Information processing apparatus, tampering detection apparatus, information processing method, tampering detection method, and computer product

A tampering detection apparatus comprising:

• a hardware processor configured to:
• receive input of encrypted data acquired by encrypting plain text data on a basis of a predetermined block length;
• generate from the received encrypted data, decrypted data that is decrypted by using a key used for encryption and decryption in common;
• extract a first code from a first area that is at an end of the decrypted data and stores a data length of padding;
• determine based on a data length indicated by the extracted first code, a second area that is of the decrypted data and stores a code calculated from the plain text data;
• extract a second code from the determined second area;
• extract a third code from a third area of the decrypted data, acquired by removing an ending area of the decrypted data, and having a data length indicated by the first code;
• generate a fourth code that is calculated from the extracted third code and of a same data length as the data length of the second code;
• judge whether the generated fourth code and the second code coincide;
• and
• output a judgment result.

An information processing method executed by a computer, the information processing method comprising:

• identifying a data length that is longer than a data length of plain text data and that is a multiple of a predetermined block length;
• calculating a data length difference of the data length of the plain text and the identified data length;
• generating a first code that indicates the calculated data length difference;
• generating a second code that is calculated from the plain text data and is of a data length that is within a remaining data length acquired by subtracting a data length of the generated first code from the data length difference;
• creating padding that includes the generated second code, has the first code at an end, and is of a length equivalent to the data length difference;
• concatenating the created padding to an end of the plain text data to generate concatenated data;
• and
• outputting the concatenated data.

A tampering detection method executed by a computer, the tampering detection method comprising:

• receiving input of encrypted data acquired by encrypting plain text data on a basis of a predetermined block length;
• generating from the received encrypted data, decrypted data that is decrypted by using a key used for encryption and decryption in common;
• extracting a first code from a first area that is at an end of the decrypted data and stores a data length of padding;
• determining based on a data length indicated by the extracted first code, a second area that is of the decrypted data and stores a code calculated from the plain text data;
• extracting a second code from the determined second area;
• extracting a third code from a third area of the decrypted data, acquired by removing an ending area of the decrypted data, and having a data length indicated by the first code;
• generating a fourth code that is calculated from the extracted third code and of a same data length as the data length of the second code;
• judging whether the generated fourth code and the second code coincide;
• and
• outputting a judgment result.

A non-transitory computer-readable recording medium storing an information processing program that causes a computer to execute a process comprising:

• identifying a data length that is longer than a data length of plain text data and that is a multiple of a predetermined block length;
• calculating a data length difference of the data length of the plain text and the identified data length;
• generating a first code that indicates the calculated data length difference;
• generating a second code that is calculated from the plain text data and is of a data length that is within a remaining data length acquired by subtracting a data length of the generated first code from the data length difference;
• creating padding that includes the generated second code, has the first code at an end, and is of a length equivalent to the data length difference;
• concatenating the created padding to an end of the plain text data to generate concatenated data;
• and
• outputting the concatenated data.

A non-transitory computer-readable recording medium storing a tampering detection program that causes a computer to execute a process comprising:

• receiving input of encrypted data acquired by encrypting plain text data on a basis of a predetermined block length;
• generating from the received encrypted data, decrypted data that is decrypted by using a key used for encryption and decryption in common;
• extracting a first code from a first area that is at an end of the decrypted data and stores a data length of padding;
• determining based on a data length indicated by the extracted first code, a second area that is of the decrypted data and stores a code calculated from the plain text data;
• extracting a second code from the determined second area;
• extracting a third code from a third area of the decrypted data, acquired by removing an ending area of the decrypted data, and having a data length indicated by the first code;
• generating a fourth code that is calculated from the extracted third code and of a same data length as the data length of the second code;
• judging whether the generated fourth code and the second code coincide;
• and
• outputting a judgment result.