A simple cipher substitutes one letter for another, one-to-one generally. Technically ‘simple ciphers’ covers three common setups, but as they are fundamentally the same, I’m including both.
The first is “ROT-13 encoding,” which despite it’s name isn’t really an encoding. ROT-13 substitutes each letter with the letter 13 away (half an alphabet), which eliminates the need to define a direction in the shift. A becomes M, M becomes A. D becomes P, P becomes D.
Second is a “Rotation Cipher,” more commonly known as a “Caesar Cipher,” which is actually a more general version of ROT-13. A Rotation Cipher would generally be named with a direction and a number. ROT Right 5, or ROT-L-9. ROT-13 is a special case because L and R would be identical (26 letters, 13 is halfway around, either direction). Rotation Ciphers are frequently put on “secret decoder rings.”
The third is the generic case of “substitution cipher.” In this instance, A may be mapped to X and then B to C, there is no relation between mappings. This can either be a 13-letter table (where, then, X would map to A and C to B) or a full 26 (C may map to A, and X would become I).
In terms of data security, none of these is any good. These are more useful for either pre-coding a message for actual encoding or encryption, or for passing insecure data. There are ways to complicate these techniques (which I will work up separately), which make them more secure.
The basic use for a Simple Cipher is to make data require some level of work to read (to slow someone down), and to make for ready manual processing. With the “decoder ring”, anyone can easily tick through an arbitrarily long message with a consistent time per letter, and a consistent process.
(This is a post I’m migrating from the first edition of this blog)