A sending apparatus generates a first initial vector, a second initial vector, and an encryption key in response to a pseudo random number. Original information is encrypted into cipher information in response to the encryption key and the second initial vector. The cipher information and the first initial vector are transmitted from the sending apparatus to a receiving apparatus. The receiving apparatus generates a first initial vector, a second initial vector, and an encryption key in response to a pseudo random number equal to that in the sending apparatus. The cipher information is decrypted back to the original information in response to the generated encryption key and the generated second initial vector. The receiving apparatus compares the received first initial vector and the generated first initial vector to check whether or not encryption/decryption-related synchronization between the sending apparatus and the receiving apparatus is normally maintained.