nym-compact-ecash 1.20.4

Nym's ecash implementation
Documentation 
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// Copyright 2024 - Nym Technologies SA <contact@nymtech.net>
// SPDX-License-Identifier: Apache-2.0

use std::borrow::Borrow;

use digest::generic_array::typenum::Unsigned;
use digest::Digest;
use nym_bls12_381_fork::Scalar;
use sha2::Sha256;

pub mod proof_spend;
pub mod proof_withdrawal;

type ChallengeDigest = Sha256;

/// Generates a Scalar [or Fp] challenge by hashing a number of elliptic curve points.
fn compute_challenge<D, I, B>(iter: I) -> Scalar
where
    D: Digest,
    I: Iterator<Item = B>,
    B: AsRef<[u8]>,
{
    let mut h = D::new();
    for point_representation in iter {
        h.update(point_representation);
    }
    let digest = h.finalize();

    // TODO: I don't like the 0 padding here (though it's what we've been using before,
    // but we never had a security audit anyway...)
    // instead we could maybe use the `from_bytes` variant and adding some suffix
    // when computing the digest until we produce a valid scalar.
    let mut bytes = [0u8; 64];
    let pad_size = 64usize
        .checked_sub(D::OutputSize::to_usize())
        .unwrap_or_default();

    bytes[pad_size..].copy_from_slice(&digest);

    Scalar::from_bytes_wide(&bytes)
}

fn produce_response(witness_replacement: &Scalar, challenge: &Scalar, secret: &Scalar) -> Scalar {
    witness_replacement - challenge * secret
}

// note: it's caller's responsibility to ensure witnesses.len() = secrets.len()
fn produce_responses<S>(witnesses: &[Scalar], challenge: &Scalar, secrets: &[S]) -> Vec<Scalar>
where
    S: Borrow<Scalar>,
{
    debug_assert_eq!(witnesses.len(), secrets.len());

    witnesses
        .iter()
        .zip(secrets.iter())
        .map(|(w, x)| produce_response(w, challenge, x.borrow()))
        .collect()
}