Permutation ciphers are cool transposition ciphers that use a code word to shuffle letters in a message.
The key to a Permutation cipher is a code word.
To encrypt a message, write your code word at the top of a grid. Beneath the codeword, write your message from left to right, moving down a column when you reach the end of a row.
For example, with code word TOMATO and message "WATCH OUT" (I've written the spaces as underscores below, so it's easier to see what's going on):
When you have written your entire message out beneath the codeword organise your columns so that your code word at the top is now in alphabetical order.
Finally, read out your cipher text left to right. Don't forget to include the spaces:
To decrypt a message, write the code word at the top of a grid but in alphabetical order (e.g. don't write
AMOOTT. Now write the ciphertext out underneath the code word, left-to-right.
Then re-arrange the columns so that the code word is in the correct order. Your ciphertext should now be back to readable english!
Making Your Own Key
This is really easy. Simply pick a new code word. A good code word is hard to guess and is a good size. A good size means that it's not so small that it's easy to see groups of letters together (e.g. ABC) or so large that your columns are really short (e.g. SUPERCALIFRAGILISTICEXPIALIDOCIOUS)
Here's a key that you can edit. When you change the key the example plaintext will be encrypted for you. Can you see how it all works?