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
For large
This means that if there exists an algorithm that can forge a new FDH signature that runs in time t, computes at most