BBS+ Verifier Contract

[man2706kum]

This PR includes the solidity verifier contract for BBS+ signature verification. The following function is implemented:

• expandMsgTo48: converts arbitrary byte msg to fixed 48 bytes. Also required for bn254 hash-to-curve
• from_okm which takes a bigint number n as 48 byte msg and does n (mod p) where p is scalar prime
• hashToScalar: converts arbitrary byte msg to bn254 scalar
• calculate_domain: calculates the domain scalar from public key and generators as mentioned in BBS Draft
• verifySignature: verifies the BBS+ signature
• All the bn254 add, scalar_mul and pairing operation used the bn254 precompile
• There are 32 generators generated in pseudo-random manner and are hardcoded in the contract. That means the upper bound on number of messages is 32-1 = 31