[dlc-dev] Oracle attestations can be simplified and be more much more efficient

Thibaut Le Guilly thibaut at cryptogarage.co.jp
Fri Feb 26 08:33:04 CET 2021


Hi all,

As I was doing some benchmarking to see how to optimize the creation of a
signature point which seemed quite slow when using numerical decomposition,
I ended up also benchmarking this proposal (with quite some help from
Lloyd, thanks!). The following results were obtained by benchmarking the
creation of signature points for all possible outcomes when using 3
oracles, 5 nonces, and 2 outcomes per nonce.

Original sequential:
        31,071,116 ns/iter (+/- 3,749,403)
Original outcome parallelized:
  11,382,976 ns/iter (+/- 4,162,747)
Factorized:
             11,695,640 ns/iter (+/- 920,184)
Factorized outcome parallelized:
 4,407,871 ns/iter (+/- 2,431,375)
Factorized oracle parallelized:
     8,657,884 ns/iter (+/- 3,446,381)
Factorized oracle and outcome parallelized:
 4,362,859 ns/iter (+/- 2,637,939)
Lloyd's technique:
          2,387,935 ns/iter (+/- 205,201)
Lloyd's technique + aggregate oracle nonces and pubkeys:     782,595
ns/iter (+/- 66,201)
Lloyd's technique + oracle parallelized
 1,380,110 ns/iter (+/- 639,166)

Outcome parallelized = computation of sigpoint for each outcome is
parallelized
Oracle parallelized = computation of sigpoint for each oracle is
parallelized

As you can see Lloyd's proposal outperforms the others by quite a lot, in
particular when aggregating oracle nonces and public keys before computing
sigpoints (which cannot be done when using hashes). We can note that the
benchmark might favor Lloyd's approach slightly as it works particularly
well for computing sigpoints of consecutive outcomes and that with
compression we don't always need to compute all sigpoints, but the
difference performance improvement is such that I don't think it will
matter.

You can find the benchmark code here:
https://github.com/p2pderivatives/rust-dlc/tree/sig-point-bench-tmp
(warning this is not pretty code, only meant for experimental purpose). Run
`cargo bench --features unstable` to run the benchmarks.

Best,

Thibaut

On Tue, Jan 26, 2021 at 10:31 AM Lloyd Fournier via dlc-dev <
dlc-dev at mailmanlists.org> wrote:

> Hi list,
>
> I want to expand on a nuance of the performance improvement. Since it only
> saves ecmults when computing the anticipation points. it won't have much of
> an effect on the performance of binary digit decomposition. If you have a
> 16 bit number you will have 16 R values and do 32 ecmults. With this
> proposal above you will instead only have to do 16 ec additions which would
> make it ~20 times faster.
>
> But let's say we have 1k CETs for this event in binary digit decomposition
> then you will have to do a few thousand EC additions to produce the
> encryption key for each CET signature (each of the CETs takes <16
> additions). The time it takes to do these additions will make the
> performance improvement offered by speeding up the anticipation point
> calculation relatively insignificant. The performance improvement is mostly
> felt when you have a single R value with many possible outcomes (i.e. not
> binary digit decomposed).
>
> Having said that I still think this is a good idea since it is simpler to
> implement and doesn't require maintaining a mangled BIP340 function to
> produce the anticipation point. This idea can be implemented with
> libsecp256k1's public API.
>
> Cheers,
>
> LL
>
>
>
> On Tue, Dec 29, 2020 at 1:23 PM Lloyd Fournier <lloyd.fourn at gmail.com>
> wrote:
>
>> Merry Christmas list,
>>
>> A Schnorr signature has the form (R, s) where
>> 1. R = r *G for some secret nonce r
>> 2. s = r + c*x where c = H(R || X || m) for a message m and some hash
>> function H
>>
>> The original DLC paper suggests re-using this structure but with R
>> published upfront (in what we call the *announcement*) to create a list of
>> anticipation points computed like P = R + c_i * X where c_i = H(R || X ||
>> m_i) where m_i indicates the i'th possible outcome of the event.
>>
>> I make the claim that c_i need not be the result of a hash function. c_i
>> can instead simply be set to i. So to attest to the 3rd outcome set c_i = 2
>> and the oracle reveals s = r + 2 * x.
>>
>> == Security
>>
>> The only security property required for the point anticipation scheme is
>> that (i) it reveals no information other than the list of points (zero
>> knowledge) (ii) if the oracle reveals the secrets for two points then they
>> lose their secret key (no equivocation).
>> Clearly (i) holds since R is still just a random point.
>> (ii) holds for the same reason as in the original -- two different c_i
>> and c_j reveals x = (s_i - s_j) / (c_i - c_j). The change from hash of
>> message to index is irrelevant.
>>
>> == Impact
>>
>> The main specification change would be to make outcome descriptors
>> describe an ordered list of outcomes rather than a set of strings. e.g. the
>> enumeration descriptor would simply assign the index to each outcome.
>>
>> The benefits of applying the above idea are:
>> 1. It would reduce the time taken to compute the list of anticipation
>> points to ~1/12 of the time currently taken since you can avoid doing any
>> ec multiplications. You can incrementally add X to each previous value
>> (starting with R) to get the next anticipation point. In the current
>> protocol you have to verify ECDSA adaptor signatures so this would still
>> lead to a time reduction of 1/2 to 1/3 since it removes 2/3 of the ecmults
>> (depends a lot on implementation).
>> 2. In the future you could capture the full speedup by not doing adaptor
>> signatures and just putting each outcome in its own Taproot branch and
>> using OP_CTV to enforce the outcome.
>> 3. It removes arbitrary structure from the scheme and is easier to
>> specify and implement (no need to talk about tagged hashes or to have a
>> BIP340 implementation).
>> 4. It gets rid of all concerns about utf8 and message encoding since
>> outcomes are represented by integers.
>>
>> The downsides are mostly rhetorical:
>> 1. We couldn't talk about "signing the outcome" because that's not what
>> we're doing anymore (actually we were never really doing that but it was at
>> least not too much of a lie since there was at least a signature at the end
>> of it). I don't have an idea about the best way to talk about it yet.
>> 2. The oracle's attestation could no longer provide useful information by
>> itself. I liked the idea that systems who didn't care about DLCs could
>> still use the oracles to get info about the world just from the signature
>> and the message it signed. You can still use oracles in this way here but
>> you *must* have the announcement with the attestation data to figure out
>> what was attested to.
>>
>> Let me know your thoughts and hope you have a good break.
>>
>> Cheers,
>>
>> LL
>>
>
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