An encryption transmission apparatus and an encryption reception apparatus
avoid attack that takes advantage of a re-transmission request. A server
apparatus encrypts a content key five times, thereby generating five
encrypted content keys, calculates a hash value of the content key, and
transmits the five encrypted content keys and the hash value. An image
playback apparatus receives the five encrypted content keys and the hash
value, decrypts the five encrypted content keys thereby generating five
content keys, calculates hash values, each corresponding to the generated
content keys, and compares the calculated hash values with the received
hash value, respectively. If at least one of the five calculated hash
values matches the received hash value, the corresponding content key is
considered correct. Conversely, if none of the five calculated hash
values matches the received hash value, it is considered a decryption
error.