The Caesar cypher is one of the earliest known and simplest cyphers. It is a type of substitution cipher in which each letter in the plaintext is ‘shifted’ a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B would become C, and so on. The method is named after Julius Caesar, who apparently used it to communicate with his generals.
Variations of this method, often using a scrambled alphabet, are the basis for decoder wheels. Even though this is a relatively simple encryption technique, when many wheels are stacked in this manner (see image below), breaking this code becomes extremely difficult. The Caesar Cypher is the basis for the most infamous encryption technique in history, the German Enigma engine.
While serving as George Washington’s secretary of state (1790-1793), Thomas Jefferson devised an elaborate and ingenious wheel cipher consisting of thirty-six cylindrical wooden pieces, each threaded onto an iron spindle. The letters of the alphabet were inscribed on the edge of each wheel in a random order. Turning these wheels, words could be scrambled and unscrambled.
As an example, the sender of the message shown in the picture, “MEET ME AT NOON,” spells the message out and then looks to any other line of text – possibly the one directly above, which on this version of the cipher begins with the letter “S.” The sender then copies the rest of the letters from that line into the correspondence to spell out “SPVZ 4A FZ ERDP.”
The recipient of the coded message would spell out these random-seeming letters on his own identical cipher and then begin looking for the one line that made sense. In this case, the line below.
Although Jefferson abandoned this design in after 1802, it was independently “re-invented” in the early 20th century. Designated as M-94, it was used by the Army and other military services from 1922 to the beginning of World War II. A short time later, Jefferson’s design was found among his papers.