NUT-14 HTLC refund path incorrectly requires preimage (spec violation) #848
Description
Summary
NUT-14 HTLC refund path requires a preimage even after locktime has passed. The spec says only a signature is needed.
Bug Location
cashu/core/nuts/nut14.py lines 24-27:
if not proof.htlcpreimage:
raise TransactionError("no HTLC preimage provided") # Checked BEFORE locktime
now = time.time()
if not htlc_secret.locktime or htlc_secret.locktime > now: # Locktime check comes AFTERThe preimage check is unconditional but should only apply before locktime.
Expected vs Actual
| Test | Expected | Actual |
|---|---|---|
| Refund with signature only (per spec) | ACCEPT | REJECT |
| Refund with dummy preimage | N/A | ACCEPT |
Proof of Concept
https://gist.github.com/Amperstrand/3cc9788f1bac4c1461e441bfa1ef47eb
Run: npx tsx nutshell_htlc_bug_e2e.ts
Fix
def verify_htlc_spending_conditions(proof: Proof) -> bool:
# ...
now = time.time()
# Refund path: locktime passed, only signature needed
if htlc_secret.locktime and htlc_secret.locktime < now:
return True
# Hashlock path: require preimage
if not proof.htlcpreimage:
raise TransactionError("no HTLC preimage provided")
# ...Note
Existing HTLC tests always include a preimage in refund scenarios, so this wasn't caught.