frost_core/

lib.rs

#![cfg_attr(not(feature = "std"), no_std)]
#![allow(non_snake_case)]
// It's emitting false positives; see https://github.com/rust-lang/rust-clippy/issues/9413
#![allow(clippy::derive_partial_eq_without_eq)]
#![deny(missing_docs)]
#![forbid(unsafe_code)]
#![deny(clippy::indexing_slicing)]
#![deny(clippy::unwrap_used)]
#![cfg_attr(docsrs, feature(doc_auto_cfg))]
#![cfg_attr(docsrs, feature(doc_cfg))]
#![doc = include_str!("../README.md")]
#![doc = document_features::document_features!()]

#[macro_use]
extern crate alloc;

use core::marker::PhantomData;

use alloc::{
    collections::{BTreeMap, BTreeSet},
    fmt::{self, Debug},
    vec::Vec,
};

use derive_getters::Getters;
#[cfg(any(test, feature = "test-impl"))]
use hex::FromHex;
use rand_core::{CryptoRng, RngCore};
use serialization::SerializableScalar;
use zeroize::Zeroize;

pub mod batch;
#[cfg(any(test, feature = "test-impl"))]
pub mod benches;
mod error;
mod identifier;
pub mod keys;
pub mod round1;
pub mod round2;
mod scalar_mul;
// We'd like to make this conditionally pub but the attribute below does
// not work yet (https://github.com/rust-lang/rust/issues/54727)
// #[cfg_attr(feature = "internals", visibility::make(pub))]
pub mod serialization;
mod signature;
mod signing_key;
#[cfg(any(test, feature = "test-impl"))]
pub mod tests;
mod traits;
mod verifying_key;

pub use error::{Error, FieldError, GroupError};
pub use identifier::Identifier;
use scalar_mul::VartimeMultiscalarMul;
// Re-export serde
#[cfg(feature = "serde")]
pub use serde;
pub use signature::Signature;
pub use signing_key::SigningKey;
pub use traits::{Ciphersuite, Element, Field, Group, Scalar};
pub use verifying_key::VerifyingKey;

/// A type refinement for the scalar field element representing the per-message _[challenge]_.
///
/// [challenge]: https://datatracker.ietf.org/doc/html/rfc9591#name-signature-challenge-computa
#[derive(Copy, Clone)]
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
pub(crate) struct Challenge<C: Ciphersuite>(
    pub(crate) <<C::Group as Group>::Field as Field>::Scalar,
);

impl<C> Challenge<C>
where
    C: Ciphersuite,
{
    /// Creates a challenge from a scalar.
    #[cfg_attr(feature = "internals", visibility::make(pub))]
    #[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
    #[allow(dead_code)]
    pub(crate) fn from_scalar(
        scalar: <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar,
    ) -> Self {
        Self(scalar)
    }

    /// Return the underlying scalar.
    #[cfg_attr(feature = "internals", visibility::make(pub))]
    #[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
    pub(crate) fn to_scalar(
        self,
    ) -> <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar {
        self.0
    }
}

impl<C> Debug for Challenge<C>
where
    C: Ciphersuite,
{
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_tuple("Secret")
            .field(&hex::encode(<<C::Group as Group>::Field>::serialize(
                &self.0,
            )))
            .finish()
    }
}

/// Generates the challenge as is required for Schnorr signatures.
///
/// Deals in bytes, so that [FROST] and singleton signing and verification can use it with different
/// types.
///
/// This is the only invocation of the H2 hash function from the [RFC].
///
/// [FROST]: https://datatracker.ietf.org/doc/html/rfc9591#name-signature-challenge-computa
/// [RFC]: https://datatracker.ietf.org/doc/html/rfc9591#name-cryptographic-hash-function
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
fn challenge<C>(
    R: &Element<C>,
    verifying_key: &VerifyingKey<C>,
    msg: &[u8],
) -> Result<Challenge<C>, Error<C>>
where
    C: Ciphersuite,
{
    let mut preimage = Vec::new();

    preimage.extend_from_slice(<C::Group>::serialize(R)?.as_ref());
    preimage.extend_from_slice(<C::Group>::serialize(&verifying_key.to_element())?.as_ref());
    preimage.extend_from_slice(msg);

