We use the terms secret code and cipher interchangeably to mean any method of encrypting data. Some people draw a subtle distinction between these terms that we don’t find useful. The earliest documented cipher is attributed to Julius Caesar. The way the Caesar cipher would work if the message were in English is as follows. Substitute for each letter of the message, the letter which is 3 letters later in the alphabet (and wrap around to A from Z). Thus an A would become a D, and so forth. For instance, DOZEN would become GRCHQ. Once you figure out what’s going on, it is very easy to read messages encrypted this way (unless, of course, the original message was in Greek). A slight enhancement to the Caesar cipher was distributed as a premium with Oval tine in the 1940s as Captain Midnight Secret Decoder rings. (Where this done today, Oval tine would probably be in violation of export controls for distributing cryptographic hardware!) The variant is to pick a secret number n between 1 and 25, instead of always using 3. Substitute for each letter of the message, the letter which is n higher (and wrap around to A from Z of course). Thus if the secret number was 1, an A would become a B, and so forth. For instance HAL would become IBM. If the secret number was 25, then IBM would become HAL. Regardless of the value of n, since there are only 26 possible ns to try, it is still very easy to break this cipher if you know it’s being used and you can recognize a message once it’s decrypted. The next type of cryptographic system developed is known as a monoalphabetic cipher, which consists of an arbitrary mapping of one letter to another letter. There are 26! possible pairings of letters, which is approximately 4×1026. [Remember, n!, which reads “n factorial”, means n(n−1)(n−2)⋅⋅⋅1.] This might seem secure, because to try all possibilities, if it took 1 microsecond to try each one, would take about 10 trillion years. However, by statistical analysis of language (knowing that certain letters and letter combinations are more common than others), it turns out to be fairly easy to break. For instance, many daily newspapers have a daily cryptogram, which is a monoalphabetic cipher, and can be broken by people who enjoy that sort of thing during their subway ride to work. An example is Cflqr’xsxsnyctm n eqxxqgsyiqulqfwdcpeqqh, erllqrxqgtiqul!