Patent ID: 7200232

Claim:
A symmetric-key decryption method performed by a computer, comprising the steps of: dividing a ciphertext, which is an input text, to generate a plurality of ciphertext blocks each having a predetermined length; generating a first random number block and a second random number block both corresponding to each of said plurality of ciphertext blocks based on a secret key that is an input value; performing decryption operations for producing plaintext blocks each corresponding to each of said plurality of ciphertext blocks; concatenating a series of said ciphertext blocks one after another sequentially to output a plaintext, which includes a message and redundancy data; and examining the redundancy data to detect whether the plaintext obtained from the ciphertext has been altered, wherein one of said decryption operations for producing a plaintext block i corresponding to a ciphertext block i (2≦i≦, i being a number indicative of ciphertext blocks) comprises: a first operation step for performing an arithmetic computation on said ciphertext block i, a second operation step for performing an arithmetic computation on a result of said first operation step performed on said ciphertext block i and said first random number block corresponding to said ciphertext block i, and a third operation step for performing an arithmetic computation on a result of said second operation step performed on said ciphertext block i and said second random number block corresponding to said ciphertext block i, to produce said plaintext block i, and wherein said first operation step performs the arithmetic computation on said ciphertext block i and a result of said second operation step performed on the ciphertext block i−1, and wherein either said first random number or said second random number is generated in complete isolation from any one of said plurality of ciphertext blocks or the result of said first operation step.