Cryptanalysis of Caesar’s Cipher

Caesar’s cipher is a little bit confusing if you’ve never seen it, but the confusion
is easy to overcome. To cryptanalyze a message using this cipher, you simply put
the encrypted text on the top row and repeatedly increase each letter one alpha-
betic position (see Figure 2-5). For example, you replace I with J, then K, then
L, and so on. Most trials produce gibberish. When you are successful, the En-
glish plaintext jumps out upon inspection.
It’s important to understand that Caesar’s cipher has only 25 possible keys.
Next, you’ll see other methods with many more possible keys.
Throughout this book the “key” concept is the same. Like Caesar and his
generals, you and your confidant share a key. The object is to make your adver-
sary work a long time trying many, many keys. Hopefully, by the time an

Figure 2-5 Caesar’s cipher cryptanalysis: Meaningful text appears after 23 tries.


adversary finds the correct decrypting key, the encrypted message has little
value. At 5:01, the value of a FIVE AM attack message is greatly reduced. We’ll
see cryptographic methods that put your adversary to work for thousands of
years, just in case unforeseen technological advances permit your adversary to
do in hours what used to take years.