signing_key.rs

//! Schnorr signature signing keys

use alloc::vec::Vec;

use rand_core::{CryptoRng, RngCore};

use crate::{
    random_nonzero, serialization::SerializableScalar, Challenge, Ciphersuite, Error, Field, Group,
    Scalar, Signature, VerifyingKey,
};

/// A signing key for a Schnorr signature on a FROST [`Ciphersuite::Group`].
#[derive(Copy, Clone, PartialEq, Eq)]
pub struct SigningKey<C>
where
    C: Ciphersuite,
{
    pub(crate) scalar: Scalar<C>,
}

impl<C> SigningKey<C>
where
    C: Ciphersuite,
{
    /// Generate a new signing key.
    pub fn new<R: RngCore + CryptoRng>(rng: &mut R) -> SigningKey<C> {
        let scalar = random_nonzero::<C, R>(rng);

        SigningKey { scalar }
    }

    /// Deserialize from bytes
    pub fn deserialize(bytes: &[u8]) -> Result<SigningKey<C>, Error<C>> {
        Self::from_scalar(SerializableScalar::deserialize(bytes)?.0)
    }

    /// Serialize `SigningKey` to bytes
    pub fn serialize(&self) -> Vec<u8> {
        SerializableScalar::<C>(self.scalar).serialize()
    }

    /// Create a signature `msg` using this `SigningKey`.
    pub fn sign<R: RngCore + CryptoRng>(&self, rng: R, message: &[u8]) -> Signature<C> {
        <C>::single_sign(self, rng, message)
    }

    /// Create a signature `msg` using this `SigningKey` using the default
    /// signing.
    #[cfg(feature = "internals")]
    pub fn default_sign<R: RngCore + CryptoRng>(&self, mut rng: R, message: &[u8]) -> Signature<C> {
        let public = VerifyingKey::<C>::from(*self);

        let (k, R) = <C>::generate_nonce(&mut rng);

        // Generate Schnorr challenge
        let c: Challenge<C> = <C>::challenge(&R, &public, message).expect("should not return error since that happens only if one of the inputs is the identity. R is not since k is nonzero. The verifying_key is not because signing keys are not allowed to be zero.");

        let z = k + (c.0 * self.scalar);

        Signature { R, z }
    }

    /// Creates a SigningKey from a scalar. Returns an error if the scalar is zero.
    pub fn from_scalar(
        scalar: <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar,
    ) -> Result<Self, Error<C>> {
        if scalar == <<C::Group as Group>::Field as Field>::zero() {
            return Err(Error::MalformedSigningKey);
        }
        Ok(Self { scalar })
    }

    /// Return the underlying scalar.
    pub fn to_scalar(self) -> <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar {
        self.scalar
    }
}

impl<C> core::fmt::Debug for SigningKey<C>
where
    C: Ciphersuite,
{
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_tuple("SigningKey").field(&"<redacted>").finish()
    }
}

impl<C> From<&SigningKey<C>> for VerifyingKey<C>
where
    C: Ciphersuite,
{
    fn from(signing_key: &SigningKey<C>) -> Self {
        VerifyingKey::new(C::Group::generator() * signing_key.scalar)
    }
}

impl<C> From<SigningKey<C>> for VerifyingKey<C>
where
    C: Ciphersuite,
{
    fn from(signing_key: SigningKey<C>) -> Self {
        VerifyingKey::<C>::from(&signing_key)
    }
}