Encryption processing apparatus and encryption processing method

An encryption processing method, executed by a processor, of processing a ciphertext,

• the ciphertext being a fully homomorphic ciphertext that has, as a plaintext associated with an integer, a value with an error having a predetermined variance added thereto within a predetermined value range and that is able to be subjected to a predetermined operation between integers without being decrypted, the method comprising:
• applying a first polynomial in which all coefficients are the same to an operation result of a homomorphic operation between a ciphertext based on a first ciphertext and a ciphertext based on a second ciphertext, to obtain a third ciphertext indicating a position of a plaintext in the operation result;
• and
• applying a predetermined polynomial to a ciphertext obtained by reducing a range of plaintext to a predetermined range in the value range by a homomorphic operation of the third ciphertext to the operation result, to perform an operation between the first ciphertext and the second ciphertext.

A non-transitory computer-readable recording medium storing a program for causing a processor to perform an encryption process of processing a ciphertext,

• the ciphertext being a fully homomorphic ciphertext that has, as a plaintext associated with an integer, a value with an error having a predetermined variance added thereto within a predetermined value range and that is able to be subjected to a predetermined operation between integers without being decrypted, the program comprising:
• applying a first polynomial in which all coefficients are the same to an operation result of a homomorphic operation between a ciphertext based on a first ciphertext and a ciphertext based on a second ciphertext, to obtain a third ciphertext indicating a position of a plaintext in the operation result;
• and
• applying a predetermined polynomial to a ciphertext obtained by reducing a range of plaintext to a predetermined range in the value range by a homomorphic addition of the third ciphertext to the operation result, to perform an operation between the first ciphertext and the second ciphertext.