Provided is a non-transitory computer readable medium. The non-transitory computer readable medium storing program code that, when is executed by a processor, causes the processor to calculate a message, based on a first cipher text, a second cipher text, and a private key, to compare a coefficient of the message with a reference value based on a prime number, to decide a coefficient of a modified message, based on a comparison result between the coefficient of the message and the reference value, and to decrypt the modified message.