lighthouse/consensus/state_processing/src/per_block_processing/signature_sets.rs

//! A `SignatureSet` is an abstraction over the components of a signature. A `SignatureSet` may be
//! validated individually, or alongside in others in a potentially cheaper bulk operation.
//!
//! This module exposes one function to extract each type of `SignatureSet` from a `BeaconBlock`.
use bls::{AggregateSignature, PublicKey, PublicKeyBytes, Signature, SignatureSet};
use ssz::DecodeError;
use std::borrow::Cow;
use tree_hash::TreeHash;
use typenum::Unsigned;
use types::{
    AbstractExecPayload, AttesterSlashingRef, BeaconBlockRef, BeaconState, BeaconStateError,
    ChainSpec, DepositData, Domain, Epoch, EthSpec, Fork, Hash256, InconsistentFork,
    IndexedAttestation, IndexedAttestationRef, IndexedPayloadAttestation, ProposerSlashing,
    SignedAggregateAndProof, SignedBeaconBlock, SignedBeaconBlockHeader,
    SignedBlsToExecutionChange, SignedContributionAndProof, SignedExecutionPayloadBid,
    SignedExecutionPayloadEnvelope, SignedRoot, SignedVoluntaryExit, SigningData, Slot,
    SyncAggregate, SyncAggregatorSelectionData,
};

pub type Result<T> = std::result::Result<T, Error>;

#[derive(Debug, PartialEq, Clone)]
pub enum Error {
    /// Signature verification failed. The block is invalid.
    SignatureInvalid(DecodeError),
    /// There was an error attempting to read from a `BeaconState`. Block
    /// validity was not determined.
    BeaconStateError(BeaconStateError),
    /// Attempted to find the public key of a validator that does not exist. You cannot distinguish
    /// between an error and an invalid block in this case.
    ValidatorUnknown(u64),
    /// Attempted to find the public key of a validator that does not exist. You cannot distinguish
    /// between an error and an invalid block in this case.
    ValidatorPubkeyUnknown(PublicKeyBytes),
    /// The `BeaconBlock` has a `proposer_index` that does not match the index we computed locally.
    ///
    /// The block is invalid.
    IncorrectBlockProposer { block: u64, local_shuffling: u64 },
    /// The public keys supplied do not match the number of objects requiring keys. Block validity
    /// was not determined.
    MismatchedPublicKeyLen { pubkey_len: usize, other_len: usize },
    /// Pubkey decompression failed. The block is invalid.
    PublicKeyDecompressionFailed,
    /// The public key bytes stored in the `BeaconState` were not valid. This is a serious internal
    /// error.
    BadBlsBytes { validator_index: u64 },
    /// The block structure is not appropriate for the fork at `block.slot()`.
    InconsistentBlockFork(InconsistentFork),
}

impl From<BeaconStateError> for Error {
    fn from(e: BeaconStateError) -> Error {
        Error::BeaconStateError(e)
    }
}

/// Helper function to get a public key from a `state`.
pub fn get_pubkey_from_state<E>(
    state: &BeaconState<E>,
    validator_index: usize,
) -> Option<Cow<'_, PublicKey>>
where
    E: EthSpec,
{
    state
        .validators()
        .get(validator_index)
        .and_then(|v| {
            let pk: Option<PublicKey> = v.pubkey.decompress().ok();
            pk
        })
        .map(Cow::Owned)
}

/// A signature set that is valid if a block was signed by the expected block producer.
pub fn block_proposal_signature_set<'a, E, F, Payload: AbstractExecPayload<E>>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signed_block: &'a SignedBeaconBlock<E, Payload>,
    block_root: Option<Hash256>,
    verified_proposer_index: Option<u64>,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let block = signed_block.message();

    let proposer_index = if let Some(proposer_index) = verified_proposer_index {
        proposer_index
    } else {
        state.get_beacon_proposer_index(block.slot(), spec)? as u64
    };
    if proposer_index != block.proposer_index() {
        return Err(Error::IncorrectBlockProposer {
            block: block.proposer_index(),
            local_shuffling: proposer_index,
        });
    }

    block_proposal_signature_set_from_parts(
        signed_block,
        block_root,
        proposer_index,
        &state.fork(),
        state.genesis_validators_root(),
        get_pubkey,
        spec,
    )
}

