Several early signature schemes were of a similar type: they involve the use of a Trapdoor permutation, such as the RSA function, or in the case of the Rabin signature scheme, computing square modulo composite, N. A Trapdoor permutation family is a family of permutations, specified by a parameter, that is easy to compute in the forward direction, but is difficult to compute in the reverse
direction without already knowing the private key"Trapdoor.