Trimethius' tableau is a good example of a polyalphabetic cipher. Most early ciphers were monoalphabetic, meaning that one cipher alphabet replaced the plaintext alphabet. A polyalphabetic cipher uses multiple alphabets to replace the plaintext. Although the same letters are used in each row, the letters of that row have a different meaning. A cryptographer enciphers a plaintext "A" in row three as a "C," but an "A" in row 23 is a "W." Trimethius' system therefore uses 26 alphabets -- one for each letter in the normal alphabet.

A person who communicates through secret writing is called a cryptographer. Cryptographers might use codes, ciphers or a combination of both to keep messages safe from others. What cryptographers create, cryptanalysts attempt to unravel. Navajo Code TalkersDuring World War II, the United States employed Navajo Native Americans to encode messages. The Navajos used a code system based on how their language translated into English. They assigned terms like "airplane" to code words such as "Da-he-tih-hi," which means "Hummingbird." To encipher words that didn't have a corresponding code word, they used an encoded alphabet. This encoded alphabet used Navajo translations of English words to represent letters; for instance, the Navajo word "wol-la-chee" meant "ant," so "wol-la-chee" could stand for the letter "a." Some letters were represented by multiple Navajo words. The Navajo language was so foreign to the Japanese, they never broke the code [source: Kahn]. In 1990, Jim Sanborn created a sculpture called Kryptos for the CIA headquarters in Langley, Va. Kryptos contains four enciphered messages, but cryptanalysts have solved only three. The final message has very few characters (either 97 or 98, depending on whether one character truly belongs to the fourth message), making it very difficult to analyze. Several people and organizations have boasted about solving the other three messages, including the CIA and the NSA.The Enigma story began in the 1920s, when the German military - using an 'Enigma' machine developed for the business market – began to communicate in unintelligible coded messages. The Enigma machine enabled its operator to type a message, then 'scramble' it using a letter substitution system, generated by variable rotors and an electric circuit. To decode the message, the recipient needed to know the exact settings of the wheels. German code experts added new plugs, circuits and features to the machine during the pre-war years, but its basic principle remained the same. Redundancy means that every language contains more characters or words than are actually needed to convey information. The rules of the English language create redundancy -- for example, no English word will begin with the letters "ng." English also relies heavily on a small number of words. Words like "the,""of,""and,""to,""a,""in,""that,""it,""is," and "I" account for more than one quarter of the text of an average message written in English [source: Kahn]. Information is an important commodity. Nations, corporations and individuals protect secret information with encryption, using a variety of methods ranging from substituting one letter for another to using a complex algorithm to encrypt a message. On the other side of the information equation are people who use a combination of logic and intuition to uncover secret information. These people are cryptanalysts, also known as code breakers.

The more complex Vigenère system didn't catch on until the 1800s, but it's still used in modern cipher machines [source: Kahn].While there are hundreds of different codes and cipher systems in the world, there are some universal traits and techniques cryptanalysts use to solve them. Patience and perseverance are two of the most important qualities in a cryptanalyst. Solving a cipher can take a lot of time, sometimes requiring you to retrace your steps or start over. It is tempting to give up when you are faced with a particuarly challenging cipher. The complex algorithms cryptographers use ensure secrecy for now. That will change ifquantum computingbecomes a reality. Quantum computers could find the factors of a large number much faster than a classic computer. If engineers build a reliable quantum computer, practically every encrypted message on the Internet will be vulnerable. To learn more about how cryptographers plan to deal with problem, read How Quantum Encryption Works. After the fall of the Roman Empire, the Western world entered what we now call the Dark Ages. During this time, scholarship declined and cryptography suffered the same fate. It wasn't until the Renaissance that cryptography again became popular. The Renaissance was not only a period of intense creativity and learning, but also of intrigue, politics, warfare and deception. In the 19th century, Thomas Jefferson proposed a new ciphering machine. It was a cylinder of discs mounted on a spindle. On the edge of each disc were the letters of the alphabet, arranged in random sequence. A cryptographer could align the discs to spell out a short message across the cylinder. He would then look at another row across the cylinder, which would appear to be gibberish, and send that to the recipient. The recipient would use an identical cylinder to spell out the series of nonsense letters, then scan the rest of the cylinder, looking for a message spelled out in English. In 1922, the United States Army adopted a device very similar to Jefferson's; other branches of the military soon followed suit [source: Kahn]. Breaking the code carved into the ceiling of the Rosslyn Chapel inScotland reveals a series of musicalpassages.