/// A signature set that is valid if a block was signed by the expected block producer.
///
/// Unlike `block_proposal_signature_set` this does **not** check that the proposer index is
/// correct according to the shuffling. It should only be used if no suitable `BeaconState` is
/// available.
pub fn block_proposal_signature_set_from_parts<'a, E, F, Payload: AbstractExecPayload<E>>(
    signed_block: &'a SignedBeaconBlock<E, Payload>,
    block_root: Option<Hash256>,
    proposer_index: u64,
    fork: &Fork,
    genesis_validators_root: Hash256,
    get_pubkey: F,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    // Verify that the `SignedBeaconBlock` instantiation matches the fork at `signed_block.slot()`.
    signed_block
        .fork_name(spec)
        .map_err(Error::InconsistentBlockFork)?;

    let block = signed_block.message();
    let domain = spec.get_domain(
        block.slot().epoch(E::slots_per_epoch()),
        Domain::BeaconProposer,
        fork,
        genesis_validators_root,
    );

    let message = if let Some(root) = block_root {
        SigningData {
            object_root: root,
            domain,
        }
        .tree_hash_root()
    } else {
        block.signing_root(domain)
    };

    Ok(SignatureSet::single_pubkey(
        signed_block.signature(),
        get_pubkey(proposer_index as usize).ok_or(Error::ValidatorUnknown(proposer_index))?,
        message,
    ))
}

pub fn bls_execution_change_signature_set<'a, E: EthSpec>(
    state: &'a BeaconState<E>,
    signed_address_change: &'a SignedBlsToExecutionChange,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>> {
    let domain = spec.compute_domain(
        Domain::BlsToExecutionChange,
        spec.genesis_fork_version,
        state.genesis_validators_root(),
    );
    let message = signed_address_change.message.signing_root(domain);
    let signing_key = Cow::Owned(
        signed_address_change
            .message
            .from_bls_pubkey
            .decompress()
            .map_err(|_| Error::PublicKeyDecompressionFailed)?,
    );

    Ok(SignatureSet::single_pubkey(
        &signed_address_change.signature,
        signing_key,
        message,
    ))
}

/// A signature set that is valid if the block proposers randao reveal signature is correct.
pub fn randao_signature_set<'a, E, F, Payload: AbstractExecPayload<E>>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    block: BeaconBlockRef<'a, E, Payload>,
    verified_proposer_index: Option<u64>,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let proposer_index = if let Some(proposer_index) = verified_proposer_index {
        proposer_index
    } else {
        state.get_beacon_proposer_index(block.slot(), spec)? as u64
    };

    let domain = spec.get_domain(
        block.slot().epoch(E::slots_per_epoch()),
        Domain::Randao,
        &state.fork(),
        state.genesis_validators_root(),
    );

    let message = block
        .slot()
        .epoch(E::slots_per_epoch())
        .signing_root(domain);

    Ok(SignatureSet::single_pubkey(
        block.body().randao_reveal(),
        get_pubkey(proposer_index as usize).ok_or(Error::ValidatorUnknown(proposer_index))?,
        message,
    ))
}

/// Returns two signature sets, one for each `BlockHeader` included in the `ProposerSlashing`.
pub fn proposer_slashing_signature_set<'a, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    proposer_slashing: &'a ProposerSlashing,
    spec: &'a ChainSpec,
) -> Result<(SignatureSet<'a>, SignatureSet<'a>)>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let proposer_index = proposer_slashing.signed_header_1.message.proposer_index as usize;

