plugins

record type	length (bytes)	value
34349341	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 33 bytes for the pubkey and 7 bytes for the signature, but requires that we change the TLV type for the signature (from ~34349337 to 34349335).

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.