Ever since the written word has been used to communicate information, people have sought ways to ensure that only the intended recipient actually understands that information. Anyone passing secret messages is likely to be using a code or cipher of some sort. The complexity of the chosen method will depend upon the degree of security required.
Probably the earliest form of mechanical device was a rod of known diameter, around which was wrapped a narrow strip of parchment, barber’s pole fashion. The message was written along the length of the rod, unwrapped, and delivered. The fragments of words and letters on the strip were meaningless until wrapped around a rod of the correct diameter.
Unlike a cipher, a code or code word contains no hidden message but merely refers the reader to a prearranged second meaning. Some large companies, diplomats, and the military use code books to instruct their operators and to keep them informed. Either real words or groups of random letters can be used, so that
A R T L E may mean ..... Abandon negotiations
B R T Y D ..... Contact head office
C S L G U ..... Competitors arriving soon.
Though rather inflexible, the code book system can be made secure if the code letters are transmitted in the form of a cipher. If words are used they must be chosen carefully so that no clue to the real meaning is even hinted at. For example, if the message is to be "You are fired," CAT would be reasonable as a code word, but GUN would not.
During World War II, coded messages were transmitted to Resistance groups in Europe. These could be broadcast on normal BBC channels for all the world to hear, including the Germans. They were even announced with words like, "And now some messages to our friends in Europe." Following this would be a coded message such as "The milkman will call in the morning." This piece of news would be read slowly and clearly and repeated in case reception was poor. To particular groups in France, it could mean that a prearranged target was to be bombed the next morning. They would then play their part by attacking antiaircraft positions in that area. The Germans suspected nothing.
Military operations from small to large were often given code names, such as Overlord and Torch. Companies researching new products code name them so that they can be discussed without giving away their true nature.
Some codes are not intended to be particularly secret, more to save time or radio space. The ten code is used by both citizens’ band radio enthusiasts and by the police, particularly in the United States. The code is simply a list of numbers following the number 10. The code 10:20 to a CB user means that the caller wants to know where you live. In practice, the question would be asked as "What’s your twenty?"
Probably the best-known code of all is the Morse code. It was developed for use with the electric telegraph well over 100 years ago. In those days before microphones had been invented, the only signal that could be sent by wire was a simple OFF or ON. Hence, the dots and dashes of Morse code.
The most commonly used letters are given short codings, E being a single dot, while the least used have more complex ones, X being dash dot dot dash. Morse code retains its popularity in radio transmission; using a very narrow band width, it can still be used when poor reception conditions preclude other forms of transmission.
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DIGITAL ENCRYPTION |
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The Data Encryption Standard (DES) of the U.S. National Bureau of Standards works by dividing text into four-bit digital types and subjecting it to 18 stages of transposition and substitution. The key generator in DES applies 16 keys (derived from a 56 bit main key) in turn to the initial permutation of the text. At each stage, the digital form message (shown symbolically in letter form) is further transposed. Without knowledge of the keys involved, it is claimed, no computer can crack the cipher generated. Users have a published algorithm, and secret paired-key generators for the main 56 bit key (actually 64 bits—eight bits are used for internal coding).
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Ciphers
Unlike a code, a cipher actually contains the secret to be transmitted. Numerical ciphers can be made that are very secure indeed. Here is a very simple example of just one method.
Suppose the message to be encoded is How It Works. First, each letter of the alphabet has a number, as follows:
By substitution, the message becomes
8 15 23 9 20 23 15 18 11 19
Next, choose ten random numbers and add them in turn to the numbers from the alphabet to get ten new numbers.
The new numbers would then be transmitted, probably in two blocks of five:
81 37 71 44 87 27 69 45 92 78
When these numbers are received, the agent refers to a copy of the random numbers. Subtracting them from the numbers received gives the real alphabet numbers and in turn the message.
In reality, these random numbers would be printed in the form of a booklet containing many tear-off pages. Both sender and agent would have identical copies. Having used one page of numbers, the sender would tear it out and destroy it, as would the agent. Each set of numbers is used only once, giving rise to the name "one time pad." When used with much more sophisticated ciphers than this, the system is virtually 100 percent secure.
