An extremely simple example of
convetional cryptography is a
substitution cipher. A
substitution cipher substitutes one piece of
information for another. This is most frequently done by offsetting letters of the alphabet. Two examples are
Captain Midnight's Secret Decoder Ring, which you may have owned when you were a kid, and
Julius Caesar's cipher. In both cases, the algorithm is to offset the
alphabet and the key is the number of characters to offset it.
For example, if we encode the word "SECRET" using Caesar's key value of 3, we offset the alphabet so that the 3rd letter down (D) begins the alphabet.
So starting with
ABCDEFGHIJKLMNOPQRSTUVWXYZ
and slidng everything up by 3, you get
DEFGHIJKLMNOPQRSTUVWXYZABC
Where D=A, E=B, F=C, and so on.
Using this scheme, the plaintext, "SECRET" encrypts as "VHFUHW." To allow someone else to read the ciphertext, you tell them that the key is 3.
Obviously, this is exceedingly weak cryptography by today's standards, but hey, it worked for Caesar, and it illustrates how conventional cryptography works.