proposal_queue::<GroupId, ProposalRef>(group_id)
                    .map_err(MergeCommitError::StorageError)?;
                self.proposal_store_mut().empty();

                Ok(())
            }
        }
    }
}

#[derive(Debug, Serialize, Deserialize)]
#[cfg_attr(any(test, feature = "test-utils"), derive(Clone, PartialEq))]
pub(crate) enum StagedCommitState {
    PublicState(Box<PublicStagedCommitState>),
    /// The group member variant of the staged commit state.
    GroupMember(Box<MemberStagedCommitState>),
}

/// Contains the changes from a commit to the group state.
#[derive(Debug, Serialize, Deserialize)]
#[cfg_attr(any(test, feature = "test-utils"), derive(Clone, PartialEq))]
pub struct StagedCommit {
    /// A queue containing the proposals associated with the commit.
    pub staged_proposal_queue: ProposalQueue,
    /// The staged commit state.
    pub(super) state: StagedCommitState,
    /// Derivation epoch this commit binds the group to on merge, when
    /// the commit was built via `CommitBuilder::vc_emulation`.
    #[cfg(feature = "virtual-clients-draft")]
    #[serde(default, alias = "vc_emulation_epoch_id")]
    // alias for backwards compatibility after renaming field
    pub(super) vc_derivation_epoch_id: Option<crate::components::vc_derivation_info::EpochId>,
    /// Whether the commit's virtual-clients Safe AAD item carries a
    /// `new_derivation_epoch` action. Set when the commit is staged, and read
    /// again at merge, which is why it is persisted with an own pending commit.
    /// Membership changes are not covered here, they are read off the proposal
    /// queue by [`Self::changes_membership`].
    #[cfg(feature = "virtual-clients-draft")]
    #[serde(default)]
    pub(super) marks_new_vc_derivation_epoch: bool,
}

impl StagedCommit {
    /// Create a new [`StagedCommit`] from the provisional group state created
    /// during the commit process.
    pub(crate) fn new(
        staged_proposal_queue: ProposalQueue,
        state: StagedCommitState,
        #[cfg(feature = "virtual-clients-draft")] vc_derivation_epoch_id: Option<
            crate::components::vc_derivation_info::EpochId,
        >,
    ) -> Self {
        StagedCommit {
            staged_proposal_queue,
            state,
            #[cfg(feature = "virtual-clients-draft")]
            vc_derivation_epoch_id,
            #[cfg(feature = "virtual-clients-draft")]
            marks_new_vc_derivation_epoch: false,
        }
    }

    /// Returns whether the commit adds or removes a member. External commits
    /// count, since the sender joins the group through them.
    #[cfg(feature = "virtual-clients-draft")]
    fn changes_membership(&self) -> bool {
        self.staged_proposal_queue
            .queued_proposals()
            .any(|queued| match queued.proposal() {
                Proposal::Add(_)
                | Proposal::Remove(_)
                | Proposal::SelfRemove
                | Proposal::ExternalInit(_) => true,
                Proposal::Update(_)
                | Proposal::PreSharedKey(_)
                | Proposal::ReInit(_)
                | Proposal::GroupContextExtensions(_)
                | Proposal::AppDataUpdate(_)
                | Proposal::AppEphemeral(_)
                | Proposal::Custom(_) => false,
            })
    }

    /// Returns the epoch that this commit moves the group into
    pub fn epoch(&self) -> GroupEpoch {
        self.group_context().epoch()
    }

    /// Returns the ratchet tree of the staged commit state.
    pub fn export_ratchet_tree(
        &self,
        crypto: &impl OpenMlsCrypto,
        original_tree: RatchetTree,
    ) -> Result<Option<RatchetTree>, TreeSyncFromNodesError> {
        match &self.state {
            StagedCommitState::PublicState(_public_staged_commit_state) => Ok(None),
            StagedCommitState::GroupMember(member_staged_commit_state) => Ok(Some(
                member_staged_commit_state.staged_diff.export_ratchet_tree(
                    crypto,
                    self.group_context().ciphersuite(),
                    original_tree,
                )?,
            )),
        }
    }

