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Cryptography according to Schneier [#!secure!#] is ``the art
of keeping messages secure''. There are two choices for communication
over networks: prevent unauthorised people gaining access to the
network, as described in Section 10.4; or
scrambling the data so that it can't be understood. If the latter
root is taken, then cryptographic technology will be used. For
multimedia streams this generally requires the use of fast encryption
using shared keys on the media streams, and public key cryptography to
distribute the shared keys for the media streams.
Figure 10.1:
Encrypting to provide confidentiality

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
19981203