[bitcoin-dev] Schnorr signatures BIP
roconnor at blockstream.io
Fri Jul 6 21:05:03 UTC 2018
Some quick comments:
> To sign:
> - Let *k = int(hash(bytes(d) || m)) mod n*[8
> - Let *R = kG*.
> - If *jacobi(y(R)) ≠ 1*, let *k = n - k*.
> - Let *e = int(hash(bytes(x(R)) || bytes(dG) || m)) mod n*.
> - The signature is *bytes(x(R)) || bytes(k + ex mod n)*.
> Can we avoid mutable variables in these specification? I know this is
commonly done in RFCs, but I think it is fairly confusing to have `k`
defined in two different ways within a single specification.
Let's let k' = k when jacobi(y(R)) = 1 and let k' = n - k when jacobi(y(R))
= -1. Note that this ensures that jacobi(y(k'G)) = 1.
Also you've sort of left it undefined what to do when k = 0. According to
the current specification, you will produce an invalid signature. The
expected result is that you should win a 1000 BTC prize.
One solution is to let k = *1 + int(hash(bytes(d) || m)) mod (n-1)*.
Alternatively you could let k' = 1 when k = 0. Or you could just make a
note that signature generation fails with this message and private key pair
when this happens.
Let *e = int(hash(bytes(x(R)) || bytes(dG) || m)) mod n*.
P = dG should probably be noted somewhere in the text. I.e. this signature
is generated for the public key P = dG.
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.
The signature is *bytes(x(R)) || bytes(k + ex mod n)*.
You haven't defined `x`. I'm guessing you mean `d` instead.
> *Jacobian coordinates*
> - *oncurve(P)* can be implemented as *y2 = x3 + 7z6 mod p*.
> oncurve(P) requires that `P` be on the curve and not infinity. You need
another condition here to ensure that `P` is not infinity.
On Fri, Jul 6, 2018 at 2:08 PM, Pieter Wuille via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:
> Hello everyone,
> Here is a proposed BIP for 64-byte elliptic curve Schnorr signatures,
> over the same curve as is currently used in ECDSA:
> It is simply a draft specification of the signature scheme itself. It
> does not concern consensus rules, aggregation, or any other
> integration into Bitcoin - those things are left for other proposals,
> which can refer to this scheme if desirable. Standardizing the
> signature scheme is a first step towards that, and as it may be useful
> in other contexts to have a common Schnorr scheme available, it is its
> own informational BIP.
> If accepted, we'll work on more production-ready reference
> implementations and tests.
> This is joint work with several people listed in the document.
> bitcoin-dev mailing list
> bitcoin-dev at lists.linuxfoundation.org
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