[bitcoin-dev] RFC: Kicking BIP-322 (message signing) into motion

[Karl-Johan Alm <karljohan-alm@garage.co.jp>]

Hello,

I noticed recently that a PR to Bitcoin Core that pretty much touched
everything my BIP-322 pull request touches (around the same
complexity) was merged without a thought given to BIP-322
compatibility, despite the BIP-322 PR being open for 2x the time. I
can only conclude from this that people dislike BIP-322 in its current
form, which the 9 month old pull request stagnating can probably
attest to.

There are several things that I can do to make this a bit more
appealing to people, which would hopefully kick the progress on this
forward. I have already put in a non-trivial amount of energy and
effort into maintaining the pull request as is, so I'd prefer if
people were harsh and unfiltered in their criticism rather than polite
and buffered, so I can beat this thing into shape (or abandon it, in
the worst case).

=============
1. People use signmessage as a way to prove funds. This is misleading
and should be discouraged; throw the sign message stuff out and
replace it entirely with a prove funds system.

I know in particular luke-jr is of this opinion, and Greg Maxwell in
https://github.com/bitcoin/bitcoin/pull/16440#issuecomment-568194168
leans towards this opinion as well, it seems.

=============
2. Use a transaction rather than a new format; make the first input's
txid the message hash to ensure the tx cannot be broadcasted. This has
the benefit of being able to provide to an existing hardware wallet
without making any modifications to its firmware.

I think Mark Friedenbach and Johnson Lau are of this opinion, except
Johnson Lau also suggests that the signature hash is modified, see
https://github.com/bitcoin/bips/pull/725#issuecomment-420040430 --
which defeats the benefit above since now hw wallets can no longer
sign.

Prusnak (I think he works at Trezor; apologies if I am mistaken) is
against this idea, and proposes (3) below:
https://github.com/bitcoin/bips/pull/725#issuecomment-420210488

=============
3. Use Trezor style

See https://github.com/trezor/trezor-mcu/issues/169

This has the benefit of already being adopted (which clearly BIP-322
is failing hard at right now), but has the drawback that we can no
longer do *generic* signing; we are stuck with the exact same
limitations as in the legacy system, which we kinda wanted to fix in
the updated version.

=============
4. Introduce OP_MESSAGEONLY

Quoting Johnson Lau at
https://github.com/bitcoin/bips/pull/725#issuecomment-420421058 :
"""
OP_MESSAGEONLY means the script following the code would never be
valid. For example, a scriptPubKey:

OP_IF OP_MESSAGEONLY <key_m> OP_ELSE <key_s> OP_ENDIF OP_CHECKSIG

For messaging purpose, OP_MESSAGEONLY is considered as OP_NOP and is
ignored. A message could be signed with either key_m or key_s.

For spending, only key_s is valid.

I don't think it is a big problem to consume a op_code. If this is a
real concern, I could modify it as follow: in message system,
OP_RETURN will pop the top stack. If top stack is msg in hex, it is
ignored. Otherwise, the script fails.
"""

=============
5. Some other solution