Subject: Re: [bitcoin-dev] Schnorr signatures BIP

From: Gregory Maxwell <[email-masked]>
Date: Fri, 6 Jul 2018 22:00:28 +0000

On Fri, Jul 6, 2018 at 9:05 PM, Russell O'Connor via bitcoin-dev
<[email-masked]> wrote:
> If the inputs to hash were reordered as hash(bytes(dG) || bytes(x(R)) || m)
> then there is an opportunity for SHA256 expander to be partially prefilled
> for a fixed public key.  This could provide a little benefit, especially
> when multiple signatures for a single public key need to be generated and/or
> verified.  If all things are otherwise equal, perhaps this alternate order
> is better.

There is a minor design preference to have message before nonce when
H() is a MD-style hash function.  Say the attacker knows some weakness
in H and can find pairs of messages m and m' so that the compression
function results in the same midstate.  He could then ask you to sign
m but get a signature that also works for m'.   If the signer
controlled R value comes first, then this doesn't work.    The pubkey
being where it is in the current design just follows from the idea
that it is just logically prepended on the message.  I don't think the
pubkey is sufficiently attacker controlled that the above argument
would apply,  so H(P || R.x || m) would be okay.

BUT, the sha256 compression function reads 64 bytes at a time. PRM
would not let you precompute a whole compression function run, but
instead would just let you hardwire part of the expander in a pubkey
dependant way-- an optimization I'm pretty confident virtually no one
would use.  (Hardwiring to a constant, yes. Hardwiring to a reused
dynamic value that comes in from the network, no)

If instead the hash function were defined as using 31 zeros then
P||R||m (or P || 31 zeros bytes || R || m, I'm not sure what would be
better), an entire midstate could be cached for different pubkeys. m
is often 32 bytes, sadly- - but the final compression run in that case
would only be the constant update with the length.... and
almost-all-zeros + constant length, is an easy optimization. (Bitcoin
core even has it for computing sha256(sha256())).

[I'm not really sure if I was clear, so I'll try TLDRing it:  I think
optimizing sha256 where part of the input is constant is realistic,
optimizing midstate reuse is realistic, optimizing where part is
reused is less realistic.  If we insert padding, and put P first, we
can make it possible to midstate cache P,  and the 'extra' compression
function run ends up with all constant input, so it could be made
faster.]