The Imitation Game [DVD]

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A clever way to hide a secret message is in plain sight. One way to do this is to use a Cardano Grille -- a piece of paper or cardboard with holes cut out of it. To cipher a message, you lay a grille on a blank sheet of paper and write out your message through the grille's holes. You fill the rest of the paper with innocent text. When your recipient receives the message, he lays an identical grille over it to see the secret text. This is a form of steganography, hiding a message within something else. 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]. One of the earliest cipher devices known is the Alberti Disc, invented by Leon Battista Alberti, in the 15th century. The device consisted of two discs, the inner one containing a scrambled alphabet and the outer one a second, truncated alphabet and the numbers 1 to 4. The outer disc rotated to match up different letters with the inner circle, which letters the cryptographer used as plaintext. The outer disc's letters then served as the cipher text. 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]. 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.

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Perhaps the most famous ciphering device was Germany's Enigma Machine from the early 20th century. The Enigma Machine resembled a typewriter, but instead of letter keys it had a series of lights with a letter stamped on each. Pressing a key caused an electric current to run through a complex system of wires and gears, resulting in a ciphered letter illuminating. For instance, you might press the key for the letter "A" and see "T" light up. This closeup shows some writings from the notebook of Enigma codebreaker Alan Turing, who played a major part in breaking codes during the Second World War. Chris Radburn/PA Images via Getty Images

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 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].

Both of these systems, the Polybius Square and the Caesar Shift, formed the basis of many future cipher systems. 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. The Germans, convinced their Enigma messages were unbreakable, used the machine for battlefield, naval, and diplomatic communications. Although the experts at Bletchley first succeeded in reading German code during the 1940 Norwegian campaign, their work only began to pay off meaningfully in 1941, when they were able to gather evidence of the planned invasion of Greece, and learn Italian naval plans for the Battle of Cape Matapan. In the autumn, the Allies gained advantage in North Africa from deciphering coded messages used by Rommel's Panzer Army. Information obtained from such high-level German sources was codenamed ULTRA. To decipher, the recipient would first look at the first letter of the encrypted message, a "K" in this case, and use the Trimethius table to find where the "K" fell in the "D" row -- remember, both the cryptographer and recipient know beforehand that the first letter of the key will always be "D," no matter what the rest of the message says. The letter at the top of that column is "H." The "H" becomes the next letter in the cipher's key, so the recipient would look at the "H" row next and find the next letter in the cipher -- a "V" in this case. That would give the recipient an "O." Following this method, the recipient can decipher the entire message, though it takes some time.

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. Cryptanalysts take advantage of any opportunity to solve a cipher. If the cryptographer used a ciphering device, a savvy cryptanalyst will try to get the same device or make one based on his theories of the cryptographer's methodology. During World War II, Polish cryptanalysts obtained an Enigma Machine and were close to figuring out Germany's ciphering system when it became too dangerous to continue. The Polish exchanged their information and technology with the Allies, who created their own Enigma Machines and deciphered many of Germany's coded messages. Another important skill to have is a strong familiarity with the language in which the plaintext is written. Trying to solve a coded message written in an unfamiliar language is almost impossible. 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. 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.Breaking the code carved into the ceiling of the Rosslyn Chapel inScotland reveals a series of musicalpassages. Modern high-level encryption methods rely on mathematical processes that are relatively simple to create, but extremely difficult to decipher. Public-key encryption is a good example. It uses two keys -- one for encoding a message and another for decoding. The encoding key is the public key, available to whomever wants to communicate with the holder of the secret key. The secret key decodes messages encrypted by the public key and vice versa. For more information on public-key encryption, see How Encryption Works. Notice that the cipher alphabet wraps around to "A" after reaching "Z." Using this cipher system, you could encipher the phrase "How Stuff Works" as "KRZ VWXII ZRUNV."

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. 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. In the late 1500s, Blaise de Vigenère proposed a polyalphabetic system that is particularly difficult to decipher. His method used a combination of the Trimethius tableau and a key. The key determined which of the alphabets in the table the decipherer should use, but wasn't necessarily part of the actual message. Let's look at the Trimethius tableau again:

One such scholar was Johannes Trimethius, who proposed laying out the alphabet in a matrix, or tableau. The matrix was 26 rows long and 26 columns wide. The first row contained the alphabet as it is normally written. The next row used a Caesar Shift to move the alphabet over one space. Each row shifted the alphabet another spot so that the final row began with "Z" and ended in "Y." You could read the alphabet normally by looking across the first row or down the first column. It looks like this: