Patent ID: 7512231

Claim:
A decryption method of decrypting a plaintext m from a secret exponent d, a public key n, and a ciphertext c in a public-key cryptosystem, said decryption method comprising steps executed by a computer, said steps comprising: (i) converting the ciphertext c into a randomized ciphertext t, (ii) converting the randomized ciphertext t into a randomized plaintext u, and (iii) converting the randomized plaintext u into the plaintext m, wherein said step (i) comprises: generating a random number r, and computing the randomized ciphertext t by using the random number r and an integer s derived from the random number r, wherein said step (ii) comprises computing the randomized plaintext u by using a value derived from the secret exponent d, wherein said step (iii) comprises computing the plaintext m by multiplying the randomized plaintext u by the integer s, wherein r eâˆ’1 is used as the integer s derived from the random number r, where e is a public exponent, and wherein said step of converting into the randomized ciphertext t comprises multiplying the ciphertext c by sr.