    Ok((
        block_header_signature_set(
            state,
            &proposer_slashing.signed_header_1,
            get_pubkey(proposer_index).ok_or(Error::ValidatorUnknown(proposer_index as u64))?,
            spec,
        ),
        block_header_signature_set(
            state,
            &proposer_slashing.signed_header_2,
            get_pubkey(proposer_index).ok_or(Error::ValidatorUnknown(proposer_index as u64))?,
            spec,
        ),
    ))
}

/// Returns a signature set that is valid if the given `pubkey` signed the `header`.
fn block_header_signature_set<'a, E: EthSpec>(
    state: &'a BeaconState<E>,
    signed_header: &'a SignedBeaconBlockHeader,
    pubkey: Cow<'a, PublicKey>,
    spec: &'a ChainSpec,
) -> SignatureSet<'a> {
    let domain = spec.get_domain(
        signed_header.message.slot.epoch(E::slots_per_epoch()),
        Domain::BeaconProposer,
        &state.fork(),
        state.genesis_validators_root(),
    );

    let message = signed_header.message.signing_root(domain);

    SignatureSet::single_pubkey(&signed_header.signature, pubkey, message)
}

/// Returns the signature set for the given `indexed_attestation`.
pub fn indexed_attestation_signature_set<'a, 'b, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signature: &'a AggregateSignature,
    indexed_attestation: IndexedAttestationRef<'b, E>,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let mut pubkeys = Vec::with_capacity(indexed_attestation.attesting_indices_len());
    for &validator_idx in indexed_attestation.attesting_indices_iter() {
        pubkeys.push(
            get_pubkey(validator_idx as usize).ok_or(Error::ValidatorUnknown(validator_idx))?,
        );
    }

    let domain = spec.get_domain(
        indexed_attestation.data().target.epoch,
        Domain::BeaconAttester,
        &state.fork(),
        state.genesis_validators_root(),
    );

    let message = indexed_attestation.data().signing_root(domain);

    Ok(SignatureSet::multiple_pubkeys(signature, pubkeys, message))
}

pub fn indexed_payload_attestation_signature_set<'a, 'b, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signature: &'a AggregateSignature,
    indexed_payload_attestation: &'b IndexedPayloadAttestation<E>,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let mut pubkeys = Vec::with_capacity(indexed_payload_attestation.attesting_indices.len());
    for &validator_idx in indexed_payload_attestation.attesting_indices.iter() {
        pubkeys.push(
            get_pubkey(validator_idx as usize).ok_or(Error::ValidatorUnknown(validator_idx))?,
        );
    }

    let epoch = indexed_payload_attestation
        .data
        .slot
        .epoch(E::slots_per_epoch());
    let domain = spec.get_domain(
        epoch,
        Domain::PTCAttester,
        &state.fork(),
        state.genesis_validators_root(),
    );

    let message = indexed_payload_attestation.data.signing_root(domain);

    Ok(SignatureSet::multiple_pubkeys(signature, pubkeys, message))
}

/// Returns the signature set for the given `indexed_attestation` but pubkeys are supplied directly
/// instead of from the state.
pub fn indexed_attestation_signature_set_from_pubkeys<'a, 'b, E, F>(
    get_pubkey: F,
    signature: &'a AggregateSignature,
    indexed_attestation: &'b IndexedAttestation<E>,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let mut pubkeys = Vec::with_capacity(indexed_attestation.attesting_indices_len());
    for &validator_idx in indexed_attestation.attesting_indices_iter() {
        pubkeys.push(
            get_pubkey(validator_idx as usize).ok_or(Error::ValidatorUnknown(validator_idx))?,
        );
    }

    let domain = spec.get_domain(
        indexed_attestation.data().target.epoch,
        Domain::BeaconAttester,
        fork,
        genesis_validators_root,
    );

    let message = indexed_attestation.data().signing_root(domain);

    Ok(SignatureSet::multiple_pubkeys(signature, pubkeys, message))
}

pub fn execution_envelope_signature_set<'a, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signed_envelope: &'a SignedExecutionPayloadEnvelope<E>,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let proposer_index = state.latest_block_header().proposer_index;
    let builder_index = signed_envelope.message.builder_index(proposer_index);
    let domain = spec.get_domain(
        state.current_epoch(),
        Domain::BeaconBuilder,
        &state.fork(),
        state.genesis_validators_root(),
    );
    let message = signed_envelope.message.signing_root(domain);
    let pubkey =
        get_pubkey(builder_index as usize).ok_or(Error::ValidatorUnknown(builder_index))?;

    Ok(SignatureSet::single_pubkey(
        &signed_envelope.signature,
        pubkey,
        message,
    ))
}

pub fn execution_payload_bid_signature_set<'a, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signed_execution_payload_bid: &'a SignedExecutionPayloadBid,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    // TODO(EIP-7732): needs to handle self building!
    let domain = spec.get_domain(
        state.current_epoch(),
        Domain::BeaconBuilder,
        &state.fork(),
        state.genesis_validators_root(),
    );
    let execution_payload_bid = &signed_execution_payload_bid.message;
    let pubkey = get_pubkey(execution_payload_bid.builder_index as usize)
        .ok_or(Error::ValidatorUnknown(execution_payload_bid.builder_index))?;
    let message = execution_payload_bid.signing_root(domain);

    Ok(SignatureSet::single_pubkey(
        &signed_execution_payload_bid.signature,
        pubkey,
        message,
    ))
}

/// Returns the signature set for the given `attester_slashing` and corresponding `pubkeys`.
pub fn attester_slashing_signature_sets<'a, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    attester_slashing: AttesterSlashingRef<'a, E>,
    spec: &'a ChainSpec,
) -> Result<(SignatureSet<'a>, SignatureSet<'a>)>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>> + Clone,
{
    Ok((
        indexed_attestation_signature_set(
            state,
            get_pubkey.clone(),
            attester_slashing.attestation_1().signature(),
            attester_slashing.attestation_1(),
            spec,
        )?,
        indexed_attestation_signature_set(
            state,
            get_pubkey,
            attester_slashing.attestation_2().signature(),
            attester_slashing.attestation_2(),
            spec,
        )?,
    ))
}

/// Returns the BLS values in a `Deposit`, if they're all valid. Otherwise, returns `None`.
pub fn deposit_pubkey_signature_message(
    deposit_data: &DepositData,
    spec: &ChainSpec,
) -> Option<(PublicKey, Signature, Hash256)> {
    let pubkey = deposit_data.pubkey.decompress().ok()?;
    let signature = deposit_data.signature.decompress().ok()?;
    let domain = spec.get_deposit_domain();
    let message = deposit_data.as_deposit_message().signing_root(domain);
    Some((pubkey, signature, message))
}

/// Returns a signature set that is valid if the `SignedVoluntaryExit` was signed by the indicated
/// validator.
pub fn exit_signature_set<'a, E, F>(
    state: &'a BeaconState<E>,
    get_pubkey: F,
    signed_exit: &'a SignedVoluntaryExit,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let exit = &signed_exit.message;
    let proposer_index = exit.validator_index as usize;

    let domain = if state.fork_name_unchecked().deneb_enabled() {
        // EIP-7044
        spec.compute_domain(
            Domain::VoluntaryExit,
            spec.capella_fork_version,
            state.genesis_validators_root(),
        )
    } else {
        spec.get_domain(
            exit.epoch,
            Domain::VoluntaryExit,
            &state.fork(),
            state.genesis_validators_root(),
        )
    };

    let message = exit.signing_root(domain);

