adiabat via bitcoin-dev

2017-05-07 06:45:00 UTC

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If / when Schnorr signatures are deployed in a future witness version, itRaw Message

may be possible to have non-interactive partial aggregation of the

signatures on a per-block basis. This could save quite a bit of space. It

*seems* not to have any security problems but this mailing list is very

good at finding vulnerabilities so that type of feedback is the main reason

I'm writing :) (A quick explanation of why this is horribly broken could

save me lots of time!)

(also sorry if this has been discussed; didn't see anything)

Quick recap / context of Schnorr sigs:

There are a bunch of private keys x1, x2, x3...

multiply by generator G to get x1G = P1, x2G = P2, x3G = P3

Everyone makes their sighash m1, m2, m3, and their random nonces k1, k2, k3.

To sign, people calculate s values:

s1 = k1 - h(m1, R1, P1)x1

s2 = k2 - h(m2, R2, P2)x2

(adding the P2 into the e hash value is not in most literature /

explanations but helps with some attacks; I beleive that's the current

thinking. Anyway it doesn't matter for this idea)

Signature 1 is [R1, s1]. Verifiers check, given P1, m1, R1, s1:

s1G =? R1 - h(m1, R1, P1)P1

You can *interactively* make aggregate signatures, which requires

co-signers to build an aggregate R value by coming up with their own k

values, sharing their R with the co-signers, adding up the R's to get a

summed R, and using that to sign.

Non-interactively though, it seems like you can aggregate half the

signature. The R values are unique to the [m, P] pair, but the s's can be

summed up:

s1 + s2 = k1 + k2 - h(m1, R1, P1)x1 - h(m2, R2, P2)x2

(s1 + s2)G = R1 + R2 - h(m1, R1, P1)P1 - h(m2, R2, P2)P2

To use this property in Bitcoin, when making transactions, wallets can sign

in the normal way, and the signature, consisting of [R, s] goes into the

witness stack. When miners generate a block, they remove the s-value from

all compatible inputs, and commit to the aggregate s-value in the coinbase

transaction (either in a new OP_RETURN or alongside the existing witness

commitment structure).

The obvious advatage is that signatures go down to 32 bytes each, so you

can fit more of them in a block, and they take up less disk and network

space. (In IBD; if a node maintains a mempool they'll need to receive all

the separate s-values)

Another advatage is that block verification is sped up. For individual

signatures, the computation involves:

e = h(m1, R1, P1) <- hash function, super fast

e*P <- point multiplication, slowest

R - e*P <- point addidion, pretty fast

s*G <- base point multiplication, pretty slow

with s-aggregate verification, the first three steps are still carried out

on each signature, but the s*G operation only needs to be done once.

Instead another point addition per signature is needed, where you have some

accumulator and add in the left side:

A += R - e*P

this can be parallelized pretty well as it's commutative.

The main downside I can see (assuming this actually works) is that it's

hard to cache signatures and quickly validate a block after it has come

in. It might not be as bad as it first seems, as validation given chached

signatures looks possible without any elliptic curve operations. Keep an

aggregate s-value (which is a scalar) for all the txs in your mempool.

When a block comes in, subtract all the s-values for txs not included in

the block. If the block includes txs you weren't aware of, request them in

the same way compact blocks works, and get the full signature for those

txs. It could be several thousand operations, but those are all bigInt

modular additions / subtractions which I believe are pretty quick in

comparison with point additions / multiplications.

There may be other complications due to the fact that the witness-txids

change when building a block. TXIDs don't change though so should be

possible to keep track of things OK.

Also you can't "fail fast" for the signature verification; you have to add

everything up before you can tell if it's correct. Probably not a big deal

as PoW check comes first, and invalid blocks are pretty uncommon and quite

costly.

Would be interested to hear if this idea looks promising.

Andrew Polestra mentioned something like this in the context of CT /

mimblewimble transactions a while ago, but it seems it may be applicable to

regular bitcoin Schnorr txs.

-Tadge