    Ok(Challenge(C::H2(&preimage[..])))
}

/// Generates a random nonzero scalar.
///
/// It assumes that the Scalar Eq/PartialEq implementation is constant-time.
pub(crate) fn random_nonzero<C: Ciphersuite, R: RngCore + CryptoRng>(rng: &mut R) -> Scalar<C> {
    loop {
        let scalar = <<C::Group as Group>::Field>::random(rng);

        if scalar != <<C::Group as Group>::Field>::zero() {
            return scalar;
        }
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, Zeroize)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serde", serde(deny_unknown_fields))]
struct Header<C: Ciphersuite> {
    /// Format version
    #[cfg_attr(
        feature = "serde",
        serde(deserialize_with = "crate::serialization::version_deserialize::<_>")
    )]
    version: u8,
    /// Ciphersuite ID
    #[cfg_attr(
        feature = "serde",
        serde(serialize_with = "crate::serialization::ciphersuite_serialize::<_, C>")
    )]
    #[cfg_attr(
        feature = "serde",
        serde(deserialize_with = "crate::serialization::ciphersuite_deserialize::<_, C>")
    )]
    ciphersuite: (),
    #[cfg_attr(feature = "serde", serde(skip))]
    phantom: PhantomData<C>,
}

impl<C> Default for Header<C>
where
    C: Ciphersuite,
{
    fn default() -> Self {
        Self {
            version: Default::default(),
            ciphersuite: Default::default(),
            phantom: Default::default(),
        }
    }
}

/// The binding factor, also known as _rho_ (ρ)
///
/// Ensures each signature share is strongly bound to a signing set, specific set
/// of commitments, and a specific message.
///
/// <https://github.com/cfrg/draft-irtf-cfrg-frost/blob/master/draft-irtf-cfrg-frost.md>
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
pub(crate) struct BindingFactor<C: Ciphersuite>(Scalar<C>);

impl<C> BindingFactor<C>
where
    C: Ciphersuite,
{
    /// Serializes [`BindingFactor`] to bytes.
    pub fn serialize(&self) -> Vec<u8> {
        SerializableScalar::<C>(self.0).serialize()
    }
}

impl<C> Debug for BindingFactor<C>
where
    C: Ciphersuite,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("BindingFactor")
            .field(&hex::encode(self.serialize()))
            .finish()
    }
}

/// A list of binding factors and their associated identifiers.
#[derive(Clone)]
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
pub(crate) struct BindingFactorList<C: Ciphersuite>(BTreeMap<Identifier<C>, BindingFactor<C>>);

impl<C> BindingFactorList<C>
where
    C: Ciphersuite,
{
    /// Create a new [`BindingFactorList`] from a map of identifiers to binding factors.
    #[cfg(feature = "internals")]
    pub fn new(binding_factors: BTreeMap<Identifier<C>, BindingFactor<C>>) -> Self {
        Self(binding_factors)
    }

    /// Get the [`BindingFactor`] for the given identifier, or None if not found.
    pub fn get(&self, key: &Identifier<C>) -> Option<&BindingFactor<C>> {
        self.0.get(key)
    }
}

/// [`compute_binding_factors`] in the spec
///
/// [`compute_binding_factors`]: https://datatracker.ietf.org/doc/html/rfc9591#name-binding-factors-computation
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
pub(crate) fn compute_binding_factor_list<C>(
    signing_package: &SigningPackage<C>,
    verifying_key: &VerifyingKey<C>,
    additional_prefix: &[u8],
) -> Result<BindingFactorList<C>, Error<C>>
where
    C: Ciphersuite,
{
    let preimages = signing_package.binding_factor_preimages(verifying_key, additional_prefix)?;

    Ok(BindingFactorList(
        preimages
            .iter()
            .map(|(identifier, preimage)| {
                let binding_factor = C::H1(preimage);
                (*identifier, BindingFactor(binding_factor))
            })
            .collect(),
    ))
}

#[cfg(any(test, feature = "test-impl"))]
impl<C> FromHex for BindingFactor<C>
where
    C: Ciphersuite,
{
    type Error = &'static str;

