# FROST (Flexible Round-Optimised Schnorr Threshold signatures)

Rust implementations of ['Two-Round Threshold Schnorr Signatures with FROST'](https://datatracker.ietf.org/doc/draft-irtf-cfrg-frost/).

Unlike signatures in a single-party setting, threshold signatures require cooperation among a
threshold number of signers, each holding a share of a common private key. The security of threshold
schemes in general assume that an adversary can corrupt strictly fewer than a threshold number of
participants.

['Two-Round Threshold Schnorr Signatures with
FROST'](https://datatracker.ietf.org/doc/draft-irtf-cfrg-frost/) presents a variant of a Flexible
Round-Optimized Schnorr Threshold (FROST) signature scheme originally defined in
[FROST20](https://eprint.iacr.org/2020/852.pdf). FROST reduces network overhead during threshold
signing operations while employing a novel technique to protect against forgery attacks applicable
to prior Schnorr-based threshold signature constructions. This variant of FROST requires two rounds
to compute a signature, and implements signing efficiency improvements described by
[Schnorr21](https://eprint.iacr.org/2021/1375.pdf). Single-round signing with FROST is not
implemented here.

## Status ⚠

The FROST specification is not yet finalized, and this codebase has not yet been audited or
released. The APIs and types in `frost-core` are subject to change.

## Usage

`frost-core` implements the base traits and types in a generic manner, to enable top-level
implementations for different ciphersuites / curves without having to implement all of FROST from
scratch. End-users should not use `frost-core` if they want to sign and verify signatures, they
should use the crate specific to their ciphersuite/curve parameters that uses `frost-core` as a
dependency.

## Example

```rust
use std::{collections::HashMap, convert::TryFrom};

use rand::thread_rng;

use frost_ristretto255::frost;

let mut rng = thread_rng();
let numsigners = 5;
let threshold = 3;
let (shares, pubkeys) =
  frost::keys::keygen_with_dealer(numsigners, threshold, &mut rng).unwrap();

// Verifies the secret shares from the dealer
let key_packages: Vec<frost::keys::KeyPackage> = shares
  .into_iter()
  .map(|share| frost::keys::KeyPackage::try_from(share).unwrap())
  .collect();

let mut nonces: HashMap<u16, Vec<frost::round1::SigningNonces>> = HashMap::new();
let mut commitments: HashMap<u16, Vec<frost::round1::SigningCommitments>> = HashMap::new();

////////////////////////////////////////////////////////////////////////////
// Round 1: generating nonces and signing commitments for each participant
////////////////////////////////////////////////////////////////////////////

for participant_index in 1..(threshold + 1) {
  // Generate one (1) nonce and one SigningCommitments instance for each
  // participant, up to _threshold_.
  let (nonce, commitment) = frost::round1::preprocess(1, participant_index as u16, &mut rng);
  nonces.insert(participant_index as u16, nonce);
  commitments.insert(participant_index as u16, commitment);
}

// This is what the signature aggregator / coordinator needs to do:
// - decide what message to sign
// - take one (unused) commitment per signing participant
let mut signature_shares: Vec<frost::round2::SignatureShare> = Vec::new();
let message = "message to sign".as_bytes();
let comms = commitments.clone().into_values().flatten().collect();
let signing_package = frost::SigningPackage::new(comms, message.to_vec());

////////////////////////////////////////////////////////////////////////////
// Round 2: each participant generates their signature share
////////////////////////////////////////////////////////////////////////////

for participant_index in nonces.keys() {
  let key_package = key_packages
    .iter()
    .find(|key_package| *participant_index == key_package.index)
    .unwrap();

  let nonces_to_use = nonces.get(participant_index).unwrap()[0];

  // Each participant generates their signature share.
  let signature_share =
    frost::round2::sign(&signing_package, &nonces_to_use, key_package).unwrap();
  signature_shares.push(signature_share);
}

////////////////////////////////////////////////////////////////////////////
// Aggregation: collects the signing shares from all participants,
// generates the final signature.
////////////////////////////////////////////////////////////////////////////

// Aggregate (also verifies the signature shares)
let group_signature_res = frost::aggregate(&signing_package, &signature_shares[..], &pubkeys);

assert!(group_signature_res.is_ok());

let group_signature = group_signature_res.unwrap();

// Check that the threshold signature can be verified by the group public
// key (the verification key).
assert!(pubkeys
  .group_public
  .verify(message, &group_signature)
  .is_ok());

// Check that the threshold signature can be verified by the group public
// key (the verification key) from SharePackage.group_public
for (participant_index, _) in nonces.clone() {
  let key_package = key_packages.get(participant_index as usize).unwrap();

  assert!(key_package
    .group_public
    .verify(message, &group_signature)
    .is_ok());
}

```