Apparatus for homomorphic encryption of categorical data and method for thereof

A method for generating a homomorphic ciphertext, comprising:

• storing a plurality of categorical data whose values are expressed as a plurality of classes in a single category;
• generating a categorical column in which each homomorphic ciphertext is placed in a respective one of a plurality of slots by homomorphically encrypting each of the plurality of categorical data;
• for each of the plurality of classes, generating a mask column in which information indicating whether categorical data of each of the plurality of slots within the categorical data column belongs to a specific class is placed in a slot corresponding to the data among the plurality of slots;
• and
• generating a homomorphic ciphertext by combining the categorical column and the plurality of mask columns.

A non-transitory computer-readable recording medium including a program for executing a method for generating a homomorphic ciphertext, wherein the method comprising:

• storing a plurality of categorical data whose values are expressed as a plurality of classes in a single category;
• generating a categorical column in which each homomorphic ciphertext is placed in a respective one of a plurality of slots by homomorphically encrypting each of the plurality of categorical data;
• for each of the plurality of classes, generating a mask column in which information indicating whether categorical data of each of the plurality of slots within the categorical data column belongs to a specific class is placed in a slot corresponding to the data among the plurality of slots;
• and
• generating a homomorphic ciphertext by combining the categorical column and the plurality of mask columns.