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Protocol Structure
As shown in Figure 16-1, SSL functions
between the application layer and transport layer. SSL is composed
of two layers: the SSL Record Protocol at the bottom layer and the
SSL Handshake protocol, SSL Change Cipher Spec Protocol, and SSL Alert
Protocol at the upper layer.
- SSL Record Protocol: divides upper-layer data into blocks, computes the data, adds Message Authentication Code (MAC), encrypts the data to form record blocks, and sends the record blocks to the peer. The SSL Record Protocol implements data transmission.
- SSL Handshake Protocol: used before application data is transmitted. The SSL Handshake Protocol negotiates a cipher suite including an encryption algorithm, a key exchange algorithm, and a MAC algorithm, authenticates server and client, and exchanges a key securely between the server and client.
- SSL Change Cipher Spec Protocol: used by both the client and server to send a Change Cipher Spec message to notify each other that subsequent packets will be protected using the new cipher suite and key.
- SSL Alert Protocol: reports alarms about the handshake or data transmission process to the remote end, so that the remote end can take corresponding measures. An alarm message conveys the severity of the message and description of the alarm.