This is a JavaScript 1.2 implementation of a letter-pair replacement solver, also known as a substitution cipher. This is the kind of puzzle where A is N, B is O, C is P, etc. It can also be called a cryptoquip or a cryptogram in the local newspaper.

To use it, you just define a "key" and the letters in the upper part will be translated and shown in the lower part. This is only a tool to help you out – it can not automatically solve the puzzle for you.

In a related vein, you can check out the ROT13 encoder which swaps letters from the first half of the alphabet with the second half (and vice versa), or the Caesar cipher where you essentialy "slide" the alphabet over to the left or right.

Result:

Keyed Caesar alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789
Or a decode alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789