Netscape NETSCAPE CONSOLE 6.0 - MANAGING SERVERS, Руководство пользователя

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The SSL and TLS Protocols
180 Managing Servers with Netscape Console • December 2001
At a minimum, SSL and TLS require a server certificate. As part of the initial
“handshake” process, the server authenticates its identity by presenting this server
certificate to the client. Using public-key encryption and digital signatures, the
client confirms that the server is, in fact, the server it claims to be. If desired, the
server can also request that the client authenticate its identity by presenting a client
certificate.
If authentication is successful, the client and server use techniques of
symmetric-key encryption to encode all the information they exchange for the
remainder of the session. Symmetric-key encryption also allows the client and
server to detect if any tampering has occurred during the transmission of data.
SSL and TLS Ciphers
The SSL and TLS protocols support a variety of different cryptographic algorithms
for use in operations such as authenticating the server and client to each other,
transmitting certificates, and establishing session keys. These algorithms are called
ciphers and are often implemented in sets called cipher suites . Clients and servers
may support different cipher suites depending on factors such as the version of SSL
or TLS they use, and company policies regarding acceptable encryption strength.
Among their other functions, the SSL and TLS protocols determine how servers
and clients negotiate which cipher suites they use to communicate.
Each new version of SSL and TLS maintains backward compatibility with earlier
versions. As a result, the SSL 2.0, SSL 3.0, and TLS protocols have several cipher
suites in common. This allows a newer client or server to communicate securely
with an older client or server. To control the level of encryption used during
communication, Administrators can enable or disable cipher suites on both clients
and servers. When a particular client and server exchange information during the
SSL or TLS handshake, they identify the strongest enabled cipher suites they have
in common and use those for the session.
Choosing SSL and TLS Ciphers
Decisions about which cipher suites an organization enables are often based on
both the sensitivity of the data involved and the speed of the cipher. A 40-bit cipher
is relatively easy to break, but very fast. A 128-bit cipher is difficult to break, but
slower than other ciphers.
Some organizations may want to disable less secure ciphers to prevent
insufficiently encrypted SSL connections. To serve the greatest number of users, it’s
a good idea for administrators to enable as broad a range of SSL cipher suites as
possible. That way, when clients or servers are dealing with each other, they can
negotiate the use of the strongest ciphers available.