Remember that our information is just bits in memory and on the wire. If this data is viewed as the representation of large numbers, then we can apply mathematical functions to the data, ie if our input text, known as cleartext, is x, our cryptographic function is f() and we have a key k, the output of f(k,x) is the encrypted text y or cyphertext. Now if we choose our mathematical function f() so that there is no easily discovered inverse mapping, ie given y, it is extremely difficult to calculate x without knowing k, but it is possible given the knowledge of k, then we have a means of encrypting our data.
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Jon CROWCROFT
1998-12-03