HTLC Invoice

An HTLC invoice is, in practical terms, an ordinary Lightning invoice - one whose payment_hash field is used to construct Hashed Time-Locked Contracts at each hop of the routed payment. Almost every Lightning payment made today is via HTLC invoices.

What makes the invoice "HTLC-based":

1. The receiver picks a random 32-byte preimage and computes its SHA-256 hash. This hash goes into the invoice as the payment_hash.
2. The sender's wallet uses the payment hash to construct HTLCs along the route - each hop is locked by the same hash.
3. To claim payment, the receiver reveals the preimage. This propagates backward through the route, settling every HTLC.

This is essentially the BOLT-11 invoice mechanism. "HTLC invoice" is a description of the cryptographic mechanism rather than a separate format - all standard Lightning invoices work this way.

Variants and successors:

• BOLT-12 offers still use HTLC-based settlement; the change is in invoice format and reusability, not the underlying cryptography.
• PTLCs (Point Time-Locked Contracts) are a proposed future replacement for HTLCs using Schnorr/Taproot-based point math instead of hash preimages. They'd give better privacy (each hop's lock looks different rather than all sharing the same hash). Not yet deployed.
• Held HTLCs / hodl invoices are HTLC invoices that the receiver deliberately doesn't settle immediately - useful for conditional payments where the receiver wants to confirm something off-chain before completing.

For everyday Lightning users, "HTLC invoice" is just "the way Lightning invoices work." The mechanism is invisible behind wallet UX. See HTLC for the underlying primitive.