while the csa algorithm uses 64-bit keys, most of the time, only 48 bits of the key are unknown, since bytes 3 and 7 are used as parity bytes in ca systems, and may be easily recalculated. this allows for possible known-plaintext attacks when combined with knowledge of the underlying plaintext structure. as the first three bytes of the pes header is always 0x000001, it would be possible to launch a brute force attack. such an attack would reveal millions of possible keys, but still few enough to make it practical to attempt decryption of other parts of the data with the same key in a second pass to recover the true key.