    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
        let v: Vec<u8> = FromHex::from_hex(hex).map_err(|_| "invalid hex")?;

        match v.try_into() {
            Ok(bytes) => <<C::Group as Group>::Field>::deserialize(&bytes)
                .map(|scalar| Self(scalar))
                .map_err(|_| "malformed scalar encoding"),
            Err(_) => Err("malformed scalar encoding"),
        }
    }
}

/// Generates a lagrange coefficient.
///
/// The Lagrange polynomial for a set of points (x_j, y_j) for 0 <= j <= k
/// is ∑_{i=0}^k y_i.ℓ_i(x), where ℓ_i(x) is the Lagrange basis polynomial:
///
/// ℓ_i(x) = ∏_{0≤j≤k; j≠i} (x - x_j) / (x_i - x_j).
///
/// This computes ℓ_j(x) for the set of points `xs` and for the j corresponding
/// to the given xj.
///
/// If `x` is None, it uses 0 for it (since Identifiers can't be 0)
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
fn compute_lagrange_coefficient<C: Ciphersuite>(
    x_set: &BTreeSet<Identifier<C>>,
    x: Option<Identifier<C>>,
    x_i: Identifier<C>,
) -> Result<Scalar<C>, Error<C>> {
    if x_set.is_empty() {
        return Err(Error::IncorrectNumberOfIdentifiers);
    }
    let mut num = <<C::Group as Group>::Field>::one();
    let mut den = <<C::Group as Group>::Field>::one();

    let mut x_i_found = false;

    for x_j in x_set.iter() {
        if x_i == *x_j {
            x_i_found = true;
            continue;
        }

        if let Some(x) = x {
            num = num * (x.to_scalar() - x_j.to_scalar());
            den = den * (x_i.to_scalar() - x_j.to_scalar());
        } else {
            // Both signs inverted just to avoid requiring Neg (-*xj)
            num = num * x_j.to_scalar();
            den = den * (x_j.to_scalar() - x_i.to_scalar());
        }
    }
    if !x_i_found {
        return Err(Error::UnknownIdentifier);
    }

    Ok(
        num * <<C::Group as Group>::Field>::invert(&den)
            .map_err(|_| Error::DuplicatedIdentifier)?,
    )
}

/// Generates the lagrange coefficient for the i'th participant (for `signer_id`).
///
/// Implements [`derive_interpolating_value()`] from the spec.
///
/// [`derive_interpolating_value()`]: https://datatracker.ietf.org/doc/html/rfc9591#name-polynomials
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
fn derive_interpolating_value<C: Ciphersuite>(
    signer_id: &Identifier<C>,
    signing_package: &SigningPackage<C>,
) -> Result<Scalar<C>, Error<C>> {
    compute_lagrange_coefficient(
        &signing_package
            .signing_commitments()
            .keys()
            .cloned()
            .collect(),
        None,
        *signer_id,
    )
}

/// Generated by the coordinator of the signing operation and distributed to
/// each signing party
#[derive(Clone, Debug, PartialEq, Eq, Getters)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serde", serde(bound = "C: Ciphersuite"))]
#[cfg_attr(feature = "serde", serde(deny_unknown_fields))]
pub struct SigningPackage<C: Ciphersuite> {
    /// Serialization header
    #[getter(skip)]
    pub(crate) header: Header<C>,
    /// The set of commitments participants published in the first round of the
    /// protocol.
    signing_commitments: BTreeMap<Identifier<C>, round1::SigningCommitments<C>>,
    /// Message which each participant will sign.
    ///
    /// Each signer should perform protocol-specific verification on the
    /// message.
    #[cfg_attr(
        feature = "serde",
        serde(
            serialize_with = "serdect::slice::serialize_hex_lower_or_bin",
            deserialize_with = "serdect::slice::deserialize_hex_or_bin_vec"
        )
    )]
    message: Vec<u8>,
}

impl<C> SigningPackage<C>
where
    C: Ciphersuite,
{
    /// Create a new `SigningPackage`
    ///
    /// The `signing_commitments` are sorted by participant `identifier`.
    pub fn new(
        signing_commitments: BTreeMap<Identifier<C>, round1::SigningCommitments<C>>,
        message: &[u8],
    ) -> SigningPackage<C> {
        SigningPackage {
            header: Header::default(),
            signing_commitments,
            message: message.to_vec(),
        }
    }

