schnorr 0.0.20

Schnorr Signatures
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//! Multi extenstion for Signature Type

use crate::{
    errors::{self, MuSigError, SchnorrError},
    BatchVerification, MuSigContext, Multimessage, PublicKey, Signature, SingleVerifier,
};
use bacteria::Transcript;
use mohan::dalek::{constants::RISTRETTO_BASEPOINT_POINT, scalar::Scalar};
use std::borrow::Borrow;
use std::iter;

/// Extension trait for `schnorr::Signature`.
pub trait Multisignature {
    /// Creates a multi-message signature.
    fn sign_multi<P, M>(
        privkeys: P,
        messages: Vec<(PublicKey, M)>,
        transcript: &mut Transcript,
    ) -> Result<Signature, SchnorrError>
    where
        M: AsRef<[u8]>,
        P: IntoIterator,
        P::Item: Borrow<Scalar>,
        P::IntoIter: ExactSizeIterator;

    /// Verifies a multi-message signature.
    fn verify_multi<M: AsRef<[u8]>>(
        &self,
        transcript: &mut Transcript,
        messages: Vec<(PublicKey, M)>,
    ) -> Result<(), SchnorrError>;

    /// Verifies a multi-message signature in a batch.
    fn verify_multi_batched<M: AsRef<[u8]>>(
        &self,
        transcript: &mut Transcript,
        messages: Vec<(PublicKey, M)>,
        batch: &mut impl BatchVerification,
    );
}

impl Multisignature for Signature {
    fn sign_multi<P, M>(
        privkeys: P,
        messages: Vec<(PublicKey, M)>,
        transcript: &mut Transcript,
    ) -> Result<Signature, SchnorrError>
    where
        M: AsRef<[u8]>,
        P: IntoIterator,
        P::Item: Borrow<Scalar>,
        P::IntoIter: ExactSizeIterator,
    {
        let mut privkeys = privkeys.into_iter().peekable();

        if messages.len() != privkeys.len() {
            return Err(errors::from_musig(MuSigError::BadArguments));
        }
        if privkeys.len() == 0 {
            return Err(errors::from_musig(MuSigError::BadArguments));
        }

        let context = Multimessage::new(messages);

        let mut rng = transcript
            .build_rng()
            // Use one key that has enough entropy to seed the RNG.
            // We can call unwrap because we know that the privkeys length is > 0.
            .rekey_with_witness_bytes(b"x_i", privkeys.peek().unwrap().borrow().as_bytes())
            .finalize(&mut rand::thread_rng());

        // Generate ephemeral keypair (r, R). r is a random nonce.
        let r = Scalar::random(&mut rng);
        // R = generator * r
        let R = (RISTRETTO_BASEPOINT_POINT * r).compress();

        // Commit the context, and commit the nonce sum with label "R"
        context.commit(transcript);
        transcript.commit_point(b"R", &R);

        // Generate signature: s = r + sum{c_i * x_i}
        let mut s = r;
        for (i, x_i) in privkeys.enumerate() {
            let mut t = transcript.clone();
            let c_i = context.challenge(i, &mut t);
            s = s + c_i * x_i.borrow();
        }

        Ok(Signature { s, R })
    }

    /// Verifies a signature for a multimessage context
    fn verify_multi<M: AsRef<[u8]>>(
        &self,
        transcript: &mut Transcript,
        messages: Vec<(PublicKey, M)>,
    ) -> Result<(), SchnorrError> {
        SingleVerifier::verify(|verifier| self.verify_multi_batched(transcript, messages, verifier))
    }

    fn verify_multi_batched<M: AsRef<[u8]>>(
        &self,
        transcript: &mut Transcript,
        messages: Vec<(PublicKey, M)>,
        batch: &mut impl BatchVerification,
    ) {
        let context = Multimessage::new(messages);
        context.commit(transcript);
        transcript.commit_point(b"R", &self.R);

        // Form the final linear combination:
        // `s * G = R + sum{c_i * X_i}`
        //      ->
        // `0 == (-s * G) + (1 * R) + sum{c_i * X_i}`
        let n = context.len();
        batch.append(
            -self.s,
            iter::once(Scalar::one())
                .chain((0..n).map(|i| context.challenge(i, &mut transcript.clone()))),
            iter::once(self.R.decompress())
                .chain((0..n).map(|i| Some(context.key(i).into_point()))),
        );
    }
}