In addition to starting with proof-of-funds instead of proof-of-receiver, it
would be nice to integrate with Taproot somehow or another. Perhaps
OP_MESSAGEONLY is the most straightforward way to do this? It might be a good
idea to have a message type after the opcode too.

On Wednesday 04 March 2020 06:23:53 Karl-Johan Alm via bitcoin-dev wrote:
> 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
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