“Because computers can carry out elaborate sequences of
calculations flawlessly, it is possible to encrypt data using
complicated algorithms such as DES – and algorithms many times more
complex than that. Thus it is not merely tempting, but reasonable
to view the problem of devising a cipher no one can break – at
least a conventional or symmetric cipher – as a problem that has
already been solved. It is seen as a problem hardly worth wasting
anyone’s time on, except as an amusing pastime.”
“But another component of communications security is key
management, and that is regarded as more difficult….”
“This means that while one message sent with that program is
hard to crack without knowledge of the key phrase, sending two
messages with the same key phrase will make both messages easy to
read. This is because (as noted above in technical language)
each key phrase produces a fixed sequence of pseudo-random numbers
that are just added to your message.”