Secretcodes(密码)keepmessagesprivate。Banks,companies,andg...

问题详情：

Secret codes (密码)keep messages private。Banks, companies, and government agencies use secret codes in doing business, especially when information is sent by computer.

   People have used secret codes for thousands of years. _31_ Code breaking never lags(落后) far behind code making. The science of creating and reading coded messages is called cryptography.

   There are three main types of cryptography. _32_ For example, the first letters of “My elephant eats too many eels” Spell out the hidden message “Meet me.”

_33_ You might represent each letter with a number, For example, 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 20 13 5.”

   A code uses symbols to replace words, phrases, or sentences. To read the message of a real code, you must have a code book. _34  For example, “bridge” might stand for “meet” and “out” might stand for “me.” The message “bridge out” would actually mean “Meet me.” _35_ However, it is also hard to keep a code book secret for long. So codes must be changed frequently.

A. It is very hard to break a code without the code book. B. In any language, some letters are used more than others. C. Only people who know the keyword can read the message. D. As long as there have been codes, people have tried to break 

them. E. You can hide a message by having the first letters of each 

word   spell it out. F. With a code book, you might write down words that would stand 

for other words. G. Another way to hide a message is to use symbols to stand for 

specific letters of the alphabet.

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