[Bitcoin-development] Small update to BIP 62

Aaron Voisine voisine at gmail.com
Sat Jul 19 19:08:13 UTC 2014

Thanks g.maxwell, your explanation of *why* you can't just generate k
in a way that the verifier can duplicate is really helpful. This also
servers as a great illustration why engineers should never try to
designing their own crypto protocols! I knew enough to know not try
that at least.

Aaron Voisine

On Fri, Jul 18, 2014 at 11:56 PM, Gregory Maxwell <gmaxwell at gmail.com> wrote:
> On Fri, Jul 18, 2014 at 9:38 PM, Aaron Voisine <voisine at gmail.com> wrote:
>> Well, you could always create a transaction with a different signature
>> hash, say, by changing something trivial like nLockTime, or changing
>> the order of inputs or outputs. Is that what you're talking about? Or
>> is there some sophistry I'm ignorant of having to do with the elliptic
>> curve math in the signature itself?
> No, though thats true too. I was talking about the properties of the DSA nonce:
> An attacker is not obligated to follow your protocol unless you can
> prevent him. You can _say_ use derandomized DSA all you like, but he
> can just not do so, there is no (reasonable) way to prove you're using
> a particular nonce generation scheme without revealing the private key
> in the process. The verifier cannot know the nonce or he can trivially
> recover your private key thus he can't just repeat the computation
> (well, plus if you're using RFC6979 the computation includes the
> private key), so short of a very fancy ZKP (stuff at the forefront of
> cryptographic/computer science) or precommiting to a nonce per public
> key (e.g. single use public keys), you cannot control how a DSA nonce
> was generated in the verifier in a way that would prevent equivalent
> but not identical signatures.
> (I believe there was some P.O.S. altcoin that was vulnerable because
> of precisely the above too— thinking specifying a deterministic signer
> would prevent someone from grinding signatures to improve their mining
> odds... there are signature systems which are naturally
> randomness-free: most hash based signatures and pairing short
> signatures are two examples that come to mind... but not DSA, schnorr,
> or any of their derivatives).

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