FROST Multisig

Overview

RadFi validators use FROST (Flexible Round-Optimized Schnorr Threshold Signatures) for decentralized signing of Bitcoin transactions. FROST replaces the legacy Taproot multisig scheme with a threshold signing protocol, increasing flexibility and resilience.

Key properties of this implementation:

• 5 signers (3-of-5 threshold)

• Distributed key generation (no single party ever holds the full private key)

• Signer key management in AWS Secrets Manager

• Support for key refresh, recovery, replacement, and removal

Benefits

• Decentralized signing: No single point of failure.

• Security: Keys are distributed, refreshed on leak, and recoverable on loss.

• Flexibility: Signers can be replaced or removed without disrupting the wallet.

• Backward compatibility: Legacy Taproot key is retained for non-signing use cases

Cryptographic Design

Key Generation

• A pubkey_package and 5 signer key pairs are generated.

• Each signer receives:

  • secret → backup seed

  • key → active signing key (derived from secret)

Signing Model

• Threshold: 3-of-5

• Signers collaborate via FROST rounds to produce a Schnorr signature.

• Signatures are valid Taproot-compatible signatures for Bitcoin transactions.

Keys Lifecycle

1. Refresh Keys

• Used when a signer key leaks.

• Distributor issues refresh shares.

• Each signer derives a new key while retaining the same wallet identity.

• Old keys are destroyed.

2. Recover a Lost Key

• If one signer loses their key, any 3 helpers can collaboratively reconstruct it.

• Recovery is a 3-step protocol using delta and sigma values.

3. Replace a Key

• A new signer can take over an old signer’s role by recovering their key, then refreshing.

4. Remove a Key

• 4 signers refresh without including the unwanted signer.