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Re: [POLL] Perfectly hiding vs perfectly binding


I think it makes sense. It's a reasonable price to pay and I like that it makes it a lot easier to scan your unspent outputs. One question: switch commitments reuse H and compute SHA256(rH). Any particular reason why we'd want yet another generator?

And we'd likely use blake2 again instead of SHA256 but that's a detail.

- Igno

> -------- Original Message --------
> Subject: Re: [Mimblewimble] [POLL] Perfectly hiding vs perfectly binding
> Local Time: August 16, 2017 7:20 PM
> UTC Time: August 16, 2017 7:20 PM
> From: apoelstra@xxxxxxxxxxxxxx
> To: Ignotus Peverell <igno.peverell@xxxxxxxxxxxxxx>
> mimblewimble@xxxxxxxxxxxxxxxxxxx <mimblewimble@xxxxxxxxxxxxxxxxxxx>
> On Wed, May 03, 2017 at 08:14:27PM -0400, Ignotus Peverell wrote:
>> Hi all,
>> I thought running a little poll could be fun and it"s on a topic that may be more emotional than technical: in the advent of Quantum Computers, or even computers of infinite power, do we prefer transactions that are perfectly hiding (one will never be able to discover their value) or perfectly binding (one will never be able to steal or create money). It"s really inconvenient, but it"s been proven we can"t have both.
>> To vote, just reply with one of these 2 lines:
>> [X] Perfectly hiding, privacy guarantees should remain true forever
>> [X] Perfectly binding, one should never be able to break transaction integrity
>> Because some arguments may be non-obvious, I"ll flesh out a few.
>> Why we"d really want perfectly binding transactions is straightforward: being able to create money out of thin air or stealing sounds pretty bad for any cryptocurrency. Note that most existing cryptocurrencies are sensitive to this right now: with a working and powerful Quantum Computer, you"d likely be able to steal a fair amount of bitcoins or even zcash. So there"s a definite advantage in offering such strong integrity guarantees.
>> On the other hand, QCs aren"t going to happen overnight. We will likely have years (many experts say decades) to prepare. Also if it was to happen right now, we"d likely have very tangible issues in other places we"re not anticipating. But *when* it happens, a chain that"s not perfectly hiding will become fully clear. So all the transaction history up to the point where we have fully quantum safe algorithms will be analyzed. And while we can adjust algos, data stays forever.
>> Cast your votes!
>> - Igno
>> P.S. I can"t promise we"ll do what the majority says (on the crypto side we have perfectly hiding, but not perfectly binding yet), but it"ll influence the direction!
> While chatting about Mimblewimble over Elements with instagibbs and gmaxwell
> and real-or-random[0], we found a scheme that should satisfy everybody, except
> for those who believe there are already quantum computers among us. (But
> they should not be using MW because they should expect their coins to be
> immediately stolen.)
> Basically our outputs should consist of the pair
> vH + rG, sha256(rJ)
> where J is a new NUMS generator, G the standard generator, and H is our asset
> ID as always. We set this up so that we can softfork a rule that only allows
> outputs to be spent by revealing rJ and an unconditional rangeproof, but prior
> to the softfork we only require ordinary rangeproofs.
> This is just switch commitments[1] with the important distinction that rJ is
> hidden behind a hash. This means that users who use the scheme correctly still
> have statistical soundness (practically speaking this is just as good as
> unconditional soundness unless sha256 is destroyed, and in fact this is the
> only form of soundness we have in the existing Elements rangeproofs because
> we generate all of our secret values from a CSPRNG).
> Then we get the usual benefits of switch commitments
> - If quantum computers seem imminent we can require users reveal rJ, do the
> unconditional rangeproof, and convert to some quantum-hard Pedersen commitment
> analogue. Hopefully one exists, I think [2] is what we want but maybe I"ve
> misunderstood it.
> - Users who want permanent privacy and are willing to sacrifice their coins in
> case the above requirement is softforked in, can use whatever gibberish they
> want in place of the hash, and they will retain their privacy even against
> quantum computers. They will have plenty of warning before any such fork, so
> they can either move their coins away or move them to upgradable outputs,
> taking the privacy hit voluntarily and knowingly and only for their most recent
> history.
> - Regardless of the above, users who generate their hashes deterministically,
> e.g. from BIP32 paths, can easily scan for and recognize their outputs when
> scanning the blockchain. This is currently quite difficult to do in MW.
> - We reduce the amount of design and code that we have to write immediately,
> letting us postpone it until we have a better idea of how people use use the
> system and what real users want as far as privacy/security.
> Cheers
> Andrew
> [0] Greg Sanders, Greg Maxwell, Tim Ruffing
> [1] https://eprint.iacr.org/2017/237.pdf
> [2] http://eprint.iacr.org/2015/628
> --
> Andrew Poelstra
> Mathematics Department, Blockstream
> Email: apoelstra at wpsoftware.net
> Web: https://www.wpsoftware.net/andrew
> "A goose alone, I suppose, can know the loneliness of geese
> who can never find their peace,
> whether north or south or west or east"
> --Joanna Newsom

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