README.md

BLS Threshold CryptoSignatures

This library provides primitives for (blind) threshold cryptography. Currently supported curves are BN254(alt_bn128) and BLS12-381.

Work In Progress: DO NOT EXPECT ANY STABLE API NOW

Group functionality

src/group.rs contains the definitions of generic trait to work with scalars of prime fields and points on elliptic curves. The following Element trait allows to get a generic implementation of a polynomial with lagrange interpolation for both scalars and points.

pub trait Element<RHS = Self>: Clone + fmt::Display + fmt::Debug + Eq {
    /// new MUST return the zero element of the group.
    fn new() -> Self;
    fn one() -> Self;
    fn add(&mut self, s2: &Self);
    fn mul(&mut self, mul: &RHS);
    fn pick<R: RngCore>(&mut self, rng: &mut R);
    fn zero() -> Self {
        Self::new()
    }
}

There is an implementation of these traits using the curve BLS12-381 in src/bls12381.rs.

Polynomial functionality

src/poly.rs contains the implementation of a polynomial suitable to be used for secret sharing schemes and the dkg protocol. It can evaluates shares and interpolate private and public shares to their corresponding polynomial.

The following (from the tests) shows how to interploate a set of private shares:

use crate::bls12381::Scalar as Sc;
fn interpolation() {
    let degree = 4;
    let threshold = degree + 1;
    let poly = Poly::<Sc, Sc>::new(degree);
    let shares = (0..threshold)
        .map(|i| poly.eval(i as u64))
        .collect::<Vec<Share<Sc>>>();
    let recovered = Poly::<Sc, Sc>::recover(threshold as usize, shares);
    let expected = poly.c[0];
    let computed = recovered.c[0];
    assert_eq!(expected, computed);
}

Curve Implementations

Curently there are two curves available, BLS12 381 and BN254. By default they are enabled both, but you can select which one you want to use using the features bls12_381 and bn254.

You can use them like this when adding the dependency to your Cargo.toml file.

# Only bls12_381
threshold = { version = "0.1", default-features = false, features = ["bls12_381"] }
# Only bn254
threshold = { version = "0.1", default-features = false, features = ["bn254"] }
# Both
threshold = { version = "0.1" }