FROST

Flexible round-optimized schnorr threshold signatures for Bitcoin.

How to Use

The FROST protocol specifies two rounds for producing a threshold signature.

Initial setup of parameters (using a trusted dealer):

This repository uses a trusted dealer method for demonstration purposes. Feel free to use your own Distributed Key Generation (DKG) protocol for generating and distributing shares.

import { create_key_group } from '@cmdcode/frost/lib'
import { random_bytes }     from '@cmdcode/frost/util'

// Generate a random secret key and message.
const seckey  = random_bytes(32).hex
const message = random_bytes(32).hex

// Configure the paramaters of the group.
const secrets   = [ seckey ]
const threshold = 2
const share_max = 3

// Generate a group of secret shares.
const group = create_key_group(secrets, threshold, share_max)

Round 1 Example (nonce commitments):

Each member that is participating in the signing round must first create a nonce commitment:

import { create_commit_pkg } from '@cmdcode/frost/lib'

// Select a threshold (t) amount of shares and create nonce commitments.
const shares  = group.shares.slice(0, threshold)
const commits = shares.map(e => create_commit_pkg(e))

Each member then distributes their nonce commitment to other members.

Round 2 Example (signing with secret shares):

Once all participating member commitments have been collected, we can now produce a partial signature:

import {
  get_commit_pkg,
  get_session_context,
  sign_msg,
  verify_partial_sig
} from '@cmdcode/frost/lib'

// Compute the context data for the signing session.
const ctx = get_session_ctx(group.pubkey, commits, message)

// Convert the share indices into iterable numbers.
const idx = ctx.indexes.map(i => Number(i) - 1)

// Collect a partial signature from each share.
const psigs = idx.map(i => {
  const share  = shares[i]
  const commit = get_commit_pkg(commits, share)
  const sig    = sign_msg(ctx, share, commit)
  if (!verify_partial_sig(ctx, commit, sig.pubkey, sig.psig)) {
    throw new Error('sig share failed validation')
  }
  return sig
})

When the partial signatures have been collected, we can aggregate them into a full signature:

import { combine_partial_sigs, verify_final_sig } from '@cmdcode/frost/lib'

// Aggregate the partial signatures into a single signature.
const signature = combine_partial_sigs(ctx, psigs)

// Check that the signature is valid.
const is_valid  = verify_final_sig(ctx, message, signature)

console.log('is valid:', is_valid)

Development and Testing

To run the test suite, use the following commands:

yarn test    # For yarn.
npm run test # For NPM.

The test suite comes bundled with Bitcoin Core (located in test/bin) for testing purposes. Depending on your computer architecture, you may have to replace these binaries with another version, or change the default configuration in test/tape.ts.

There are code examples located in test/examples for performing various protocols via FROST and DKG. You can run a test file via the following command:

yarn load test/example/<example_name>.ts

Feel free to check them out!

Resources

ZF FROST Book
A guide to the FROST protocol.
https://frost.zfnd.org/index.html

FROST draft specification
A draft specification of the FROST protocol from the IETF.
https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-15.html

ZCash FROST GitHub
A rust implementation of the IETF FROST draft spec, in rust.
https://github.com/ZcashFoundation/frost

FROST BIP340
A draft implemenation of the FROST protocol for BIP340.
https://github.com/jesseposner/FROST-BIP340

Draft BIP for Secure DKG
A draft proposal for secure DKG in FROST, provided by Blockstream Research.
https://github.com/BlockstreamResearch/bip-frost-dkg

ROAST GitHub
A naive implementation of the ROAST protocol, written in rust.
https://github.com/robot-dreams/roast

FROST Whitepaper
The white-paper for FROST: Flexible Round-Optimized Schnorr Threshold Signatures
https://eprint.iacr.org/2020/852.pdf

ROAST Whitepaper
A white-paper for ROAST: Robust Asynchronous Schnorr Threshold Signatures.
https://eprint.iacr.org/2022/550.pdf