Adjustment-value-attached block cipher apparatus, cipher generation method and recording medium

An computer-implemented cipher generation method performed by an adjustment-value-attached block cipher apparatus including an encryption section performing encryption by using a block cipher system, the method comprising:

• accepting plaintext, an adjustment value and instruction information which indicates a first cipher processing operation or a second cipher processing operation to be performed by at least a computer processor;
• and
• using at least the computer processor to perform the steps of:
• dividing the adjustment value into a first adjustment value and into a second adjustment value,
• encrypting the first adjustment value using the encryption section to generate a cipher for offset,
• determining an offset value on the basis of the cipher for offset and the second adjustment value,
• determining the sum of the offset value and the plaintext, and
• encrypting the sum using the encryption section to generate a first ciphertext when the instruction information indicates the first cipher processing operation, and
• encrypting the plaintext using the encryption section to generate a second ciphertext when the instruction information indicates the second cipher processing operation;
• wherein
• generating the first ciphertext comprises fixing the least significant bit of the first adjustment value to 0 and fixing the least significant bit of the plaintext used in the second cipher processing to 1 so that the first adjustment value is different from the plaintext used in the second cipher processing operation.

A non-transitory computer-readable recording medium in which a program is recorded, the program causing a computer to function as:

• an encryption section performing encryption by using a block cipher system;
• an input section accepting plaintext, an adjustment value and instruction information which indicates a first cipher processing operation or a second cipher processing operation;
• and
• a generation section dividing the adjustment value into a first adjustment value and into a second adjustment value, encrypting the first adjustment value using the encryption section to generate a cipher for offset, determining an offset value on the basis of the cipher for offset and the second adjustment value, determining the sum of the offset value and the plaintext, and encrypting the sum using the encryption section to generate a first ciphertext when the instruction information indicates the first cipher processing operation, and encrypting the plaintext using the encryption section to generate a second ciphertext when the instruction information indicates the second cipher processing operation;
• wherein
• the generation section fixes the least significant bit of the first adjustment value to 0 and fixes the least significant bit of the plaintext used in the second cipher processing to 1 so that the first adjustment value is different from the plaintext used in the second cipher processing operation.