[bitcoin-dev] The first successful Zero-Knowledge Contingent Payment

Tier Nolan tier.nolan at gmail.com
Fri Feb 26 23:33:51 UTC 2016

That is very interesting.

There has been some recent discussion about atomic cross chain transfers
between Bitcoin and legacy altcoins.  For this purpose a legacy altcoin is
one that has strict IsStandard() rules and none of the advanced script

It has a requirement that Bob sends Alice a pair [hash_of_bob_private_key,
bob_public_key].  Bob has to prove that the hash is actually the result of
hashing the private key that matches bob_public_key.

This can be achieved with a cut-and-choose scheme.  It uses a fee so that
an attacker loses money on average.  It is vulnerable to an attacker who
doesn't mind losing money as long as the target loses money too.

Bob would have to prove that he has an x such that

xG = <bob_public_key>
hash(x) = hash_of_bob_private_key

Is the scheme fast enough such that an elliptic curve multiply would be
feasible?  You mention 20 seconds for 5 SHA256 operations, so I am guessing

On Fri, Feb 26, 2016 at 11:06 PM, Sergio Demian Lerner via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Congratulations!
> It a property of the SKCP system that the person who performed the trusted
> setup cannot extract any information from a proof?
> In other words, is it proven hard to obtain information from a proof by
> the buyer?
> On Fri, Feb 26, 2016 at 6:42 PM, Gregory Maxwell via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
>> I am happy to announce the first successful Zero-Knowledge Contingent
>> Payment (ZKCP) on the Bitcoin network.
>> ZKCP is a transaction protocol that allows a buyer to purchase
>> information from a seller using Bitcoin in a manner which is private,
>> scalable, secure, and which doesn’t require trusting anyone: the
>> expected information is transferred if and only if the payment is
>> made. The buyer and seller do not need to trust each other or depend
>> on arbitration by a third party.
>> Imagine a movie-style “briefcase swap” (one party with a briefcase
>> full of cash, another containing secret documents), but without the
>> potential scenario of one of the cases being filled with shredded
>> newspaper and the resulting exciting chase scene.
>> An example application would be the owners of a particular make of
>> e-book reader cooperating to purchase the DRM master keys from a
>> failing manufacturer, so that they could load their own documents on
>> their readers after the vendor’s servers go offline. This type of sale
>> is inherently irreversible, potentially crosses multiple
>> jurisdictions, and involves parties whose financial stability is
>> uncertain–meaning that both parties either take a great deal of risk
>> or have to make difficult arrangement. Using a ZKCP avoids the
>> significant transactional costs involved in a sale which can otherwise
>> easily go wrong.
>> In today’s transaction I purchased a solution to a 16x16 Sudoku puzzle
>> for 0.10 BTC from Sean Bowe, a member of the Zcash team, as part of a
>> demonstration performed live at Financial Cryptography 2016 in
>> Barbados. I played my part in the transaction remotely from
>> California.
>> The transfer involved two transactions:
>> 8e5df5f792ac4e98cca87f10aba7947337684a5a0a7333ab897fb9c9d616ba9e
>> 200554139d1e3fe6e499f6ffb0b6e01e706eb8c897293a7f6a26d25e39623fae
>> Almost all of the engineering work behind this ZKCP implementation was
>> done by Sean Bowe, with support from Pieter Wuille, myself, and Madars
>> Virza.
>> Read more, including technical details at
>> https://bitcoincore.org/en/2016/02/26/zero-knowledge-contingent-payments-announcement/
>> [I hope to have a ZKCP sudoku buying faucet up shortly. :) ]
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