The Internet Encyclopedia (Volume 3)

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268 SECURESOCKETSLAYER(SSL)

Figure 8: SSL connection process.

to ensure message integrity. The third requirement, au-

thentication, is provided by the handshake protocol in its

requirement of at least a server’s certificate.

The record protocol processes a message by first

breaking the message into fragments of equal fixed

size, padding the last fragment as needed. The next

step is optional compression of each fragment. Once

the compression is completed, a MAC is computed for

each fragment and appended to the fragment. The result

is then encrypted using the key and algorithm agreed

upon by the client and server. An SSL record header

is appended. Then this segment is passed to the TCP

layer for processing. The received data are processed by

the receiving protocol in the reverse process: data are

decrypted, verified by means of the MAC, and decom-

pressed if necessary, the fragments are reassembled, and

the result is then passed on to the destination application.

This process is depicted in Figure 8.

TLS—Transport Layer Security

TLS is an IETF attempt to specify an Internet standard

version for SSL. The current proposed standard for TLS

is defined in RFC 2246 (2002).

The proposed TLS standard is very similar to SSLv3.

The TLS record format is identical to the SSL record for-

mat. There are a few differences between SSL and TLS.

Some of these are how MAC computations are carried out,

how pseudorandom functions are used, including addi-

tional alert codes and client certificate types, and how cer-

tificateverification and finished message are carried out.

The details of these differences are discussed in Stallings

(2000).

SSL and TLS Protocols: Details

The preceding sections provide an overview of how a se-

cure channel is set up and used. A better understanding of

this process is obtained when a detailed examination of

this process is presented. It is informative to work through

each step of Figure 7 and detail how the protocols work to

set up the secure channel. The following is an adaptation

of information that may be found in specification docu-

ments for SSL (Netscape Communications, 1996, 1998).

Handshake Protocol

Of the four protocol that make up SSL and TLS, the hand-

shake protocol is the most critical. This protocol is respon-

sible for setting up the connection. It uses a sequence of

messages that allows the client and server to authenti-

cate each other and agree upon encryption and MAC

algorithms and their associated keys.

The format of the handshake protocol is simple and is

depicted in Figure 9 below. The type field of the handshake

protocol indicates one of 10 messages listed in Table 1 be-

low. Length is the length of the message in bytes. Content

is the parameters associated with the message type (cf.

Table 1).

Step 1of Figure 7 is theClientHellomessage. Its pa-

rameters are

versionThe version of the SSL protocol by which the

client wishes to communicate during this session. This

should be the most recent version supported by the

client.

randomA client-generated random structure. This is a

value 32 bytes long. The first four bytes are the time

Figure 9: Handshake protocol layout.