Month: March 2009

Since the dawn of human civilization, people have needed to communicate secretly, because as long as there have been generals, diplomats, revolutionaries, and businessmen, there have been other generals, diplomats, counter-revolutionaries and businessmen who wanted to know their secrets.

In humanity’s early days, people needing to communicate secretly often relied on trusted messengers to relay information. The problem with this approach is obvious: a messenger can get lost or killed, he can be bribed, he can be tortured to reveal the message, or he can be searched for any messages hidden on his person. Sometimes using a trusted messenger isn’t even very practical – Herodotus, an ancient Greek historian, wrote about a ruler who shaved a slave’s head, tattooed a message on his now bald head, then waited for the slave’s hair to grow back before sending him on his mission. One wonders how “pressing” a secret message is if one has to wait two months before it can be sent.

The ancients certainly had other methods of sending secret messages. The Spartans used a long strip of fabric called a scytale to send secret messages: the strip was wrapped around a wooden rod, and a message was written on it. The person receiving the message would simply wrap the strip around another rod of identical diameter, and the message could be read. Transposition ciphers, in which letters are shifted by a certain amount, were also popular. Julius Caesar is thought to be the father of the Caesar Cipher, in which all the letters of the alphabet are shifted by three letters; thus, A becomes C, B becomes D, and C becomes E, and so on.

Transposition ciphers were easily the most popular type of cipher in the ancient world… but it all came crashing down in the 9th century, when Arab mathematician Al-Kindi discovered the science of frequency analysis. Simply put, frequency analysis looks at how often letters appear in a language, and applies those percentages to a ciphered text. For instance, the most common letter in English is the letter e. If the most common letter in an encrypted English language message is x, chances are good that x represents e. You can then move on to the next  most common letter (in English, this is a) and apply it to the second most common letter in the encrypted message. Continue down the line and you’ll eventually decode the message. This can be simplified further by the use of cribs, which are “cheats” that code breakers can use. For example, if you’re trying to crack a message sent by a general to one of his subordinates, you can safely assume that certain words – like “troops”, “attack” or “movement” – are contained in the letter, and you can look for them in the text. This can cut out a lot of the keyspace (the possible combinations) you have to search to decrypt a message.

Continue reading “Thomas Jefferson: Spy”