Patent ID: 7321658

Claim:
A padding application method for applying a padding scheme that ensures the security of cryptosystems not using random numbers, to cryptosystems in which a random number is used to create a ciphertext and is susceptible to recovery at a receiving end, the method comprising the steps of: converting input information into a bit string with a prescribed length or less according to the padding scheme; converting the bit string into a first bit string and a second bit string based on a prescribed conversion rule; and supplying an encryption function with the first bit string as data input and the second bit string as random number input; wherein the prescribed conversion rule is a map to map the bit string having a prescribed length or less to the element of the direct product of the set of the first bit strings and the set of the second bit strings, and satisfies the following conditions: the map is injective; the map and inverse map thereof are computable by a polynomial time; and the encryption function whose domain is the direct product is a one-way function.