Английский язык: Аннотирование и реферирование текста

217 Since it is impractical to keep secret the algorithms that are used to encrypt and decrypt data, these algorithms are designed so that their operation depends on a certain data item called the key. It is the key that is kept secret. Even if you know all the details of the encrypting and decrypting algorithms, you cannot decrypt any messages unless you know the key that was used when they were encrypted. For instance, the National Bureau of Standards has adopted an algorithm for encrypting and decrypting the data processed by federal agencies. The details of the algorithm have been published in the Fed- eral Register 2 . Plans are under way to incorporate the algorithm in spe- cial purpose microprocessors, which anyone can purchase and install in his computer. So the algorithm is available to anyone who bothers to look it up or buy one of the special purpose microprocessors. But the operation of the algorithm is governed by a sixty-four-bit key. Since there are about 1022 possible sixty-four-bit keys, no one is likely to discover the cor- rect one by chance. And, without the correct key, knowing the algo- rithm is useless. A recent important development involves what are called public- key cryptosystems. In a public-key cryptosystem, each person using the system has two keys, a public key and a private key. Each person's public key is published in a directory for all to see; each person's private key is kept secret. Messages encrypted with a person's public key can be decrypted with that person's (but no one else's) private key. Messages encrypted with a person's private key can be decrypted with that person's (but no one else's) public key. Protection through Software . The software of a computer sys- tem, particularly the operating system, can be designed to prevent unau- thorized access to the files stored on the system. The protection scheme uses a special table called a security matrix.