plugins

The Noise plugin allows sending and receiving private messages through the Lightning Network. It is implemented on top to c-lightning's createonion and sendonion RPC methods that allow delivering custom payloads to a specific node, as well as the htlc_accepted hook which can be used to extract the message from the onion payload.

You can send a message using the following RPC method:

lightning-cli sendmsg 02a5deaa47804c518bb4a1c6f04a85b92b796516bd32c4114a51b00d73e251f999 "Hello world 👋"

In addition a message can also be accompanied by a payment (using the keysend protocol draft) by specifying an amount of millisatoshis as the last argument:

lightning-cli sendmsg 02a5deaa47804c518bb4a1c6f04a85b92b796516bd32c4114a51b00d73e251f999 "Here's my rent" 31337

You can read the last message received using the following command:

lightning-cli recvmsg last_id

The last_id indicates the last message we read, so we can retrieve each message individually. If you'd just like to wait for the next message then do not specify any last_id.

Todo

Persist messages across restarts
Use rpc_command to intercept any payment listing and add the keysend payments to it. (No longer needed, since keysends are handled correctly by listpays.)

Protocol

The protocol was heavily inspired by the WhatSat protocol:

record type	length (bytes)	value
5482373484	32	key send preimage
34349334	variable	chat message
34349335	65	compressed signature + recovery id
34349339	33	sender pubkey
34349343	8	timestamp in nano seconds since unix epoch (big endian encoded)

The key differences are that we don't explicitly pass the sender pubkey, since we can recover that from the signature itself, and we use the compressed 64 byte signature, instead of the DER encoded signature. This saves us 39 bytes for the pubkey (5 byte type, 1 byte length, 33 byte value) and about 6 bytes for the signature, but requires that we change the TLV type for the signature (from 34349337 to 34349335). More could be achieved by giving keysend a smaller type which currently is 9 bytes and could get down to 1 byte. We'll need to wait for the spec to catch up 😉

The signature is computed by serializing all other TLV fields, hex-encoding the resulting TLV payload, and signing it using lightning-cli signmessage returning the zbase32 encoded signature. The signature consists of a 1 byte recovery ID and the 64 byte raw signature.