To: Greg Sanders <gsanders87@gmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
References: <CAP6ruDR0GrLRNb4TTub+wqpwVPyzHggbomV48kLZU3tvubH73Q@mail.gmail.com>
	<CABaSBaxjGLmiM0+zTk2PoGTt1zEao-k0ADLkT47vx+mcnPACJw@mail.gmail.com>
	<CAB3F3Dv1kuJdu8veNUHa4b58TvWy=BT6zfxdhqEPBQ8rjDfWtA@mail.gmail.com>
From: Jochen Hoenicke <hoenicke@gmail.com>
Message-ID: <5f67d70d-a432-7826-22df-4207580aa1d2@gmail.com>
Date: Mon, 21 Aug 2017 23:36:24 +0200
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Subject: Re: [bitcoin-dev] [BIP Proposal] Partially Signed Bitcoin
 Transaction (PSBT) format
Precedence: list

On 21.08.2017 20:12, Greg Sanders via bitcoin-dev wrote:
> To fix this I consulted with andytoshi and got something we think works
> for both cases:
> 
> 1) When a signing device receives a partially signed transaction, all
> inputs must come with a ownership proof:
> - For the input at address A, a signature over H(A || x) using the key
> for A. 'x' is some private fixed key that only the signing device
> knows(most likely some privkey along some unique bip32 path).
> - For each input ownership proof, the HW wallet validates each signature
> over the hashed message, then attempts to "decode" the hash by applying
> its own 'x'. If the hash doesn't match, it cannot be its own input.
> - Sign for every input that is yours

Interesting, basically a proof of non-ownership :), a proof that the
hardware wallet doesn't own the address.

But shouldn't x be public, so that the device can verify the signature?
Can you expand on this, what is exactly signed with which key and how is
it checked?

One also has to make sure that it's not possible to reuse signatures as
ownership proof that were made for a different purpose.

  Jochen