We humans have always enjoyed our secrets. Some of us purchase specially-tinted screens to hide what we do on our phone or in our car. Our voices hush to whispers when we talk about our personal affairs. There are even diaries with special locks to conceal the stories of our daily lives. Perhaps the most secretive technique we humans have come up with is cryptography.

Cryptography is the practice of masking a message into a garbled mess that can only be deciphered with some additional information that ideally only the recipient knows. Our computers and phones encrypt all of our messages so that even if a hacker intercepts it, it would be useless. Encryption is not a product of the modern world. It has been around for at least 3000 years.

The ancient Greeks and Romans used simple ciphers to hide their messages. The most famous is the Caesar cipher, an encryption technique Julius Caesar used to hide his messages. A Caesar cipher shifts the letters of a message back by a constant amount. The word “Julius” becomes “Mxolxv” with a key shift of 3. 3 letters back from M is J, 3 letters back from x is u, etc.

This type of cipher is easy enough to figure out given enough time. If a message is long enough, code breakers can analyze the letter frequency to decrypt the Caesar cipher. If a “t” appears as frequently as the letter “e” does in the English language, then “e” was probably substituted with “t.” A more sophisticated cipher was needed. The Vigenère cipher was the solution.

In this cipher, letters do not always match one to one with another letter. The letter “e” could be replaced by “d” or “z” depending on its position. To account for position, a key word and a table of Caesar ciphers is used.

Suppose you want to encrypt the message “defend.” We will use the word “fly” as the key word. To start, we must “expand” the key word to equal the number of letters in the message. To do this, we simply repeat the letters of the word “fly” in order until we have the same number of letters in the word “defend.”

Message: DEFEND

Key: FLYFLY

In the following table, you can look up the letter of your encrypted text by finding the intersection of the row with the key letter and column with the message letter (or vice versa). We can see that “d” from the message and “f” from the key intersect at “i,” so this is the first letter of our encrypted message. We continue doing this for the remaining five letters. When we do this, our encrypted message is “ipdjyb.”

Notice how this method of encryption allows the letter “e” to be encrypted as both “p” and “j.” To decrypt the message, we simply do the reverse of encryption

Encrypted Message: IPDJYB

Key: FLYFLY

We look for the letter in row that contains the first letter of the expanded key: “f.” We continue along the row until we run into the first letter of the encrypted message: “i.” Finally, we go upward to the letter in the column, which is our decrypted letter “d.” We continue this for the remaining 5 letters.

This is quite tedious, so use a computer to make this process faster. Follow this link to encrypt and decrypt Vigenère ciphers without all the laborious work of referencing the table. Even though all Vigenère ciphers are encrypted and decrypted using the same table, the recipient must know the key word in order to decipher the message. Computers do cheapen this cipher because they have the capacity to try every word in the English language as a key until they decrypt something resembling a human message. Encryption techniques have since become far more complex in the war between code makers and code breakers. The turf of this war is now all on cyberspace, but using the old manual forms on encryption may still have a use: passing notes in class that a teacher cannot read.