    Ok(SignatureSet::single_pubkey(
        &signed_exit.signature,
        get_pubkey(proposer_index).ok_or(Error::ValidatorUnknown(proposer_index as u64))?,
        message,
    ))
}

pub fn signed_aggregate_selection_proof_signature_set<'a, E, F>(
    get_pubkey: F,
    signed_aggregate_and_proof: &'a SignedAggregateAndProof<E>,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let slot = signed_aggregate_and_proof.message().aggregate().data().slot;

    let domain = spec.get_domain(
        slot.epoch(E::slots_per_epoch()),
        Domain::SelectionProof,
        fork,
        genesis_validators_root,
    );
    let message = slot.signing_root(domain);
    let signature = signed_aggregate_and_proof.message().selection_proof();
    let validator_index = signed_aggregate_and_proof.message().aggregator_index();
    Ok(SignatureSet::single_pubkey(
        signature,
        get_pubkey(validator_index as usize).ok_or(Error::ValidatorUnknown(validator_index))?,
        message,
    ))
}

pub fn signed_aggregate_signature_set<'a, E, F>(
    get_pubkey: F,
    signed_aggregate_and_proof: &'a SignedAggregateAndProof<E>,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let target_epoch = signed_aggregate_and_proof
        .message()
        .aggregate()
        .data()
        .target
        .epoch;

    let domain = spec.get_domain(
        target_epoch,
        Domain::AggregateAndProof,
        fork,
        genesis_validators_root,
    );
    let message = signed_aggregate_and_proof.message().signing_root(domain);
    let signature = signed_aggregate_and_proof.signature();
    let validator_index = signed_aggregate_and_proof.message().aggregator_index();

    Ok(SignatureSet::single_pubkey(
        signature,
        get_pubkey(validator_index as usize).ok_or(Error::ValidatorUnknown(validator_index))?,
        message,
    ))
}

pub fn signed_sync_aggregate_selection_proof_signature_set<'a, E, F>(
    get_pubkey: F,
    signed_contribution_and_proof: &'a SignedContributionAndProof<E>,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let slot = signed_contribution_and_proof.message.contribution.slot;

    let domain = spec.get_domain(
        slot.epoch(E::slots_per_epoch()),
        Domain::SyncCommitteeSelectionProof,
        fork,
        genesis_validators_root,
    );
    let selection_data = SyncAggregatorSelectionData {
        slot,
        subcommittee_index: signed_contribution_and_proof
            .message
            .contribution
            .subcommittee_index,
    };
    let message = selection_data.signing_root(domain);
    let signature = &signed_contribution_and_proof.message.selection_proof;
    let validator_index = signed_contribution_and_proof.message.aggregator_index;

    Ok(SignatureSet::single_pubkey(
        signature,
        get_pubkey(validator_index as usize).ok_or(Error::ValidatorUnknown(validator_index))?,
        message,
    ))
}

pub fn signed_sync_aggregate_signature_set<'a, E, F>(
    get_pubkey: F,
    signed_contribution_and_proof: &'a SignedContributionAndProof<E>,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(usize) -> Option<Cow<'a, PublicKey>>,
{
    let epoch = signed_contribution_and_proof
        .message
        .contribution
        .slot
        .epoch(E::slots_per_epoch());

    let domain = spec.get_domain(
        epoch,
        Domain::ContributionAndProof,
        fork,
        genesis_validators_root,
    );
    let message = signed_contribution_and_proof.message.signing_root(domain);
    let signature = &signed_contribution_and_proof.signature;
    let validator_index = signed_contribution_and_proof.message.aggregator_index;

    Ok(SignatureSet::single_pubkey(
        signature,
        get_pubkey(validator_index as usize).ok_or(Error::ValidatorUnknown(validator_index))?,
        message,
    ))
}

