calculates a new value for hash using the ghash // algorithm. func paddedGHASH(hash *[16]byte, data []byte, productTable *[256]byte) { if siz := len(data) - (len(data) % gcmBlockSize); siz > 0 { gcmHash(hash[:], productTable, data[:], siz) data = data[siz:] } if len(data) > 0 { var s [16]byte copy(s[:], data) gcmHash(hash[:], productTable, s[:], len(s)) } } // auth calculates GHASH(ciphertext, additionalData), masks the result with // tagMask and writes the result to out. func auth(out, ciphertext, aad...