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From: Luke Dashjr <luke@dashjr.org>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] {sign|verify}message replacement
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On Thursday 15 March 2018 7:36:48 AM Karl Johan Alm wrote:
> On Wed, Mar 14, 2018 at 12:36 PM, Luke Dashjr <luke@dashjr.org> wrote:
> > 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.
> 
> Re-reading this, I think what you mean is it should be possible to
> create a proof for (a) specific UTXO(s), hence "inputs". That sounds
> pretty useful, yeah!

Not necessarily specific UTXOs (that would contradict fungibility, as well as 
be impossible for hot/cold wallet separation), but just to prove funds are 
available. The current sign message cannot be used to prove present possession 
of funds, only that you receive funds.