What’s A Cipher?

“Cipher” really just means “encryption technique”. A cipher is a set of steps for encrypting information.

The first example they gave us in my Cryptography class was the “Caesar Cipher”, which got its name from the fact that Julius Caesar used it to encrypt his communications.

This text has been encrypted with the Caesar Cipher:

PHHW PH DW FKLSRWOH DW QRRQ

In cryptography terms, the encrypted message is called “ciphertext”.

Here’s the text before it was encrypted (a.k.a. “plaintext”):

MEET ME AT CHIPOTLE AT NOON

Do you see what the transformation was?

The encryption is performed by shifting each letter in the plaintext three places down the alphabet. I take the M and convert it: M -> N -> O -> P. I take the E and convert it: E -> F -> G -> H. And so on.

If I hit the end of the alphabet, I wrap back around to the beginning. So if I were to transform a Y, the conversion would look like this: Y -> Z -> A -> B.

I could pick any number of spaces to shift my plaintext characters into ciphertext characters. Here I picked 3. That’s my “key”. The rules for a Caesar Cipher are the same regardless of the key I pick, but the output is different depending on the key. If someone knows the key, they can easily decrypt the ciphertext, shifting each letter by 3 in the other direction.

The state of information security in 50 BC was not so hot. At least by current standards. The Caesar Cipher is about as simple a way of encrypting information as one could imagine. It’s easy to encrypt your information with it, but super easy to crack, even if you don’t know the key. At most, you have to try 26 keys before you’re successful.

Modern encryption is much more sophisticated, of course. But we have to start somewhere.