frost/frost-secp256k1/src/keys/repairable.rs

//! Repairable Threshold Scheme
//!
//! Implements the Repairable Threshold Scheme (RTS) from <https://eprint.iacr.org/2017/1155>.
//! The RTS is used to help a signer (participant) repair their lost share. This is achieved
//! using a subset of the other signers know here as `helpers`.

use std::collections::HashMap;

// This is imported separately to make `gencode` work.
// (if it were below, the position of the import would vary between ciphersuites
//  after `cargo fmt`)
use crate::{frost, Ciphersuite, CryptoRng, Identifier, RngCore, Scalar};
use crate::{Error, Secp256K1Sha256};

use super::{SecretShare, VerifiableSecretSharingCommitment};

/// Step 1 of RTS.
///
/// Generates the "delta" values from `helper_i` to help `participant` recover their share
/// where `helpers` contains the identifiers of all the helpers (including `helper_i`), and `share_i`
/// is the share of `helper_i`.
///
/// Returns a HashMap mapping which value should be sent to which participant.
pub fn repair_share_step_1<C: Ciphersuite, R: RngCore + CryptoRng>(
    helpers: &[Identifier],
    share_i: &SecretShare,
    rng: &mut R,
    participant: Identifier,
) -> Result<HashMap<Identifier, Scalar>, Error> {
    frost::keys::repairable::repair_share_step_1(helpers, share_i, rng, participant)
}

/// Step 2 of RTS.
///
/// Generates the `sigma` values from all `deltas` received from `helpers`
/// to help `participant` recover their share.
/// `sigma` is the sum of all received `delta` and the `delta_i` generated for `helper_i`.
///
/// Returns a scalar
pub fn repair_share_step_2(deltas_j: &[Scalar]) -> Scalar {
    frost::keys::repairable::repair_share_step_2::<Secp256K1Sha256>(deltas_j)
}

/// Step 3 of RTS
///
/// The `participant` sums all `sigma_j` received to compute the `share`. The `SecretShare`
/// is made up of the `identifier`and `commitment` of the `participant` as well as the
/// `value` which is the `SigningShare`.
pub fn repair_share_step_3(
    sigmas: &[Scalar],
    identifier: Identifier,
    commitment: &VerifiableSecretSharingCommitment,
) -> SecretShare {
    frost::keys::repairable::repair_share_step_3(sigmas, identifier, commitment)
}

#[cfg(test)]
mod tests {

    use lazy_static::lazy_static;
    use rand::thread_rng;
    use serde_json::Value;

    use crate::Secp256K1Sha256;

    lazy_static! {
        pub static ref REPAIR_SHARE: Value =
            serde_json::from_str(include_str!("../../tests/helpers/repair-share.json").trim())
                .unwrap();
    }

    #[test]
    fn check_repair_share_step_1() {
        let rng = thread_rng();

        frost_core::tests::repairable::check_repair_share_step_1::<Secp256K1Sha256, _>(rng);
    }

    #[test]
    fn check_repair_share_step_2() {
        frost_core::tests::repairable::check_repair_share_step_2::<Secp256K1Sha256>(&REPAIR_SHARE);
    }

    #[test]
    fn check_repair_share_step_3() {
        let rng = thread_rng();
        frost_core::tests::repairable::check_repair_share_step_3::<Secp256K1Sha256, _>(
            rng,
            &REPAIR_SHARE,
        );
    }
}