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
and slidng everything up by 3, you get
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.