Answer: Shift Cipher

Today, we take extremely sophisticated encryption for granted. Millennia ago, however, encryption was a decidedly simpler affair.

One of the earliest and best known examples of encryption is the shift cipher, used by Julius Caesar, famed Roman statesman and general. In fact, the shift cipher is so closely associated with Caesar that it is often referred to as the “Caesar Cipher”. He used this cipher to send sensitive military messages.

As the name “shift cipher” implies, the cipher was simply a transposition of letters with their neighbors a certain number of spaces away. Imagine, if you will, that you wrote out all the letters of the alphabet on two strips of paper and then lined them up so that A was above A, B was above B, and so forth. This would be a zero shift cipher, because there would be no change in the cipher from the regular use of the alphabet. Now imagine that you slid the lower strip of paper three letters to the right out of alignment with the top strip (cutting off the floating letters at the end and bringing them to the front). A becomes X, B becomes Y, C becomes Z, and D, having picked up the first letter of the alphabet due to the three space shift, is now A.

It’s worth noting that this method is not the same as other ciphers (where every letter may be reassigned to a new, relatively random letter and A might be Z, but B might be V). A shift cipher is always applied like a slide rule, wherein the letters are simply realigned in their same order. In this way, the recipient doesn’t need to possess a decryption table with your substitution cipher on it, they simply need to know what the “shift number” is.

Now, by modern standards, that level of encryption is laughable and could be cracked by a child with even the most passing familiarity with secret decoder rings or a Boy Scout manual. But when encryption was in its absolute infancy, Caesar’s cryptographic trick was more than sufficient to secure his communications. In fact, such simple ciphers remained in use until the advent of sophisticated mechanical cipher machines in the 20th century—in the photo shown here, we see a cipher disc used by the Confederate army in the mid-19th century using the same shift cipher employed by Julius Caesar almost two millennia earlier.