Web Security, Privacy & Commerce, 2nd Edition

SSL/TLS Protocols

SSL and TLS protocols are specific types of messages that are sent using the record layer. The SSL v3.0/TLS standard defines three protocols:

• The Handshake protocol, which performs the initial key negotiation.

• The Alert protocol, which sends important messages about the state of the SSL/TLS connection from one side to the other.

• The ChangeCipherSpec protocol, which changes the encryption system currently in use.

• The application data protocol, which sends user data.

Handshake Protocol

The SSL Handshake protocol is used to authenticate the SSL server to the client (and optionally the client to the server) and to agree upon an initial encryption algorithm and keys. The Handshake protocol is described in the next major section.

Alert Protocol

Alerts are a specific type of message that can be transmitted by the SSL/TLS record layer. Alerts consist of two parts: an AlertLevel and an AlertDescription. Both are coded as single 8-bit numbers.

The SSL v3.0 and TLS 1.0 specifications define two alert levels.

Alert level	Level name	Meaning
1	Warning	SSL warnings indicate a problem that is not fatal.
2	Fatal	SSL fatal alerts immediately terminate the current SSL session.

SSL v3.0 and TLS 1.0 define the following alerts.

SSL alert number	TLS alert number	Alert name	Meaning
0	0	close_notify	Indicates that the sender will not send any more information. If a close_notify is sent with a warning alert level, the session may be resumed. If a close_notify is sent with a fatal alert level, the session may ...