FROST Trusted Dealer Demo

A CLI demo for running trusted dealer key generation with the Zcash Foundation's Rust implementation of Two-Round Threshold Schnorr Signatures with FROST. The trusted dealer is trusted to:

1. generate good randomness
2. delete secret values after distributing shares to each participant
3. keep secret values confidential.

This will be part of a set of demos and a proof of concept application that uses the FROST libraries and reference implementation. The purpose of these demos is to:

1. identify gaps in our documentation
2. provide usage examples for developer facing documentation
3. provide reference implementations for developers wanting to use FROST in a “real world” scenario.

This demo uses the (Ed25519, SHA-512) ciphersuite. The crate can be found here.

About FROST (Flexible Round-Optimised Schnorr Threshold signatures)

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 presents a variant of a Flexible Round-Optimized Schnorr Threshold (FROST) signature scheme originally defined in FROST20. 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. Single-round signing with FROST is not implemented here.

Status ⚠

The Trusted Dealer demo is a WIP.

Usage

NOTE: This is for demo purposes only and should not be used in production.

You will need to have Rust and Cargo installed.

To run:

1. Clone the repo. Run git clone https://github.com/ZcashFoundation/frost-zcash-demo.git
2. Run cargo install
3. Run cargo run

On startup, the Trusted Dealer demo will prompt for:

1. Minimum number of signers (>= 2) i.e. The threshold number of signers for the secret sharing scheme.

2. Maximum number of signers i.e. the number of shares to generate

> cargo run
   Finished dev [unoptimized + debuginfo] target(s) in 0.05s
   Running 'target/debug/frost-trusted-dealer-demo
The minimum number of signers: (2 or more)
2
The maximum number of signers: (must be greater than minimum number of signers)
5

The dealer CLI will then use that data to generate:

1. The group public key
2. A commitment to the secret
3. Each signer's public key
4. Each signer's secret share
5. An identifier for each signer

In a "real world" scenario, the secret share should be delivered securely to each participant. For the purposes of the Trusted Dealer demo only, the above data will be output to the terminal.

In the next round, all signers will receive the data but only the individual signers will receive their personal secret share.

Using the output

To generate a key package the participant requires:

• The signer's identifier
• The signer's secret share
• The signer's public key
• The public signing key that represents the entire group

The commitment is used to verify the signer's secret share and to generate the group commitment.

The dealer should use a secure broadcast channel to ensure each participant has a consistent view of this commitment and each participant must verify the commitments. The trusted dealer MUST delete the secret_key (used during calculation) and secret_share upon completion. Use of this method for key generation requires a mutually authenticated secure channel between the dealer and participants to send secret key shares, wherein the channel provides confidentiality and integrity. Mutually authenticated TLS is one possible deployment option.

NOTE: A signer is a participant that generates a signing share which is aggregated with the signing shares of other signers to obtain the final group signature.

Developer information

Pre-commit checks

1. Run cargo make all

Coverage

Test coverage checks are performed in the pipeline. This is configured here: .github/workflows/coverage.yaml To run these locally:

1. Install coverage tool by running cargo install cargo-llvm-cov
2. Run cargo make cov (you may be asked if you want to install llvm-tools-preview, if so type Y)