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- use curve25519_dalek::edwards::EdwardsPoint;
- use curve25519_dalek::traits::Identity;
- use curve25519_dalek::scalar::Scalar;
- use curve25519_dalek::constants::ED25519_BASEPOINT_POINT;
- use rand::rngs::ThreadRng;
- pub type Secret = Scalar;
- #[derive(Copy, Clone)]
- pub struct Share {
- index: u32,
- value: Scalar,
- }
- pub struct Commitment {
- coms: Vec<EdwardsPoint>,
- }
- /// Create secret shares for a given secret.
- pub fn generate_shares(secret: Secret, numshares: u32, threshold: u32) -> Result<(Commitment, Vec<Share>), &'static str> {
- if threshold < 1 { return Err("Threshold cannot be 0") }
- if numshares < 1 { return Err("Number of shares cannot be 0") }
- if threshold > numshares { return Err("Threshold cannot exceed numshares") }
- let numcoeffs = (threshold-1) as usize;
- let mut coeffs: Vec<Scalar> = Vec::with_capacity(numcoeffs);
- let mut rng: ThreadRng = rand::thread_rng();
- let mut shares: Vec<Share> = Vec::with_capacity(numshares as usize);
- let mut comm: Commitment = Commitment { coms: Vec::with_capacity(threshold as usize) };
- for _ in 0..numcoeffs {
- coeffs.push(Scalar::random(&mut rng));
- }
- for share_index in 1..numshares+1 {
- // Evaluate the polynomial with `secret` as the constant term
- // and `coeffs` as the other coefficients at the point x=share_index
- // using Horner's method
- let scalar_index = Scalar::from(share_index);
- let mut value = Scalar::zero();
- for i in (0..numcoeffs).rev() {
- value += coeffs[i];
- value *= scalar_index;
- }
- value += secret;
- shares.push(Share{ index: share_index, value: value });
- }
- comm.coms.push(ED25519_BASEPOINT_POINT * secret);
- for c in coeffs {
- comm.coms.push(ED25519_BASEPOINT_POINT * c);
- }
- Ok((comm, shares))
- }
- /// Verify that a share is consistent with a commitment.
- pub fn verify_share(share: Share, commitment: &Commitment) -> Result<bool, &'static str> {
- let f_result = ED25519_BASEPOINT_POINT * share.value;
- let x = Scalar::from(share.index);
- let (_, result) = commitment.coms.iter().fold(
- (Scalar::one(), EdwardsPoint::identity()),
- |(x_to_the_i, sum_so_far), comm_i|
- (x_to_the_i * x, sum_so_far + x_to_the_i * comm_i));
- let is_valid = f_result == result;
- Ok(is_valid)
- }
- /// Reconstruct the secret from enough (at least the threshold) already-verified shares.
- pub fn reconstruct_secret(shares: &Vec<Share>) -> Result<Secret, &'static str> {
- let numshares = shares.len();
- if numshares < 1 { return Err("No shares provided"); }
- let mut lagrange_coeffs: Vec<Scalar> = Vec::with_capacity(numshares);
- for i in 0..numshares {
- let mut num = Scalar::one();
- let mut den = Scalar::one();
- for j in 0..numshares {
- if j==i { continue; }
- num *= Scalar::from(shares[j].index);
- den *= Scalar::from(shares[j].index) - Scalar::from(shares[i].index);
- }
- if den == Scalar::zero() {
- return Err("Duplicate shares provided");
- }
- lagrange_coeffs.push(num * den.invert());
- }
- let mut secret = Scalar::zero();
- for i in 0..numshares {
- secret += lagrange_coeffs[i] * shares[i].value;
- }
- return Ok(secret)
- }
- /// Create and apply a proactive update to a master commitment.
