CYPHER SYSTEM, ENCRYPTION METHOD, DECRYPTION METHOD AND PROGRAM

1 . A cryptographic system comprising:

an encryption apparatus including a memory and a processor configured to encrypt a plaintext into a ciphertext,

wherein the processor of the encryption apparatus executes:

generating first information resulting from encryption of the plaintext by an encryption function of a predetermined block cipher using a first secret key;

generating second information resulting from encryption of a preset adjustment value by the encryption function using a second secret key; and

generating the ciphertext by encrypting an arithmetic operation result of a bitwise exclusive OR of the first information and the second information by the encryption function using the first secret key.

2 . The cryptographic system according to claim 1 , further comprising:

a decryption apparatus including a memory and a processor configured to decrypt the ciphertext, the decryption apparatus executes:

generating third information resulting from decryption of the ciphertext by a decryption function using the first secret key, the decryption function corresponding to the encryption function;

generating the second information resulting from encryption of the adjustment value by the encryption function using the second secret key; and

for generating the plaintext by decrypting the arithmetic operation result of the bitwise exclusive OR of the third information and the second information by the decryption function using the first secret key.

3 . The cryptographic system according to claim 2 , wherein

the generating of the ciphertext is executed by encrypting the arithmetic operation result of the bitwise exclusive OR of the first information and the second information by the encryption function using a third secret key instead of the first secret key; and

the generating of the third information results from decryption of the ciphertext by the decryption function using the third secret key instead of the first secret key.

4 . An encryption method executed by an encryption apparatus including a memory and a processor configured to encrypt a plaintext into a ciphertext, the encryption method comprising:

generating first information resulting from encryption of the plaintext by an encryption function of a predetermined block cipher using a first secret key;

generating second information resulting from encryption of a preset adjustment value by the encryption function using a second secret key; and

generating the ciphertext by encrypting an arithmetic operation result of a bitwise exclusive OR of the first information and the second information by the encryption function using the first secret key.

5 . A decryption method executed by a decryption apparatus including a memory and a processor configured to decrypt a ciphertext resulting from encryption of a plaintext by an encryption apparatus, the decryption method comprising:

generating third information resulting from decryption of the ciphertext by a decryption function using a first secret key, the decryption function corresponding to an encryption function of a predetermined block cipher;

generating second information resulting from encryption of the preset adjustment value by the encryption function using a second secret key; and

generating the plaintext by decrypting the arithmetic operation result of the bitwise exclusive OR of the third information and the second information by the decryption function using the first secret key.

6 . A non-transitory computer-readable recording medium having computer-readable instructions stored thereon, which when executed, cause a computer including a memory and a processor to execute the generating of the first information, the generating of the second information, and the generating of the ciphertext in the encryption apparatus included in the cryptographic system according to claim 1 .