Verifier for ECDSA Secp256r1 signatures.
Verifies a ECDSA signature over the secp256r1 curve.
inputs:
x coordinate of public key as 32 bytes
y coordinate of public key as 32 bytes
the signature, as a 64 bytes array
The signature internally will be represented as (r, s),
where r and s are fixed-sized big endian scalar values.
As the secp256r1 has a 256-bit modulus, we have a 64 byte signature
while r and s will both be 32 bytes.
We expect s to be normalized. This means given the curve's order,
s should be less than or equal to order / 2.
This is done to prevent malleability.
For more context regarding malleability you can reference BIP 0062.