- pub fn update_shares(old_commitment: &Commitment, old_shares: &Vec<Share>) -> Result<(Commitment, Vec<Share>), &'static str> {
- let num_shares = old_shares.len() as u32;
- let threshold = old_commitment.coms.len() as u32;
- let res = generate_shares(Scalar::zero(), num_shares, threshold);
- if !res.is_ok() {
- return res;
- }
- let (update_commitments, update_shares) = res.unwrap();
- let mut new_shares: Vec<Share> = Vec::with_capacity(num_shares as usize);
- for old_share in old_shares {
- let update_share = update_shares[(old_share.index-1) as usize];
- let new_share = Share{
- index: old_share.index,
- value: old_share.value + update_share.value,
- };
- new_shares.push(new_share);
- }
- let mut new_commitments: Commitment = Commitment { coms: Vec::with_capacity(threshold as usize) };
- for i in 0..old_commitment.coms.len() {
- let new_commitment = old_commitment.coms[i] + update_commitments.coms[i];
- new_commitments.coms.push(new_commitment);
- }
- Ok((new_commitments, new_shares))
- }
- #[cfg(test)]
- mod tests {
- use crate::vss::*;
- #[test]
- fn share_simple() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (com, shares) = res.unwrap();
- assert!(shares.len() == 5);
- assert!(com.coms.len() == 2);
- let mut recshares: Vec<Share> = Vec::new();
- recshares.push(shares[1]);
- recshares.push(shares[3]);
- let recres = reconstruct_secret(&recshares);
- assert!(recres.is_ok());
- assert_eq!(recres.unwrap(), s);
- }
- #[test]
- fn share_not_enough() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (_, shares) = res.unwrap();
- let mut recshares: Vec<Share> = Vec::new();
- recshares.push(shares[1]);
- let recres = reconstruct_secret(&recshares);
- assert!(recres.is_ok());
- assert_ne!(recres.unwrap(), s);
- }
- #[test]
- fn share_dup() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (_, shares) = res.unwrap();
- let mut recshares: Vec<Share> = Vec::new();
- recshares.push(shares[1]);
- recshares.push(shares[1]);
- let recres = reconstruct_secret(&recshares);
- assert!(recres.is_err());
- assert_eq!(recres.err(), Some("Duplicate shares provided"));
- }
- #[test]
- fn share_badparams() {
- let s = Secret::from(42u32);
- {
- let res = generate_shares(s, 5, 0);
- assert!(res.is_err());
- assert_eq!(res.err(), Some("Threshold cannot be 0"));
- }
- {
- let res = generate_shares(s, 0, 3);
- assert!(res.is_err());
- assert_eq!(res.err(), Some("Number of shares cannot be 0"));
- }
- {
- let res = generate_shares(s, 1, 3);
- assert!(res.is_err());
- assert_eq!(res.err(), Some("Threshold cannot exceed numshares"));
- }
- }
- #[test]
- fn share_commitment_valid() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 8, 3);
- assert!(res.is_ok());
- let (com, shares) = res.unwrap();
- for share in shares {
- let is_valid = verify_share(share, &com);
- assert!(is_valid.is_ok());
- assert!(is_valid.unwrap());
- }
- }
- #[test]
- fn share_commitment_invalid() {
- let s1 = Secret::from(42u32);
- let s2 = Secret::from(42u32);
- let res1 = generate_shares(s1, 8, 3);
- assert!(res1.is_ok());
- let (_, shares1) = res1.unwrap();
- let res2 = generate_shares(s2, 8, 3);
- assert!(res2.is_ok());
- let (com2, _) = res2.unwrap();
- for share in shares1 {
- // test that commitments to a different set of shares fails
- let is_valid = verify_share(share, &com2);
- assert!(is_valid.is_ok());
- assert_ne!(is_valid.unwrap(), true);
- }
- }
- #[test]
- fn share_update_valid() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (com, shares) = res.unwrap();
- let update_res = update_shares(&com, &shares);
- assert!(update_res.is_ok());
- let (update_com, update_shares) = update_res.unwrap();
- for share in update_shares {
- let is_valid = verify_share(share, &update_com);
- assert!(is_valid.is_ok());
- assert!(is_valid.unwrap());
- }
- }
- #[test]
- fn share_update_invalid() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (com, shares) = res.unwrap();
- let update_res = update_shares(&com, &shares);
- assert!(update_res.is_ok());
- let (update_com, update_shares) = update_res.unwrap();
- // test that the old shares are not valid with the updated commitment
- for share in shares {
- let is_valid = verify_share(share, &update_com);
- assert!(is_valid.is_ok());
- assert!(!is_valid.unwrap());
- }
- // assert that we can recover the original secret using the updated shares
- let recres = reconstruct_secret(&update_shares);
- assert!(recres.is_ok());
- assert_eq!(recres.unwrap(), s);
- }
- #[test]
- fn share_update_valid_multiple_updates() {
- let s = Secret::from(42u32);
- let res = generate_shares(s, 5, 2);
- assert!(res.is_ok());
- let (com, shares) = res.unwrap();
- for _ in 0..5 {
- let update_res = update_shares(&com, &shares);
- assert!(update_res.is_ok());
- let (update_com, update_shares) = update_res.unwrap();
- for share in update_shares {
- let is_valid = verify_share(share, &update_com);
- assert!(is_valid.is_ok());
- assert!(is_valid.unwrap());
- }
- }
- }
- }
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