Function schnorrProve

Schnorr proof: prove knowledge of x where y = g^x, without revealing x.

Protocol:

1. Prover picks random k, sends a = g^k
2. Challenge: c = H(g, y, a) (Fiat-Shamir for non-interactive)
3. Response: z = k - c·x mod p Verification: g^z · y^c == a