Hello,

Thanks for starting a discussion about this idea.

A few comments inline:

On Wed, Mar 14, 2018 at 1:09 AM, Karl Johan Alm via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hello,
>
> I am considering writing a replacement for the message signing tools
> that are currently broken for all but the legacy 1xx addresses. The
> approach (suggested by Pieter Wuille) is to do a script based
> approach. This does not seem to require a lot of effort for
> implementing in Bitcoin Core*. Below is my proposal for this system:
>
> A new structure SignatureProof is added, which is a simple scriptSig &
> witnessProgram container that can be serialized. This is passed out
> from/into the signer/verifier.
>

You need a bit more logic to deal with softforks and compatibility. The
question is which script validation flags you verify with:
* If you make them fixed, it means signatures can't evolve with new address
types being introduced that rely on new features.
* If you make it just consensus flags (following mainnet), it means that
people with old software will see future invalid signatures as always
valid; this is probably not acceptable.
* If you make it standardness flags, you will get future valid signatures
that fail to verify.

One solution is to include a version number in the signature, which
explicitly corresponds to a set of validation flags. When the version
number is something a verifier doesn't know, it can be reported as
inconclusive (it's relying on features you don't know about).

An solution is to verify twice; once with all consensus rules you know
about, and once with standardness rules. If they're both valid, the
signature is valid. If they're both invalid, the signature is invalid. If
they're different (consensus valid but standardness invalid), you report
the signature validation as inconclusive (it's relying on features you
don't know about). This approach works as long as new features only use
previous standardness-invalid scripts, but perhaps a version number is
still needed to indicate the standardness flags.

RPC commands:
>
> sign <address> <message> [<prehashed>=false]
>

Why not extend the existing signmessage/verifymessage RPC? For legacy
addresses it can fall back to the existing signature algorithm, while using
the script-based approach for all others.


>
> Generates a signature proof for <message> using the same method that
> would be used to spend coins sent to <address>.**
>
> verify <address> <message> <proof> [<prehashed>=false]
>
> Deserializes and executes the proof using a custom signature checker
> whose sighash is derived from <message>. Returns true if the check
> succeeds, and false otherwise. The scriptPubKey is derived directly
> from <address>.**
>
> Feedback welcome.
>
> -Kalle.
>
>
(**) If <prehashed> is true, <message> is the sighash, otherwise
> sighash=sha256d(message).
>

That's very dangerous I'm afraid. It could be used to trick someone into
signing off on an actual transaction, if you get them to sign a "random
looking" prehashed message. Even if you have a prehashed message, there is
no problem with treating it as hex input to a second hashing step, so I
think the prehashed option isn't needed. It's why the existing message
signing functionality always forcibly prefixes "Bitcoin signed message:",
to avoid signing something that unintentionally corresponds to a message
intended for another goal.

Cheers,

-- 
Pieter