lightning-blips/blip-0003.md

```
bLIP: 3
Title: Keysend
Status: Active
Author: Valentine Wallace <vwallace@protonmail.com>
Created: 2021-12-08
License: CC0
```

## Abstract

Keysend is a type of lightning payment that does not require the payee to
provide an invoice. Instead, the payer includes their payer-selected payment
preimage in the TLV onion.

This bLIP serves to document what is already well-supported in the wild (see
[Reference Implementations](#reference-implementations)), for posterity and so
that new implementations don't have to reverse-engineer keysend from existing
implementations.

## Copyright

This bLIP is licensed under the CC0 license.

## Specification

Sender:

* MUST include a TLV record keyed by type `5482373484` with a TLV value of a
  randomly generated and cryptographically-secure 32-byte value that serves as
  the HTLC payment preimage
* MUST NOT reuse a previously used preimage
* MUST NOT reuse a preimage from a previously paid invoice
* MUST set the `payment_hash` field in the `update_add_htlc` message to match
  the SHA256 hash of the payment preimage
* SHOULD only send payments to nodes advertising the `keysend` feature bit

Receiver:

* if failing the payment due to not wanting to accept keysend payments or if
  the preimage does not match the payment hash, SHOULD error with
  `PERM|15 incorrect_or_unknown_payment_details`.

## Motivation

A convenience of layer 1 bitcoin is being able to spontaneously send to a
bitcoin address with no advance work required on the part of the payee. Keysend
brings the convenience of spontaneous payments to layer 2 (with a few caveats,
see the [Drawbacks](#keysend-drawbacks) section).

As previously mentioned, keysend also has the advantage of already being
supported by major implementations. So regardless of whether it is the best
solution to spontaneous payments, it's a good idea to have some form of
official documentation for it.

## Rationale

Design decisions for keysend were largely made by the original lnd keysend
implementation (e.g. the choice of `5482373484` for the TLV type).

* The TLV type `5482373484` was chosen at random
* The rationale behind having a feature bit is that it's nicer to opt-in to
  receiving keysend payments: without a feature bit, the only way for senders to
  explicitly know that receivers support keysend is by attempting a keysend
  payment and seeing whether or not it fails.

## Keysend Drawbacks

* Inability for the payee to specify their preferred `min_final_cltv_expiry`.
  This is an issue because payer and payee may have differing security
  requirements, which could lead to payment failures if the payee considers the
  payer's choice of `min_final_cltv_expiry` too high or too low.
* Loss of being able to use the preimage and invoice signature as
  proof-of-payment. With regular lightning payments, payers are able to use (1)
  the payment preimage and (2) the invoice signature in a situation where they
  need to prove that they made a payment (the invoice signature is necessary to
  prove that the payer was not an intermediary hop). This proof cannot be
  provided for keysend payments because the preimage is provided by the payer to
  begin with and no invoice is being paid.
* Keysend is expected to be deprecated in favor of a more well-developed
  spontaneous payment solution like
  [Offers](https://github.com/lightningnetwork/lightning-rfc/pull/798) or
  [AMP](https://github.com/lightningnetwork/lightning-rfc/pull/658)

## Reference Implementations

* LDK: <https://github.com/rust-bitcoin/rust-lightning/pull/967>
* C-Lightning: <https://github.com/ElementsProject/lightning/blob/master/plugins/keysend.c>
* lnd original keysend PR: <https://github.com/lightningnetwork/lnd/pull/3795>
* Eclair: <https://github.com/ACINQ/eclair/pull/1485>