    /// Get a signing commitment by its participant identifier, or None if not found.
    pub fn signing_commitment(
        &self,
        identifier: &Identifier<C>,
    ) -> Option<round1::SigningCommitments<C>> {
        self.signing_commitments.get(identifier).copied()
    }

    /// Compute the preimages to H1 to compute the per-signer binding factors
    // We separate this out into its own method so it can be tested
    #[cfg_attr(feature = "internals", visibility::make(pub))]
    #[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
    #[allow(clippy::type_complexity)]
    pub fn binding_factor_preimages(
        &self,
        verifying_key: &VerifyingKey<C>,
        additional_prefix: &[u8],
    ) -> Result<Vec<(Identifier<C>, Vec<u8>)>, Error<C>> {
        let mut binding_factor_input_prefix = Vec::new();

        // The length of a serialized verifying key of the same cipersuite does
        // not change between runs of the protocol, so we don't need to hash to
        // get a fixed length.
        binding_factor_input_prefix.extend_from_slice(verifying_key.serialize()?.as_ref());

        // The message is hashed with H4 to force the variable-length message
        // into a fixed-length byte string, same for hashing the variable-sized
        // (between runs of the protocol) set of group commitments, but with H5.
        binding_factor_input_prefix.extend_from_slice(C::H4(self.message.as_slice()).as_ref());
        binding_factor_input_prefix.extend_from_slice(
            C::H5(&round1::encode_group_commitments(self.signing_commitments())?[..]).as_ref(),
        );
        binding_factor_input_prefix.extend_from_slice(additional_prefix);

        Ok(self
            .signing_commitments()
            .keys()
            .map(|identifier| {
                let mut binding_factor_input = Vec::new();

                binding_factor_input.extend_from_slice(&binding_factor_input_prefix);
                binding_factor_input.extend_from_slice(identifier.serialize().as_ref());
                (*identifier, binding_factor_input)
            })
            .collect())
    }
}

#[cfg(feature = "serialization")]
impl<C> SigningPackage<C>
where
    C: Ciphersuite,
{
    /// Serialize the struct into a Vec.
    pub fn serialize(&self) -> Result<Vec<u8>, Error<C>> {
        serialization::Serialize::serialize(&self)
    }

    /// Deserialize the struct from a slice of bytes.
    pub fn deserialize(bytes: &[u8]) -> Result<Self, Error<C>> {
        serialization::Deserialize::deserialize(bytes)
    }
}

/// The product of all signers' individual commitments, published as part of the
/// final signature.
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
pub(crate) struct GroupCommitment<C: Ciphersuite>(pub(crate) Element<C>);

impl<C> GroupCommitment<C>
where
    C: Ciphersuite,
{
    /// Return the underlying element.
    #[cfg_attr(feature = "internals", visibility::make(pub))]
    #[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
    pub(crate) fn to_element(self) -> <C::Group as Group>::Element {
        self.0
    }
}

/// Generates the group commitment which is published as part of the joint
/// Schnorr signature.
///
/// Implements [`compute_group_commitment`] from the spec.
///
/// [`compute_group_commitment`]: https://datatracker.ietf.org/doc/html/rfc9591#name-group-commitment-computatio
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
fn compute_group_commitment<C>(
    signing_package: &SigningPackage<C>,
    binding_factor_list: &BindingFactorList<C>,
) -> Result<GroupCommitment<C>, Error<C>>
where
    C: Ciphersuite,
{
    let identity = <C::Group as Group>::identity();

    let mut group_commitment = <C::Group as Group>::identity();

    // Number of signing participants we are iterating over.
    let n = signing_package.signing_commitments().len();

    let mut binding_scalars = Vec::with_capacity(n);

    let mut binding_elements = Vec::with_capacity(n);

    for (commitment_identifier, commitment) in signing_package.signing_commitments() {
        // The following check prevents a party from accidentally revealing their share.
        // Note that the '&&' operator would be sufficient.
        if identity == commitment.binding.value() || identity == commitment.hiding.value() {
            return Err(Error::IdentityCommitment);
        }

        let binding_factor = binding_factor_list
            .get(commitment_identifier)
            .ok_or(Error::UnknownIdentifier)?;

