In cryptography, the Full Domain Hash (FDH) is an RSA-based signature scheme that follows the hash-and-decrypt paradigm. It is provably secure (i.e, existentially unforgeable against adaptive chosen plaintext 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 it to the secret RSA exponent. Thus hashing and padding are combined into a single step. It is roughly the digital signature analogue of RSA-OAEP.

Asymmetric-key cryptosystems