Netscape Certificate Management System 6.2, Руководство администратора

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Encryption and Decryption
800 Netscape Certificate Manager System Administrator’s Guide • June 2003
Symmetric-Key Encryption
With symmetric-key encryption, the encryption key can be calculated from the
decryption key and vice versa. With most symmetric algorithms, the same key is
used for both encryption and decryption, as shown in Figure J-1.
Figure J-1 Symmetric-Key Encryption
Implementations of symmetric-key encryption can be highly efficient, so that users
do not experience any significant time delay as a result of the encryption and
decryption. Symmetric-key encryption also provides a degree of authentication,
since information encrypted with one symmetric key cannot be decrypted with any
other symmetric key. Thus, as long as the symmetric key is kept secret by the two
parties using it to encrypt communications, each party can be sure that it is
communicating with the other as long as the decrypted messages continue to make
sense.
Symmetric-key encryption is effective only if the symmetric key is kept secret by
the two parties involved. If anyone else discovers the key, it affects both
confidentiality and authentication. A person with an unauthorized symmetric key
not only can decrypt messages sent with that key, but can encrypt new messages
and send them as if they came from one of the two parties who were originally
using the key.
Symmetric-key encryption plays an important role in the SSL protocol, which is
widely used for authentication, tamper detection, and encryption over TCP/IP
networks. SSL also uses techniques of public-key encryption, which is described in
the next section.