An encryption/decryption apparatus and an encryption/decryption method thereof are provided. A data encryption/decryption unit performs an encryption/decryption operation to a digital data and thus generates an encryption/decryption power signal corresponding to the encryption/decryption operation. A complementary power generating unit generates a complementary power signal corresponding to the encryption/decryption power signal. The encryption/decryption apparatus outputs the complementary power signal and the encryption/decryption power signal as a power signal, wherein a sum of the complementary power signal and the encryption/decryption power signal is a fixed value.