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From: Karl Johan Alm <karljohan-alm@garage.co.jp>
Date: Wed, 14 Mar 2018 04:09:20 -0400
Message-ID: <CALJw2w5=g-FL+MZ08DEoLxVzOKbSXeKu50drE1b4P0JZJpdTyA@mail.gmail.com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: [bitcoin-dev] {sign|verify}message replacement
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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.

RPC commands:

sign <address> <message> [<prehashed>=false]

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.

(*) Looks like you can simply use VerifyScript with a new signature
checker class. (h/t Nicolas Dorier)
(**) If <prehashed> is true, <message> is the sighash, otherwise
sighash=sha256d(message).