Ciphertext comparison system, ciphertext comparison method, ciphertext generation apparatus, ciphertext comparison apparatus, and control methods and control programs of ciphertext generation apparatus and ciphertext comparison apparatus

A control method of a ciphertext generation apparatus, comprising:

• generating a derived key based on a main key and a document;
• generating an auxiliary derived key based on the main key, the document, and the derived key;
• generating, based on an identifier of the document, the derived key, and the auxiliary derived key, an identifier-specific ciphertext in which the identifier is encrypted;
• and
• generating, based on the identifier and the derived key, a relative value ciphertext in which a relative value generated from the main key, the document, and the derived key is encrypted,
• wherein a character string including the identifier-specific ciphertext and the relative value ciphertext is generated as a ciphertext for the document.

A non-transitory computer readable medium storing a control program of a ciphertext generation apparatus for causing a computer to execute a method, comprising:

• generating a derived key based on a main key and a document;
• generating an auxiliary derived key based on the main key, the document, and the derived key;
• generating, based on an identifier of the document, the derived key, and the auxiliary derived key, an identifier-specific ciphertext in which the identifier is encrypted;
• and
• generating, based on the identifier and the derived key, a relative value ciphertext in which a relative value generated from the main key, the document, and the derived key is encrypted,
• wherein a character string including the identifier-specific ciphertext and the relative value ciphertext is generated as a ciphertext for the document.

A ciphertext comparison system comprising:

• a derived key generator that generates a derived key based on a main key and a document;
• an auxiliary derived key generator that generates an auxiliary derived key based on the main key, the document, and the derived key;
• an identifier-specific ciphertext generator that generates, based on an identifier of the document, the derived key, and the auxiliary derived key, an identifier-specific ciphertext in which the identifier is encrypted;
• a relative value ciphertext generator that generates, based on the identifier and the derived key, a relative value ciphertext in which a relative value generated from the main key, the document, and the derived key is encrypted;
• and
• a ciphertext comparator that determines a magnitude relationship between a first document as an original of a first ciphertext and a second document as an original of a second ciphertext with maintaining these documents in a ciphertext form, using a first derived key generated from the first document by said derived key generator, a first identifier of the first document, a first ciphertext including a first identifier-specific ciphertext in which the first identifier is encrypted by said identifier-specific ciphertext generator and a first relative value ciphertext in which a first relative value is encrypted by said relative value ciphertext generator, a second identifier of the second document, and the second ciphertext including a second identifier-specific ciphertext in which the second identifier is encrypted by said identifier-specific ciphertext generator and a second relative value ciphertext in which a second relative value is encrypted by said relative value ciphertext generator.

A ciphertext comparison method comprising:

• generating a derived key based on a main key and a document;
• generating an auxiliary derived key based on the main key, the document, and the derived key;
• generating, based on an identifier of the document, the derived key, and the auxiliary derived key, an identifier-specific ciphertext in which the identifier is encrypted;
• generating, based on the identifier and the derived key, a relative value ciphertext in which a relative value generated from the main key, the document, and the derived key is encrypted;
• and
• determining a magnitude relationship between a first document as an original of a first ciphertext and a second document as an original of a second ciphertext with maintaining these documents in a ciphertext form, using a first derived key generated from the first document in the generating the derived key, a first identifier of the first document, the first ciphertext including a first identifier-specific ciphertext in which the first identifier is encrypted in the generating the identifier-specific ciphertext and a first relative value ciphertext in which a first relative value is encrypted in the generating the relative value ciphertext, a second identifier of the second document, and the second ciphertext including a second identifier-specific ciphertext in which the second identifier is encrypted in the generating the identifier-specific ciphertext and a second relative value ciphertext in which a second relative value is encrypted in the generating the relative value ciphertext.