Crate vsss_rs

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Verifiable Secret Sharing Schemes are using to split secrets into multiple shares and distribute them among different entities, with the ability to verify if the shares are correct and belong to a specific set. This crate includes Shamir’s secret sharing scheme which does not support verification but is more of a building block for the other schemes.

This crate supports Feldman and Pedersen verifiable secret sharing schemes.

Feldman and Pedersen are similar in many ways. It’s hard to describe when to use one over the other. Indeed both are used in http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.134.6445&rep=rep1&type=pdf.

Feldman reveals the public value of the verifier whereas Pedersen’s hides it.

Feldman and Pedersen are different from Shamir when splitting the secret. Combining shares back into the original secret is identical across all methods and is available for each scheme for convenience.

This crate is no-standard compliant and uses const generics to specify sizes.

This crate supports any number as the maximum number of shares to be requested. Anything higher than 255 is pretty ridiculous but if such a use case exists please let me know. This said, any number of shares can be requested since identifiers can be any size.

Shares are represented as byte arrays. Shares can represent finite fields or groups depending on the use case. In the simplest case, the first byte is reserved for the share identifier (x-coordinate) and everything else is the actual value of the share (y-coordinate). However, anything can be used as the identifier as long as it implements the ShareIdentifier trait.

When specifying share sizes, use the field size in bytes + 1 for the identifier in the easiest case.

To split a p256 secret using Shamir

use vsss_rs::{*, shamir};
use elliptic_curve::ff::PrimeField;
use p256::{NonZeroScalar, Scalar, SecretKey};

let mut osrng = rand_core::OsRng::default();
let sk = SecretKey::random(&mut osrng);
let nzs = sk.to_nonzero_scalar();
let res = shamir::split_secret::<Scalar, u8, Vec<u8>>(2, 3, *nzs.as_ref(), &mut osrng);
assert!(res.is_ok());
let shares = res.unwrap();
let res = combine_shares(&shares);
assert!(res.is_ok());
let scalar: Scalar = res.unwrap();
let nzs_dup =  NonZeroScalar::from_repr(scalar.to_repr()).unwrap();
let sk_dup = SecretKey::from(nzs_dup);
assert_eq!(sk_dup.to_bytes(), sk.to_bytes());

To split a k256 secret using Shamir

use vsss_rs::{*, shamir};
use elliptic_curve::ff::PrimeField;
use k256::{NonZeroScalar, Scalar, ProjectivePoint, SecretKey};

let mut osrng = rand_core::OsRng::default();
let sk = SecretKey::random(&mut osrng);
let secret = *sk.to_nonzero_scalar();
let res = shamir::split_secret::<Scalar, u8, Vec<u8>>(2, 3, secret, &mut osrng);
assert!(res.is_ok());
let shares = res.unwrap();
let res = combine_shares(&shares);
assert!(res.is_ok());
let scalar: Scalar = res.unwrap();
let nzs_dup = NonZeroScalar::from_repr(scalar.to_repr()).unwrap();
let sk_dup = SecretKey::from(nzs_dup);
assert_eq!(sk_dup.to_bytes(), sk.to_bytes());

Feldman or Pedersen return extra information for verification using their respective verifiers

use vsss_rs::{*, feldman};
use bls12_381_plus::{Scalar, G1Projective};
use elliptic_curve::ff::Field;

let mut rng = rand_core::OsRng::default();
let secret = Scalar::random(&mut rng);
let res = feldman::split_secret::<G1Projective, u8, Vec<u8>>(2, 3, secret, None, &mut rng);
assert!(res.is_ok());
let (shares, verifier) = res.unwrap();
for s in &shares {
    assert!(verifier.verify_share(s).is_ok());
}
let res = combine_shares(&shares);
assert!(res.is_ok());
let secret_1: Scalar = res.unwrap();
assert_eq!(secret, secret_1);

Curve25519 is not a prime field but this crate does support it using features=["curve25519"] which is enabled by default. This feature wraps curve25519-dalek libraries so they can be used with Shamir, Feldman, and Pedersen.

Here’s an example of using Ed25519 and x25519

#[cfg(feature = "curve25519")] {
use curve25519_dalek::scalar::Scalar;
use rand::Rng;
use ed25519_dalek::SigningKey;
use vsss_rs::{curve25519::WrappedScalar, *};
use x25519_dalek::StaticSecret;

let mut osrng = rand::rngs::OsRng::default();
let sc = Scalar::hash_from_bytes::<sha2::Sha512>(&osrng.gen::<[u8; 32]>());
let sk1 = StaticSecret::from(sc.to_bytes());
let ske1 = SigningKey::from_bytes(&sc.to_bytes());
let res = shamir::split_secret::<WrappedScalar, u8, Vec<u8>>(2, 3, sc.into(), &mut osrng);
assert!(res.is_ok());
let shares = res.unwrap();
let res = combine_shares(&shares);
assert!(res.is_ok());
let scalar: WrappedScalar = res.unwrap();
assert_eq!(scalar.0, sc);
let sk2 = StaticSecret::from(scalar.0.to_bytes());
let ske2 = SigningKey::from_bytes(&scalar.0.to_bytes());
assert_eq!(sk2.to_bytes(), sk1.to_bytes());
assert_eq!(ske1.to_bytes(), ske2.to_bytes());
}

Re-exports§

Modules§

Macros§

  • vsss_arr_impl
    Create a no-std verifiable secret sharing scheme with size $num using fixed arrays The arguments in order are: The vsss name The name for a pedersen secret sharing scheme result The maximum threshold allowed The maximum number of shares allowed

Structs§

Enums§

  • Error
    Errors during secret sharing

Traits§

Functions§

  • combine_shares
    Reconstruct a secret from shares created from split_secret. The X-coordinates operate in F The Y-coordinates operate in F
  • combine_shares_group
    Reconstruct a secret from shares created from split_secret. The X-coordinates operate in F The Y-coordinates operate in G

Type Aliases§