A Caesar shift cipher is a monoalphabetic substitution cipher where the cipher alphabet is merely a shifting of the plain alphabet.

An example of an english cipher alphabet with a Caesar shift of 1:

```Plain    a b c d e f g h i j k l m n o p q r s t u v w x y z
Cipher   B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
```

This type of cipher is named after Julius Caesar because the first documented use of it (or any substitution cipher) for military purposes was by him.

ROT13 is a type of Caesar shift cipher with a Caesar shift of 13.

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.