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From: Luke Dashjr <luke@dashjr.org>
To: Karl Johan Alm <karljohan-alm@garage.co.jp>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Date: Wed, 14 Mar 2018 12:36:47 +0000
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Subject: Re: [bitcoin-dev] {sign|verify}message replacement
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I don't see a need for a new RPC interface, just a new signature format.

Ideally, it should support not only just "proof I receive at this address", 
but also "proof of funds" (as a separate feature) since this is a popular 
misuse of the current message signing (which doesn't actually prove funds at 
all). To do this, it needs to be capable of signing for multiple inputs.

Preferably, it should also avoid disclosing the public key for existing or 
future UTXOs. But I don't think it's possible to avoid this without something 
MAST-like first. Perhaps it can be a MAST upgrade later on, but the new 
signature scheme should probably be designed with it in mind.

Luke


On Wednesday 14 March 2018 8:09:20 AM Karl Johan Alm via bitcoin-dev 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.
> 
> 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).
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