Definition of `onion_message` should be moved to BOLT-0002

[gijswijs]

The onion_message is to onion_message_packet what the update_add_htlc is to onion_routing_packet. It's even explained like that in https://github.com/lightning/bolts/blob/master/04-onion-routing.md#onion-decryption

Yet, update_add_htlc is part of BOLT-0002 but somehow onion_message is part of BOLT-0004. To be consistent the onion_message should be part of the peer protocol (BOLT-0002), and the onion_message_packet should be part of the onion routing (BOLT-0004).

I would be happy to create a PR for it, but I created the issue first to solicit feedback and see if there's enough support for such a change.

[tnull]

Would make sense to me to move the Onion Messages section (with the caveat that I have neither been working on OM nor BOLT12 integration, so might be lacking some context). It might of course be a bit painful to break all the spec links out there, so not sure if it's really worth the effort.

[ziggie1984]

Can somebody provide also background why we have 2 layers in the Onion_Message and put the blinding point at the first layer and did not include it in the onion_message_packet itself ? Is the reason because we want to be able to combine blinded paths otherwise the HMAC would be wrong if we hot-swap the blinded key ?

type: 513 (onion_message) (option_onion_messages)

data:

[point:path_key]
[u16:len]
[len*byte:onion_message_packet]
type: onion_message_packet

data:

[byte:version]
[point:public_key]
[...*byte:onionmsg_payloads]
[32*byte:hmac]

Why did we not go with

type: 513 (onion_message) (option_onion_messages)

data:

[u16:len]
[len*byte:onion_message_packet]
type: onion_message_packet

data:

[byte:version]
[point:public_key]
[point:path_key]
[...*byte:onionmsg_payloads]
[32*byte:hmac]

Because the only the onionmsg_payloads is encrypted anyways similar to the Payment message where only the

type: onion_packet
data:
[byte:version]
[point:public_key]
[1300*byte:hop_payloads]
[32*byte:hmac]