How to Encrypt a Key Cipher
Choose a word to be your KEY word. Make sure the word you select has at least five or six letters in it and has no repeating letters. For example the word APPLE will not work with this cipher because there are two P's in the word. In our example, we will use the word MAPLE for our key.
Write the Key word on a piece of paper. Leave a little bit of space between each letter as you write down each letter of your Key.
Next, draw a column under each letter of the Key word.
Fill in the columns by writing your PLAINTEXT message one letter at a time.
Any empty spaces should be filled in with a NULL. A NULL is usually represented by the letter X.
Rewrite the columns so that the letters in your Key word are in alphabetical order.
In our example, the word MAPLE will be rearranged to read AELMP.
To see your encrypted message, copy the letters in each column from top to bottom, but DO NOT include the letters in the Key word.
CONGRATULATIONS! You have successfully created a KEY CIPHER!
How to Decrypt a Key Cipher
Find out what the KEY word is from the person who sent you the message. Write the letters from the Key word in alphabetical order on a piece of paper.
Draw a column under each letter of the Key word.
Starting under the letter A and writing from left to right, fill in the grid with the letters from the ciphertext.
Once the grid is filled in, rearrange the columns so the Key word is spelled out correctly. Reading from left to right, you should be able to see the plaintext message.
CONGRATULATIONS! You have successfully deciphered a KEY CIPHER!