        // Collect the binding commitments and their binding factors for one big
        // multiscalar multiplication at the end.
        binding_elements.push(commitment.binding.value());
        binding_scalars.push(binding_factor.0);

        group_commitment = group_commitment + commitment.hiding.value();
    }

    let accumulated_binding_commitment: Element<C> =
        VartimeMultiscalarMul::<C>::vartime_multiscalar_mul(binding_scalars, binding_elements);

    group_commitment = group_commitment + accumulated_binding_commitment;

    Ok(GroupCommitment(group_commitment))
}

////////////////////////////////////////////////////////////////////////////////
// Aggregation
////////////////////////////////////////////////////////////////////////////////

/// Aggregates the signature shares to produce a final signature that
/// can be verified with the group public key.
///
/// `signature_shares` maps the identifier of each participant to the
/// [`round2::SignatureShare`] they sent. These identifiers must come from whatever mapping
/// the coordinator has between communication channels and participants, i.e.
/// they must have assurance that the [`round2::SignatureShare`] came from
/// the participant with that identifier.
///
/// This operation is performed by a coordinator that can communicate with all
/// the signing participants before publishing the final signature. The
/// coordinator can be one of the participants or a semi-trusted third party
/// (who is trusted to not perform denial of service attacks, but does not learn
/// any secret information). Note that because the coordinator is trusted to
/// report misbehaving parties in order to avoid publishing an invalid
/// signature, if the coordinator themselves is a signer and misbehaves, they
/// can avoid that step. However, at worst, this results in a denial of
/// service attack due to publishing an invalid signature.

pub fn aggregate<C>(
    signing_package: &SigningPackage<C>,
    signature_shares: &BTreeMap<Identifier<C>, round2::SignatureShare<C>>,
    pubkeys: &keys::PublicKeyPackage<C>,
) -> Result<Signature<C>, Error<C>>
where
    C: Ciphersuite,
{
    // Check if signing_package.signing_commitments and signature_shares have
    // the same set of identifiers, and if they are all in pubkeys.verifying_shares.
    if signing_package.signing_commitments().len() != signature_shares.len() {
        return Err(Error::UnknownIdentifier);
    }

    if !signing_package.signing_commitments().keys().all(|id| {
        #[cfg(feature = "cheater-detection")]
        return signature_shares.contains_key(id) && pubkeys.verifying_shares().contains_key(id);
        #[cfg(not(feature = "cheater-detection"))]
        return signature_shares.contains_key(id);
    }) {
        return Err(Error::UnknownIdentifier);
    }

    // Encodes the signing commitment list produced in round one as part of generating [`BindingFactor`], the
    // binding factor.
    let binding_factor_list: BindingFactorList<C> =
        compute_binding_factor_list(signing_package, &pubkeys.verifying_key, &[])?;
    // Compute the group commitment from signing commitments produced in round one.
    let group_commitment = compute_group_commitment(signing_package, &binding_factor_list)?;

    // The aggregation of the signature shares by summing them up, resulting in
    // a plain Schnorr signature.
    //
    // Implements [`aggregate`] from the spec.
    //
    // [`aggregate`]: https://datatracker.ietf.org/doc/html/rfc9591#name-signature-share-aggregation
    let mut z = <<C::Group as Group>::Field>::zero();

    for signature_share in signature_shares.values() {
        z = z + signature_share.to_scalar();
    }

    let signature = Signature {
        R: group_commitment.0,
        z,
    };

    // Verify the aggregate signature
    let verification_result = pubkeys
        .verifying_key
        .verify(signing_package.message(), &signature);

    // Only if the verification of the aggregate signature failed; verify each share to find the cheater.
    // This approach is more efficient since we don't need to verify all shares
    // if the aggregate signature is valid (which should be the common case).
    #[cfg(feature = "cheater-detection")]
    if verification_result.is_err() {
        detect_cheater(
            group_commitment,
            pubkeys,
            signing_package,
            signature_shares,
            &binding_factor_list,
        )?;
    }

