From aj at erisian.com.au  Wed Sep 25 19:29:58 2019
From: aj at erisian.com.au (Anthony Towns)
Date: Thu, 26 Sep 2019 05:29:58 +1000
Subject: [Lightning-dev] Selling timestamps (via payment points and
 scalars + Pedersen commitments ) [try2]
In-Reply-To: <_MpLAUdyzQPsMnVOswO5Eux8XUxCbJIiBIDEp4gwni9ru8r4Lj0bKD5oM3W0mvphadyx23kIXDT_hzma-NkIUrRYKwch4qplLHALx9qVxcw=@protonmail.com>
References: <CAHoOKg+69EbucDMHwVXk3ak_qiNo-3s5mzh=rhwbD0n+t=cgOw@mail.gmail.com>
	<CAHoOKgLSX=c=yriNmiu+TN6J8ZVS-_Ex1+CnRQiNQ7ZvzeVYPQ@mail.gmail.com>
	<20190925094312.iu43ij5k4r4viyol@erisian.com.au>
	<_MpLAUdyzQPsMnVOswO5Eux8XUxCbJIiBIDEp4gwni9ru8r4Lj0bKD5oM3W0mvphadyx23kIXDT_hzma-NkIUrRYKwch4qplLHALx9qVxcw=@protonmail.com>
Message-ID: <20190925192958.tt7k3lkvyr6elv4k@erisian.com.au>

On Wed, Sep 25, 2019 at 01:30:39PM +0000, ZmnSCPxj wrote:
> > Since it's off chain, you could also provide R and C and a zero knowledge
> > proof that you know an r such that:
> > R = SHA256( r )
> > C = SHA256( x || r )

> > in which case you could do it with lightning as it exists today.
> I can insist on paying only if the server reveals an `r` that matches some known `R` such that `R = SHA256(r)`, as currently in Lightning network.
> However, how would I prove, knowing only `R` and `x`, and that there exists some `r` such that `R = SHA256(r)`, that `C = SHA256(x || r)`?

If you know x and r, you can generate C and R and a zero knowledge proof
of the relationship between x,C,R that doesn't reveal r (eg, I think
you could do that with bulletproofs). Unfortunately that zkp already
proves that C was generated based on x, so you get your timestamp for
free. Ooops. :(

Cheers,
aj