The key is a shared secret type key right? So it doesn't
really matter what the key is as the encrypted output will
probably be on the same level of breakability with any key.
I don't think there is a good key or bad key in this case.
If it's a technical question on the mechanism of generating
key then then the example in the perldoc for the module
should be sufficient. Need to pack enough bytes to generate
required number of bits (max 448 bits = 56 bytes).