    /// Returns the Add proposals that are covered by the Commit message as in iterator over [QueuedAddProposal].
    pub fn add_proposals(&self) -> impl Iterator<Item = QueuedAddProposal<'_>> {
        self.staged_proposal_queue.add_proposals()
    }

    /// Returns the Remove proposals that are covered by the Commit message as in iterator over [QueuedRemoveProposal].
    pub fn remove_proposals(&self) -> impl Iterator<Item = QueuedRemoveProposal<'_>> {
        self.staged_proposal_queue.remove_proposals()
    }

    /// Returns the Update proposals that are covered by the Commit message as in iterator over [QueuedUpdateProposal].
    pub fn update_proposals(&self) -> impl Iterator<Item = QueuedUpdateProposal<'_>> {
        self.staged_proposal_queue.update_proposals()
    }

    /// Returns the PresharedKey proposals that are covered by the Commit message as in iterator over [QueuedPskProposal].
    pub fn psk_proposals(&self) -> impl Iterator<Item = QueuedPskProposal<'_>> {
        self.staged_proposal_queue.psk_proposals()
    }

    #[cfg(feature = "extensions-draft")]
    /// Returns the AppEphemeral proposals that are covered by the Commit message as an iterator
    /// over [`QueuedAppEphemeralProposal`].
    pub fn queued_app_ephemeral_proposals(
        &self,
    ) -> impl Iterator<Item = QueuedAppEphemeralProposal<'_>> {
        self.staged_proposal_queue.app_ephemeral_proposals()
    }
    // NOTE: this is not a default proposal type
    #[cfg(feature = "extensions-draft")]
    /// Returns the AppDataUpdate proposals that are covered by the Commit message as an iterator
    /// over [`QueuedAppDataUpdateProposal`].
    pub fn app_data_update_proposals(
        &self,
    ) -> impl Iterator<Item = QueuedAppDataUpdateProposal<'_>> {
        self.staged_proposal_queue.app_data_update_proposals()
    }

    /// Returns an iterator over all [`QueuedProposal`]s.
    pub fn queued_proposals(&self) -> impl Iterator<Item = &QueuedProposal> {
        self.staged_proposal_queue.queued_proposals()
    }

    /// Returns the leaf node of the (optional) update path.
    pub fn update_path_leaf_node(&self) -> Option<&LeafNode> {
        match self.state {
            StagedCommitState::PublicState(ref public_state) => {
                public_state.update_path_leaf_node()
            }
            StagedCommitState::GroupMember(ref group_member_state) => {
                group_member_state.update_path_leaf_node.as_ref()
            }
        }
    }

    /// Returns the credentials that the caller needs to verify are valid.
    pub fn credentials_to_verify(&self) -> impl Iterator<Item = &Credential> {
        let update_path_leaf_node_cred = if let Some(node) = self.update_path_leaf_node() {
            vec![node.credential()]
        } else {
            vec![]
        };

        update_path_leaf_node_cred
            .into_iter()
            .chain(
                self.queued_proposals()
                    .flat_map(|proposal: &QueuedProposal| match proposal.proposal() {
                        Proposal::Update(update_proposal) => {
                            vec![update_proposal.leaf_node().credential()].into_iter()
                        }
                        Proposal::Add(add_proposal) => {
                            vec![add_proposal.key_package().leaf_node().credential()].into_iter()
                        }
                        Proposal::GroupContextExtensions(gce_proposal) => gce_proposal
                            .extensions()
                            .iter()
                            .flat_map(|extension| {
                                match extension {
                                    Extension::ExternalSenders(external_senders) => {
                                        external_senders
                                            .iter()
                                            .map(|external_sender| external_sender.credential())
                                            .collect()
                                    }
                                    _ => vec![],
                                }
                                .into_iter()
                            })
                            // TODO: ideally we wouldn't collect in between here, but the match arms
                            //       have to all return the same type. We solve this by having them all
                            //       be vec::IntoIter, but it would be nice if we just didn't have to
                            //       do this.
                            //       It might be possible to solve this by letting all match arms
                            //       evaluate to a dyn Iterator.
                            .collect::<Vec<_>>()
                            .into_iter(),
                        _ => vec![].into_iter(),
                    }),
            )
    }

