Device, method, and program for format-preserving encryption, and device, method, and program for decryption

A format-preserving encryption method of a format-preserving encryption device comprising a processor and a memory that encrypts an input plain text and outputs a cipher text in a same data format as the plain text, the memory storing instructions to be executed by the processor, thereby causing the processor to execute the method, the method comprising:

• encrypting, by an unit block tweakable encryption unit, a specific digit of the plain text by using numerical values of remaining respective digits excluding the specific digit of the plain text and an external tweak that is input concurrently with the plain text;
• permutating, by a block permutation unit, respective digits including the encrypted specific digit of the plain text per digit based on a predetermined rule;
• causing, by an encryption counter update unit, processing of the encryption of a specific digit of the plain text and the permutation per digit to repeat for a predetermined number of times;
• encrypting, by an AES encryptor, a numerical value that concatenates remaining respective digits excluding the specific digit of the plain text and the external tweak using a prestored encryption key and outputting an output value of a specific number of bits;
• generating a temporary encryption key using an output value of the AES encryptor;
• and
• encrypting the specific digit of the plain text utilizing the temporary encryption key.

A format-preserving decryption method of a format-preserving decryption device comprising a processor and a memory that decrypts an input cipher text and outputs a plain text in a same data format as the cipher text, the memory storing instructions to be executed by the processor, thereby causing the processor to execute the method, the method comprising:

• decrypting, by an unit block tweakable decryption unit, a specific digit of the cipher text by using numerical values of remaining respective digits excluding the specific digit of the cipher text and an external tweak that is input concurrently with the cipher text;
• reverse permutating, by a block reverse permutation unit, respective digits including the decrypted specific digit of the cipher text per digit based on a predetermined rule;
• causing, by a decryption counter update unit, processing of the decryption of a specific digit of the cipher text and the reverse permutation per digit to repeat for a predetermined number of times;
• decrypting, by an AES decryptor, a numerical value that concatenates remaining respective digits excluding the specific digit of the cipher text and the external tweak using a prestored decryption key and outputting an output value of a specific number of bits;
• generating a temporary decryption key using an output value of the AES decryptor;
• and
• decrypting the specific digit of the cipher text utilizing the temporary decryption key.

A computer-implemented apparatus comprising a processor and a non-transitory computer readable storage medium storing instructions to be executed by the processor, the apparatus encrypting an input plain text and outputting a cipher text in a same data format as the plain text, the instructions causing the processor to execute procedures of:

• encrypting a specific digit of the plain text by using numerical values of remaining respective digits excluding the specific digit of the plain text and an external tweak that is input concurrently with the plain text;
• permutating respective digits including the encrypted specific digit of the plain text per digit based on a predetermined rule;
• causing processing of the encryption of a specific digit of the plain text and the permutation per digit to repeat for a predetermined number of times;
• encrypting, by an AES encryptor, a numerical value that concatenates remaining respective digits excluding the specific digit of the plain text and the external tweak using a prestored encryption key and outputting an output value of a specific number of bits;
• generating a temporary encryption key using an output value of the AES encryptor;
• and
• encrypting the specific digit of the plain text utilizing the temporary encryption key.

A computer-implemented apparatus comprising a processor and a non-transitory computer readable storage medium storing instructions to be executed by the processor decrypting an input cipher text and outputting a plain text in a same data format as the cipher text, the instructions causing the processor to execute procedures of:

• decrypting a specific digit of the cipher text by using numerical values of remaining respective digits excluding the specific digit of the cipher text and an external tweak that is input concurrently with the cipher text;
• reverse permutating respective digits including the decrypted specific digit of the cipher text per digit based on a predetermined rule;
• causing processing of the decryption of a specific digit of the cipher text and the reverse permutation per digit to repeat for a predetermined number of times;
• decrypting, by an AES decryptor, a numerical value that concatenates remaining respective digits excluding the specific digit of the cipher text and the external tweak using a prestored decryption key and outputting an output value of a specific number of bits;
• generating a temporary decryption key using an output value of the AES decryptor;
• and
• decrypting the specific digit of the cipher text utilizing the temporary decryption key.