Full Domain Hash
In cryptography, the Full Domain Hash (FDH) is an RSA-based signature scheme that follows the "hash-and-sign" paradigm. It is provably secure (i.e., is existentially unforgeable under adaptive chosen-message attacks) in the random oracle model. FDH involves hashing a message using a function whose image size equals the size of the RSA modulus, and then raising the result to the secret RSA exponent. Exact security of full domain hash. In the random oracle model, if RSA is formula_1-secure, then the full domain hash RSA signature scheme is formula_2-secure where, For large formula_4  this reduces to formula_5. This means that if there exists an algorithm that can forge a new FDH signature that runs in time "t", computes at most formula_6 hashes, asks for at most formula_4 signatures and succeeds with probability formula_8, then there must also exist an algorithm that breaks RSA with probability formula_9 in time formula_10.