NIP-NONCE-COMMITMENT: Host-Committed Nonces for NIP-46 Remote Signing

NIP-NONCE-COMMITMENT: Host-Committed Nonces for NIP-46 Remote Signing

draft optional

Two additional NIP-46 methods, sign_event_commit and sign_event_reveal, that let a client force a remote signer to commit to its BIP-340 signing nonce before the client reveals randomness of its own. The final nonce is a function of both, so a signer cannot choose its nonces freely, and the covert channel by which malicious signer firmware could leak key material through signature nonces is reduced from a broadcast to, at worst, a low-rate abort channel.

The key words MUST, MUST NOT, SHOULD, SHOULD NOT and MAY are used as defined in RFC 2119.

Motivation

A NIP-46 signer is trusted with an nsec precisely so that nothing else has to be. That trust has one channel left open: the signature itself. BIP-340 lets a signer add 32 bytes of auxiliary randomness to its nonce derivation, and nothing in the protocol constrains that choice. Malicious firmware can grind aux_rand until the resulting signature encodes a few chosen bits, and leak an entire seed over a few dozen perfectly valid signatures. The user sees normal events, normally signed. No log, no display, no policy engine notices anything.

This is not hypothetical caution. The hardware wallet world has already been through it: Blockstream's Anti-Exfil protocol exists because a compromised signing device leaking through nonces was judged a realistic threat for Bitcoin, and hardware Nostr signers now hold keys of comparable value to their owners. Open source and reproducible builds shrink the window for malicious firmware but cannot close it; a verifier can only check the build they verified, not the one that shipped.

Sign-to-contract closes the channel cryptographically. The signer commits to its nonce point before it learns randomness the client contributes; the final nonce is the committed point tweaked by a hash of both. The signer cannot grind what it cannot predict.

Protocol

Both methods travel as ordinary NIP-46 requests inside kind 24133 envelopes. P is the signer's x-only public key, m the event id to be signed, n the secp256k1 group order, and G its generator. TaggedHash is as defined in BIP-340.

Round one: sign_event_commit

The client draws 32 fresh random bytes rho and sends its hash, keeping rho secret:

{
  "id": "<request-id>",
  "method": "sign_event_commit",
  "params": ["<event-json>", "<hex SHA256(rho)>"]
}

The signer derives its nonce k0 per BIP-340 nonce derivation, with the received commitment t included in the hashed input alongside the message, so that k0 is bound to both. It computes R0 = k0·G and replies with the 33-byte compressed encoding:

{
  "id": "<request-id>",
  "result": "{\"commitment\": \"<hex R0>\", \"ref\": \"<opaque>\"}"
}

The signer MUST NOT produce a signature for m at this stage, MUST bind the pending state to (m, t), and MUST discard the pending state after a timeout (60 seconds is RECOMMENDED) or after a single reveal attempt, successful or not. k0 MUST be fresh per request and MUST never be reused.

Round two: sign_event_reveal

The client reveals its randomness:

{
  "id": "<request-id-2>",
  "method": "sign_event_reveal",
  "params": ["<ref>", "<hex rho>"]
}

The signer verifies SHA256(rho) = t for the pending request, and MUST refuse with an error otherwise. It then computes

e_c = int(TaggedHash("NonceCommit/tweak", bytes(R0) || rho)) mod n
k   = (k0 + e_c) mod n

and completes an otherwise standard BIP-340 signature for m using k as the secret nonce (including the even-Y normalisation BIP-340 requires). The result is the signed event, exactly as sign_event would return it.

Client verification

The signature is valid BIP-340; any verifier accepts it. The client that ran the protocol additionally checks the commitment:

r == x(R0 + e_c·G)

where r is the first 32 bytes of the signature and e_c is recomputed from R0 and rho. Negation for even-Y does not change the x-coordinate, so the equation holds for a compliant signer regardless of parity. If the check fails, the client MUST treat the signer as malicious: discard the signature, refuse further requests over that connection, and surface the failure to the user. A commitment mismatch is not an error condition to retry; it is the attack this protocol exists to catch.

What this does and does not achieve

The signer chooses k0 knowing t but not rho, so it cannot predict the final nonce at commitment time; grinding signatures into a covert channel stops working. What remains is an abort channel: a malicious signer that dislikes the final nonce can refuse to answer the reveal, leaking at most a fraction of a bit per interaction at the cost of visible failures. This is the same residual accepted by Anti-Exfil, and no interactive scheme does better without a second signing party.

The protection is only as good as the client's rho. A client whose randomness the signer can predict has rebuilt the original problem, and a client that reuses rho MUST be considered broken.

Threshold signing (FROST-style bunkers) also removes unilateral nonce choice, by ensuring no single party holds the key at all. The two approaches serve different deployments: threshold schemes need multiple keyholders and a coordination protocol, while this one protects the common case of a single hardware signer with two extra messages and no new keys.

Clients MUST fall back to plain sign_event when a signer does not implement these methods, and SHOULD indicate to the user that nonce commitment is not in effect. A signer that advertises the methods and then fails commitment checks is worse than one that never offered them; see above.

Costs

One extra round trip per signature. Over relays this doubles signing latency, which is material for interactive use and immaterial for anything batched; over local transports (USB-bridged signers, LAN bunkers) it is microseconds. Clients MAY reserve the protocol for high-value events and MAY apply it per connection rather than per signature. Signers carry one pending (m, t, k0) per in-flight request; the state is small, bounded and expiring.

Reference implementation

Planned for Heartwood (heartwood-esp32) as a heartwood_-prefixed method pair first, promoted to the unprefixed names here if adopted more widely. The protocol requires nothing exotic of a signer: one point multiplication and one tagged hash beyond a standard BIP-340 signature.