128 lines
5.1 KiB
Markdown
128 lines
5.1 KiB
Markdown
# FROST (Flexible Round-Optimised Schnorr Threshold signatures)
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Rust implementations of ['Two-Round Threshold Schnorr Signatures with FROST'](https://datatracker.ietf.org/doc/draft-irtf-cfrg-frost/).
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Unlike signatures in a single-party setting, threshold signatures require cooperation among a
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threshold number of signers, each holding a share of a common private key. The security of threshold
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schemes in general assume that an adversary can corrupt strictly fewer than a threshold number of
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participants.
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['Two-Round Threshold Schnorr Signatures with
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FROST'](https://datatracker.ietf.org/doc/draft-irtf-cfrg-frost/) presents a variant of a Flexible
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Round-Optimized Schnorr Threshold (FROST) signature scheme originally defined in
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[FROST20](https://eprint.iacr.org/2020/852.pdf). FROST reduces network overhead during threshold
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signing operations while employing a novel technique to protect against forgery attacks applicable
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to prior Schnorr-based threshold signature constructions. This variant of FROST requires two rounds
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to compute a signature, and implements signing efficiency improvements described by
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[Schnorr21](https://eprint.iacr.org/2021/1375.pdf). Single-round signing with FROST is not
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implemented here.
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## Status ⚠
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The FROST specification is not yet finalized, and this codebase has not yet been audited or
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released. The APIs and types in `frost-core` are subject to change.
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## Usage
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`frost-core` implements the base traits and types in a generic manner, to enable top-level
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implementations for different ciphersuites / curves without having to implement all of FROST from
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scratch. End-users should not use `frost-core` if they want to sign and verify signatures, they
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should use the crate specific to their ciphersuite/curve parameters that uses `frost-core` as a
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dependency.
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## Example
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```rust
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use std::{collections::HashMap, convert::TryFrom};
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use rand::thread_rng;
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use frost_ristretto255::frost;
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let mut rng = thread_rng();
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let numsigners = 5;
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let threshold = 3;
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let (shares, pubkeys) =
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frost::keys::keygen_with_dealer(numsigners, threshold, &mut rng).unwrap();
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// Verifies the secret shares from the dealer
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let key_packages: Vec<frost::keys::KeyPackage> = shares
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.into_iter()
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.map(|share| frost::keys::KeyPackage::try_from(share).unwrap())
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.collect();
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let mut nonces: HashMap<u16, Vec<frost::round1::SigningNonces>> = HashMap::new();
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let mut commitments: HashMap<u16, Vec<frost::round1::SigningCommitments>> = HashMap::new();
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////////////////////////////////////////////////////////////////////////////
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// Round 1: generating nonces and signing commitments for each participant
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////////////////////////////////////////////////////////////////////////////
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for participant_index in 1..(threshold + 1) {
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// Generate one (1) nonce and one SigningCommitments instance for each
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// participant, up to _threshold_.
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let (nonce, commitment) = frost::round1::preprocess(1, participant_index as u16, &mut rng);
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nonces.insert(participant_index as u16, nonce);
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commitments.insert(participant_index as u16, commitment);
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}
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// This is what the signature aggregator / coordinator needs to do:
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// - decide what message to sign
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// - take one (unused) commitment per signing participant
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let mut signature_shares: Vec<frost::round2::SignatureShare> = Vec::new();
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let message = "message to sign".as_bytes();
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let comms = commitments.clone().into_values().flatten().collect();
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let signing_package = frost::SigningPackage::new(comms, message.to_vec());
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////////////////////////////////////////////////////////////////////////////
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// Round 2: each participant generates their signature share
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////////////////////////////////////////////////////////////////////////////
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for participant_index in nonces.keys() {
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let key_package = key_packages
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.iter()
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.find(|key_package| *participant_index == key_package.index)
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.unwrap();
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let nonces_to_use = nonces.get(participant_index).unwrap()[0];
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// Each participant generates their signature share.
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let signature_share =
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frost::round2::sign(&signing_package, &nonces_to_use, key_package).unwrap();
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signature_shares.push(signature_share);
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}
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////////////////////////////////////////////////////////////////////////////
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// Aggregation: collects the signing shares from all participants,
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// generates the final signature.
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////////////////////////////////////////////////////////////////////////////
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// Aggregate (also verifies the signature shares)
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let group_signature_res = frost::aggregate(&signing_package, &signature_shares[..], &pubkeys);
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assert!(group_signature_res.is_ok());
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let group_signature = group_signature_res.unwrap();
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// Check that the threshold signature can be verified by the group public
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// key (the verification key).
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assert!(pubkeys
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.group_public
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.verify(message, &group_signature)
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.is_ok());
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// Check that the threshold signature can be verified by the group public
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// key (the verification key) from SharePackage.group_public
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for (participant_index, _) in nonces.clone() {
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let key_package = key_packages.get(participant_index as usize).unwrap();
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assert!(key_package
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.group_public
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.verify(message, &group_signature)
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.is_ok());
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}
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```
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