    /// Returns `true` if the member was removed through a proposal covered by this Commit message
    /// and `false` otherwise.
    //
    // Sibling-resync external commits intentionally land in `GroupMember`
    // rather than `PublicState`, even though the proposal queue contains a
    // Remove of our own leaf. The new leaf carries our state forward, so
    // `self_removed()` returns `false` and `merge_staged_commit` keeps the
    // group active.
    pub fn self_removed(&self) -> bool {
        matches!(self.state, StagedCommitState::PublicState(_))
    }

    /// Returns the [`GroupContext`] of the staged commit state.
    pub fn group_context(&self) -> &GroupContext {
        match self.state {
            StagedCommitState::PublicState(ref ps) => ps.staged_diff().group_context(),
            StagedCommitState::GroupMember(ref gm) => gm.group_context(),
        }
    }
    /// Consume this [`StagedCommit`] and return the internal [`StagedCommitState`].
    pub(crate) fn into_state(self) -> StagedCommitState {
        self.state
    }

    /// Returns the [`EpochAuthenticator`] of the staged commit state if the
    /// owner of the originating group state is a member of the group. Returns
    /// `None` otherwise.
    pub fn epoch_authenticator(&self) -> Option<&EpochAuthenticator> {
        if let StagedCommitState::GroupMember(ref gm) = self.state {
            Some(gm.group_epoch_secrets.epoch_authenticator())
        } else {
            None
        }
    }

    /// Returns the [`ResumptionPskSecret`] of the staged commit state if the
    /// owner of the originating group state is a member of the group. Returns
    /// `None` otherwise.
    pub fn resumption_psk_secret(&self) -> Option<&ResumptionPskSecret> {
        if let StagedCommitState::GroupMember(ref gm) = self.state {
            Some(gm.group_epoch_secrets.resumption_psk())
        } else {
            None
        }
    }

    /// Safely exports a secret for the given `component_id` from the epoch the
    /// staged commit moves to, before the commit is merged.
    ///
    /// This is needed by components that feed a secret exported from one
    /// commit into the processing of a related commit, e.g. a PSK derived
    /// from one group's staged commit and consumed by another group's key
    /// schedule.
    #[cfg(feature = "extensions-draft")]
    pub fn safe_export_secret(
        &mut self,
        crypto: &impl OpenMlsCrypto,
        component_id: ComponentId,
    ) -> Result<ExportedSecret<StagedCommitSafeExport>, StagedSafeExportSecretError> {
        let ciphersuite = self.group_context().ciphersuite();
        let StagedCommitState::GroupMember(ref mut staged_commit) = self.state else {
            return Err(StagedSafeExportSecretError::NotGroupMember);
        };
        let Some(application_export_tree) = staged_commit.application_export_tree.as_mut() else {
            return Err(StagedSafeExportSecretError::Unsupported);
        };
        let secret =
            application_export_tree.safe_export_secret(crypto, ciphersuite, component_id)?;
        Ok(ExportedSecret::new(secret))
    }

    /// Exports a secret from the epoch that the staged commit moves to.
    /// Returns [`ExportSecretError::KeyLengthTooLong`] if the requested
    /// key length is too long.
    /// Returns [`ExportSecretError::GroupStateError(MlsGroupStateError::UseAfterEviction)`]
    /// if the commit removed us from the group.
    ///
    /// [`ExportSecretError::GroupStateError(MlsGroupStateError::UseAfterEviction)`]: MlsGroupStateError::UseAfterEviction
    pub fn export_secret<CryptoProvider: OpenMlsCrypto>(
        &self,
        crypto: &CryptoProvider,
        label: &str,
        context: &[u8],
        key_length: usize,
    ) -> Result<ExportedSecret<StagedCommitExport>, ExportSecretError> {
        if key_length > u16::MAX as usize {
            log::error!("Got a key that is larger than u16::MAX");
            return Err(ExportSecretError::KeyLengthTooLong);
        }

        match &self.state {
            StagedCommitState::PublicState(_public_staged_commit_state) => Err(
                ExportSecretError::GroupStateError(MlsGroupStateError::UseAfterEviction),
            ),
            StagedCommitState::GroupMember(member_staged_commit_state) => Ok(ExportedSecret::new(
                member_staged_commit_state
                    .group_epoch_secrets
                    .exporter_secret()
                    .derive_exported_secret(
                        self.group_context().ciphersuite(),
                        crypto,
                        label,
                        context,
                        key_length,
                    )
                    .map_err(LibraryError::unexpected_crypto_error)?,
            )),
        }
    }
}

