Threshold Signing (FROST)

Overview

Cosmictron supports FROST (Flexible Round-Optimized Schnorr Threshold Signatures) for threshold signing of audit bundles and critical events. FROST allows M-of-N key holders to jointly produce a valid signature without any single party knowing the full signing key.

Use cases

• Audit bundle authorization — require 3-of-5 compliance officers to sign an audit export before it is accepted by regulators
• Key rotation authorization — require M-of-N approvals before the signing key can be rotated
• High-value event signing — selectively apply threshold signing to events above a risk threshold

How FROST works

FROST is a two-round protocol:

1. Round 1 (Commitment) — each participant generates a nonce and publishes a commitment
2. Round 2 (Response) — each participant computes a partial signature; the coordinator aggregates M partial signatures into a single valid Schnorr signature

The resulting signature is indistinguishable from a single-party Ed25519/Schnorr signature and can be verified with the aggregated public key.

Configuration

[compliance.threshold_signing]
enabled = true
threshold = 3          # M — minimum signers required
total_shares = 5       # N — total key shares distributed
aggregated_pubkey_path = "/data/keys/frost-agg.pub"

Distributing key shares

Key shares must currently be distributed out-of-band. Generate shares:

cosmictron-cli keys generate-threshold \
  --threshold 3 \
  --shares 5 \
  --output-dir /tmp/frost-shares/

This produces share-1.key through share-5.key. Distribute each share to a different key holder via a secure channel.

Caution

Do not store all shares on the same machine. The threshold scheme only provides protection if shares are held by independent parties.

Distributed Key Generation (DKG) Coming Soon

The current approach requires a trusted dealer to generate shares. DKG (Distributed Key Generation) eliminates the trusted dealer: participants generate shares collaboratively so that no single party ever knows the full secret key.

DKG is on the roadmap and will ship as part of the multi-region feature set.

Signing an audit bundle with threshold

# Coordinator: initiate threshold signing
cosmictron-cli audit threshold-sign \
  --bundle audit-jan-2026.bundle \
  --threshold-config /data/keys/frost-config.toml

# Participant (run on each key holder's machine):
cosmictron-cli frost participate \
  --round1 round1-commitment.json \
  --share share-1.key \
  --output round2-response-1.json

# Coordinator: aggregate and finalize
cosmictron-cli frost aggregate \
  --responses round2-response-*.json \
  --bundle audit-jan-2026.bundle

Verifying a threshold-signed bundle

cosmictron-cli audit verify-bundle audit-jan-2026.bundle --check threshold

The verifier checks:

• The aggregated signature is valid for the bundle hash
• The aggregated public key matches the registered threshold public key