Either of the two.
The second rule, if mutually agreeable, is a slightly better rune for quickly detecting transmission errors.
In most pragmatic implementations that I have lately seen, this technique isn't used. The 56-bit key is exchanged in a packet that includes a true CRC checksum of the key material. Even an 8-bit CRC applied to 56 bits of data is much more likely to detect a garbled key than a byte-wise parity check. But it's more complex for a hardware device to perform, and if the DES implementation is in a hardware chip, the rules of that chip must in any case be followed.
Last edited by sundialsvcs; 08-28-2012 at 09:04 AM.