/// This struct is used internally by [`StagedCommit`] to encapsulate all the modified group state.
#[derive(Debug, Serialize, Deserialize)]
#[cfg_attr(any(test, feature = "test-utils"), derive(Clone, PartialEq))]
pub(crate) struct MemberStagedCommitState {
    group_epoch_secrets: GroupEpochSecrets,
    message_secrets: MessageSecrets,
    staged_diff: StagedPublicGroupDiff,
    new_keypairs: Vec<EncryptionKeyPair>,
    new_leaf_keypair_option: Option<EncryptionKeyPair>,
    update_path_leaf_node: Option<LeafNode>,
    #[cfg(feature = "extensions-draft")]
    #[serde(default)]
    // This is `None` only if the group was stored using an older version of
    // OpenMLS that did not support the application exporter.
    application_export_tree: Option<ApplicationExportTree>,
    // The new leaf index to install on the receiving group at merge time
    // when this staged commit is a sibling-resync external commit (a VC
    // external commit from a sibling emulator that inline-removes the
    // receiver's existing leaf). `None` for all other commit kinds.
    #[cfg(feature = "virtual-clients-draft")]
    #[serde(default)]
    new_own_leaf_index: Option<LeafNodeIndex>,
}

impl MemberStagedCommitState {
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn new(
        group_epoch_secrets: GroupEpochSecrets,
        message_secrets: MessageSecrets,
        staged_diff: StagedPublicGroupDiff,
        new_keypairs: Vec<EncryptionKeyPair>,
        new_leaf_keypair_option: Option<EncryptionKeyPair>,
        update_path_leaf_node: Option<LeafNode>,
        #[cfg(feature = "extensions-draft")] application_export_tree: ApplicationExportTree,
        #[cfg(feature = "virtual-clients-draft")] new_own_leaf_index: Option<LeafNodeIndex>,
    ) -> Self {
        Self {
            group_epoch_secrets,
            message_secrets,
            staged_diff,
            new_keypairs,
            new_leaf_keypair_option,
            update_path_leaf_node,
            #[cfg(feature = "extensions-draft")]
            application_export_tree: Some(application_export_tree),
            #[cfg(feature = "virtual-clients-draft")]
            new_own_leaf_index,
        }
    }

    /// Get the staged [`GroupContext`].
    pub(crate) fn group_context(&self) -> &GroupContext {
        self.staged_diff.group_context()
    }
}

#[cfg(all(test, feature = "virtual-clients-draft"))]
mod tests {
    use super::validate_vc_external_init_secret;
    use crate::{
        components::vc_derivation_info::VirtualClientsError, group::errors::StageCommitError,
    };

    /// The two spec MUSTs behind `validate_vc_external_init_secret`: a
    /// sibling external commit whose derivation info omits the external init
    /// secret is rejected, and a carried init secret on a commit without an
    /// ExternalInit proposal is rejected. The conforming combinations pass.
    #[test]
    fn external_init_secret_presence_is_validated() {
        let malformed: Result<(), StageCommitError> =
            Err(VirtualClientsError::DerivationInfoMalformed.into());

        // Sibling external commit without a carried init secret.
        assert_eq!(
            validate_vc_external_init_secret(true, true, false),
            malformed
        );
        // Carried init secret on a commit without an ExternalInit proposal.
        assert_eq!(
            validate_vc_external_init_secret(false, false, true),
            malformed
        );
        assert_eq!(
            validate_vc_external_init_secret(true, false, true),
            malformed
        );

        // Conforming: external commit carrying the secret, and a regular
        // commit carrying none.
        assert_eq!(validate_vc_external_init_secret(true, true, true), Ok(()));
        assert_eq!(validate_vc_external_init_secret(false, true, false), Ok(()));
        assert_eq!(
            validate_vc_external_init_secret(false, false, false),
            Ok(())
        );
    }
}