    #[cfg(not(feature = "cheater-detection"))]
    verification_result?;

    Ok(signature)
}

/// Optional cheater detection feature
/// Each share is verified to find the cheater
#[cfg(feature = "cheater-detection")]
fn detect_cheater<C: Ciphersuite>(
    group_commitment: GroupCommitment<C>,
    pubkeys: &keys::PublicKeyPackage<C>,
    signing_package: &SigningPackage<C>,
    signature_shares: &BTreeMap<Identifier<C>, round2::SignatureShare<C>>,
    binding_factor_list: &BindingFactorList<C>,
) -> Result<(), Error<C>> {
    // Compute the per-message challenge.
    let challenge = crate::challenge::<C>(
        &group_commitment.0,
        &pubkeys.verifying_key,
        signing_package.message().as_slice(),
    )?;

    // Verify the signature shares.
    for (identifier, signature_share) in signature_shares {
        // Look up the public key for this signer, where `signer_pubkey` = _G.ScalarBaseMult(s[i])_,
        // and where s[i] is a secret share of the constant term of _f_, the secret polynomial.
        let verifying_share = pubkeys
            .verifying_shares
            .get(identifier)
            .ok_or(Error::UnknownIdentifier)?;

        verify_signature_share_precomputed(
            *identifier,
            signing_package,
            binding_factor_list,
            signature_share,
            verifying_share,
            challenge,
        )?;
    }

    // We should never reach here; but we return an error to be safe.
    Err(Error::InvalidSignature)
}

/// Verify a signature share for the given participant `identifier`,
/// `verifying_share` and `signature_share`; with the `signing_package`
/// for which the signature share was produced and with the group's
/// `verifying_key`.
///
/// This is not required for regular FROST usage but might useful in certain
/// situations where it is desired to verify each individual signature share
/// before aggregating the signature.
pub fn verify_signature_share<C: Ciphersuite>(
    identifier: Identifier<C>,
    verifying_share: &keys::VerifyingShare<C>,
    signature_share: &round2::SignatureShare<C>,
    signing_package: &SigningPackage<C>,
    verifying_key: &VerifyingKey<C>,
) -> Result<(), Error<C>> {
    // Encodes the signing commitment list produced in round one as part of generating [`BindingFactor`], the
    // binding factor.
    let binding_factor_list: BindingFactorList<C> =
        compute_binding_factor_list(signing_package, verifying_key, &[])?;
    // Compute the group commitment from signing commitments produced in round one.
    let group_commitment = compute_group_commitment(signing_package, &binding_factor_list)?;

    // Compute the per-message challenge.
    let challenge = crate::challenge::<C>(
        &group_commitment.to_element(),
        verifying_key,
        signing_package.message().as_slice(),
    )?;

    verify_signature_share_precomputed(
        identifier,
        signing_package,
        &binding_factor_list,
        signature_share,
        verifying_share,
        challenge,
    )
}

/// Similar to [`verify_signature_share()`] but using a precomputed
/// `binding_factor_list` and `challenge`.
#[cfg_attr(feature = "internals", visibility::make(pub))]
#[cfg_attr(docsrs, doc(cfg(feature = "internals")))]
fn verify_signature_share_precomputed<C: Ciphersuite>(
    signature_share_identifier: Identifier<C>,
    signing_package: &SigningPackage<C>,
    binding_factor_list: &BindingFactorList<C>,
    signature_share: &round2::SignatureShare<C>,
    verifying_share: &keys::VerifyingShare<C>,
    challenge: Challenge<C>,
) -> Result<(), Error<C>> {
    let lambda_i = derive_interpolating_value(&signature_share_identifier, signing_package)?;

    let binding_factor = binding_factor_list
        .get(&signature_share_identifier)
        .ok_or(Error::UnknownIdentifier)?;

    let R_share = signing_package
        .signing_commitment(&signature_share_identifier)
        .ok_or(Error::UnknownIdentifier)?
        .to_group_commitment_share(binding_factor);

    signature_share.verify(
        signature_share_identifier,
        &R_share,
        verifying_share,
        lambda_i,
        &challenge,
    )?;

    Ok(())
}