#[allow(clippy::too_many_arguments)]
pub fn sync_committee_contribution_signature_set_from_pubkeys<'a, E, F>(
    get_pubkey: F,
    pubkey_bytes: &[PublicKeyBytes],
    signature: &'a AggregateSignature,
    epoch: Epoch,
    beacon_block_root: Hash256,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
    F: Fn(&PublicKeyBytes) -> Option<Cow<'a, PublicKey>>,
{
    let mut pubkeys = Vec::with_capacity(E::SyncSubcommitteeSize::to_usize());
    for pubkey in pubkey_bytes {
        pubkeys.push(get_pubkey(pubkey).ok_or(Error::ValidatorPubkeyUnknown(*pubkey))?);
    }

    let domain = spec.get_domain(epoch, Domain::SyncCommittee, fork, genesis_validators_root);

    let message = beacon_block_root.signing_root(domain);

    Ok(SignatureSet::multiple_pubkeys(signature, pubkeys, message))
}

pub fn sync_committee_message_set_from_pubkeys<'a, E>(
    pubkey: Cow<'a, PublicKey>,
    signature: &'a AggregateSignature,
    epoch: Epoch,
    beacon_block_root: Hash256,
    fork: &Fork,
    genesis_validators_root: Hash256,
    spec: &'a ChainSpec,
) -> Result<SignatureSet<'a>>
where
    E: EthSpec,
{
    let domain = spec.get_domain(epoch, Domain::SyncCommittee, fork, genesis_validators_root);

    let message = beacon_block_root.signing_root(domain);

    Ok(SignatureSet::single_pubkey(signature, pubkey, message))
}

/// Signature set verifier for a block's `sync_aggregate` (Altair and later).
///
/// The `slot` should be the slot of the block that the sync aggregate is included in, which may be
/// different from `state.slot()`. The `block_root` should be the block root that the sync aggregate
/// signs over. It's passed in rather than extracted from the `state` because when verifying a batch
/// of blocks the `state` will not yet have had the blocks applied.
///
/// Returns `Ok(None)` in the case where `sync_aggregate` has 0 signatures. The spec
/// uses a separate function `eth2_fast_aggregate_verify` for this, but we can equivalently
/// check the exceptional case eagerly and do a `fast_aggregate_verify` in the case where the
/// check fails (by returning `Some(signature_set)`).
pub fn sync_aggregate_signature_set<'a, E, D>(
    decompressor: D,
    sync_aggregate: &'a SyncAggregate<E>,
    slot: Slot,
    block_root: Hash256,
    state: &'a BeaconState<E>,
    spec: &ChainSpec,
) -> Result<Option<SignatureSet<'a>>>
where
    E: EthSpec,
    D: Fn(&'a PublicKeyBytes) -> Option<Cow<'a, PublicKey>>,
{
    // Allow the point at infinity to count as a signature for 0 validators as per
    // `eth2_fast_aggregate_verify` from the spec.
    if sync_aggregate.sync_committee_bits.is_zero()
        && sync_aggregate.sync_committee_signature.is_infinity()
    {
        return Ok(None);
    }

    let committee_pubkeys = &state
        .get_built_sync_committee(slot.epoch(E::slots_per_epoch()), spec)?
        .pubkeys;

    let participant_pubkeys = committee_pubkeys
        .iter()
        .zip(sync_aggregate.sync_committee_bits.iter())
        .filter_map(|(pubkey, bit)| {
            if bit {
                Some(decompressor(pubkey))
            } else {
                None
            }
        })
        .collect::<Option<Vec<_>>>()
        .ok_or(Error::PublicKeyDecompressionFailed)?;

    let previous_slot = slot.saturating_sub(1u64);

    let domain = spec.get_domain(
        previous_slot.epoch(E::slots_per_epoch()),
        Domain::SyncCommittee,
        &state.fork(),
        state.genesis_validators_root(),
    );

    let message = SigningData {
        object_root: block_root,
        domain,
    }
    .tree_hash_root();

    Ok(Some(SignatureSet::multiple_pubkeys(
        &sync_aggregate.sync_committee_signature,
        participant_pubkeys,
        message,
    )))
}