الأنظمة الشفرية السرية

Secret Codes
Secret codes keep messages private. Codes are very useful for sending secret military information during wartime. Banks, companies, and government agencies also use secret codes in doing business, especially when information is sent by computer.
People have used secret codes for thousands of years. As long as there have been codes, people have tried to break them. Code breaking never lags far behind code making.
HIDDEN WRITING
The science of creating and reading coded messages is called cryptography (pronounced krip-TOG-ruh-fee). The word means “hidden writing.”
There are three main types of hidden writing. They are concealed messages, ciphers, and codes. However, all three are commonly called codes.
CONCEALED MESSAGES
You can conceal (hide) a message by having the first letters of each word spell it out. For example, the first letters of “My elephant eats too many eels,” spell out the hidden message “Meet me.”
Concealed messages, like this one, are easy to break. Once the secret is out, anyone can read them.
CODES CALLED CIPHERS
Another way to hide a message is to use symbols to stand for specific letters of the alphabet. You might represent each letter with a number, for example. Although we call this a code, experts call it a cipher.
Let’s number the letters of the alphabet, in order, from 1 to 26. If we substitute a number for each letter, the message “Meet me” would read “13 5 5 20 13 5.” This is called a substitution cipher.
In a transposition cipher, the letters in a message are rearranged in some way. We could, for example, reverse each pair of letters in “Meet me.” Then our message would read “Emte em.”
CODES AND CODEBOOKS
A code uses symbols to replace words, phrases, or sentences. To read the message of a real code, you must have a code book. However, to work out the cipher above, you don’t need a code book. All you need to know is the system used for substitution or rearrangement.
With a code book, you might write down words that would stand for other words. For example, “bridge” might stand for meet and “out” might stand for me. The message “Bridge out” would actually mean “Meet me.”
It is very hard to break a code without the code book. However, it is also hard to keep a code book secret for long. So codes must be changed frequently.
CODE BREAKING
Ciphers are easy to use and remember. A simple cipher, however, is also easy to break. In any language, some letters are used more than others. In English, e is the most common letter, followed by t and a. Some letters frequently occur together, such as th and ch. By studying several messages, a code breaker can easily figure out a simple cipher.
Complex ciphers use a different cipher, or code, for each letter in a message. The letters in a keyword reveal which cipher is used for each letter. Only people who know the keyword can read the message. Codes can be almost unbreakable if they use combinations of ciphers and change the keyword often.
During World War II (1939-1945), math experts in Britain broke the German military code. Britain was at war with Germany. German military forces coded messages with a device called an Enigma machine. Enigma used gears and wheels to create millions of different ciphers. But British mathematicians and code breakers figured out the cipher and uncovered German military plans.
COMPUTERS AND CODES
Computers can create ciphers many thousands of times more complex than the Enigma machine could. Computer scientists develop complicated formulas for coding messages. They use keywords that are long strings of symbols. The computer’s speed and power creates messages that are very hard to break. But powerful computers can be used to break codes, too. Any code that can be made can also be broken.