Allow signing messages on liquid

[jmastr]

I can sign a message on Bitcoin:

$ green-cli --network electrum-testnet getnewaddress                                                                                                                                                                                                                                                             
2N3t2fZsYE6QVBAoG3Z8cpq2oadsp6Smv2h
$ green-cli --network electrum-testnet signmessage 2N3t2fZsYE6QVBAoG3Z8cpq2oadsp6Smv2h "Hello"                                                                                                                                                                                                                   
{
  "signature": "H29dlu1Ce+icNUMY6XfcgK6+z5140vbWHfQ1BoVpOWnDQa99pnojqLdUodJIrb4hKfDsbxob7Ap+xvvi3hYS16M="
}

But doing the same thing on liquid yields to an error:

$ green-cli --network electrum-testnet-liquid getnewaddress                                                                                                                                                                                                                                                      
vjTwMefCmqyvk7yFRN7ixCaEJRz9XKA9ie8nJDoo7TfQDF9jHZ3mEGvNXhNsHwriM7hqVtLZqDtT8SRe
$ green-cli --network electrum-testnet-liquid signmessage vjTwMefCmqyvk7yFRN7ixCaEJRz9XKA9ie8nJDoo7TfQDF9jHZ3mEGvNXhNsHwriM7hqVtLZqDtT8SRe "Hello"                                                                                                                                                               
...
RuntimeError: OrderedDict([('action', 'sign_message'), ('error', 'assertion failure: ga_auth_handlers.cpp:2101:Invalid network'), ('name', 'sign_message'), ('status', 'error')])

[jmastr]

Maybe related to #23

[jmastr]

After my patch:

$ green-cli --network electrum-testnet-liquid signmessage vjTxYGsEPr728szaqx8dFL8gnhVSeTXmJuTTBcG8S5TVfzc1an1kAfQgCVqrqtNcJiGMvjDkHM3Ku4nx "Hello"
{
  "signature": "IDBd37GMBxRZbmV8DVlJnWSpgwK/HqkKoUpEDaj4oNiZEezyE59OKWxnGf1guXkHE7l8kdu2YWy0NKD+FYywpMU="
}

[jmastr]

With Jade too:

$ green-cli --network electrum-testnet-liquid --auth jade signmessage vjTyrmZQapgFd6aVGnyHhNvsgV1QaLTBt8F3GuMtv1wkWnvhhaU3eVdJbwvW14c98GQ3tMkAeVbtLt7p "Hello"
{
  "signature": "IO12QqcX2qgfetouM/wScTCQBSBrq64zC4FsK8svPa1uShykW1lxMaJi2/3FvQTg4+5Q1WGI9uOvfy1hN1dm+vU="
}

[jmastr]

Hello @LeoComandini Would you be so kind to review this and maybe even release this as 0.70.1? That would really help out our project.

[LeoComandini]

Hi @jmastr,

Your MR works however it has a couple of issues.

How do you verify the signed message?
With Bitcoin you can do it with Bitcoin Core (p2pkh only) or Electrum (p2pkh, p2sh-p2wpkh, p2wpkh).
With Liquid you can only do it with Elements Core (p2pkh only).
Thus we should restrict signing only to p2pkh accounts.
Or alternatively provide some way to verify the produced signatures.

In your PR the message is prefixed with "Bitcoin Signed Message:\n" (nothing has changed so it does the same as bitcoin).
Even though that's what Elements Core does, it's questionable.

Also should signmessage completely ignore the blinding key in the address?

Anyway I'll try to get this forward,
although given the other priorities,
it might take a bit.

[jmastr]

Hi @LeoComandini Thanks for coming back to me that quickly. I try to answer your remarks one by one.

In your PR the message is prefixed with "Bitcoin Signed Message:\n" (nothing has changed so it does the same as bitcoin).
Even though that's what Elements Core does, it's questionable.

For my use case I need the compatibility to Elements Core, so this is intentional, although questionable. I agree.

With Liquid you can only do it with Elements Core (p2pkh only).

I am no expert on address types. With green-cli I use e.g. vjTyrmZQapgFd6aVGnyHhNvsgV1QaLTBt8F3GuMtv1wkWnvhhaU3eVdJbwvW14c98GQ3tMkAeVbtLt7p (p2sh) and with elements-cli I use dumpprivkey with e.g. tlq1qq289prur7etvnyysln69fue5qjlca84ft559gv3carnm9q0pl23tg5dxw04eqc2udcexnm43lltxf5q5mv2tke0pf6qtpauky (p2wpkh) and then signmessagewithprivkey to sign the message.

Also should signmessage completely ignore the blinding key in the address?

Again I need the compatibility to Elements Core, but I don't know what they do internally.

[jmastr]

My use case is to map some arbitrary data to a user's private/public key. I can outline the verification process, but it is rather cumbersome.

Another idea that comes to mind is to use OP_RETURN to write an arbitrary message into an scriptPubKey transaction output, but I don't know if this is even possible on Liquid or only on Bitcoin. Trezor wallet for example offers this:

[LeoComandini]

@jmastr raised ElementsProject/elements#1319 to try to get the discussion going