Allied Telesis AT-S60 - Manual (Page 383)

Chapter 22: Secure Sockets Layer (SSL)
Section III: Security Features 382
All application data messages are authenticated by SSL with a message
authentication code (MAC). The MAC is a checksum that is created by the
sender and is sent as part of the encrypted message. The recipient re-
calculates the MAC, and if the values match, the sender’s identity is
verified. The MAC also ensures that the message has not been tampered
with by a third party because any change to the message changes the
MAC.
SSL uses asymmetrical (Public Key) encryption to establish a connection
between client and server, and symmetrical (Secret Key) encryption for
the data transfer phase. For more information about public keys and
encryption, see Chapter 22, Public Key Infrastructure (PKI) on page
357 and Chapter 20, Encryption on page 340.
User
Verification
An SSL connection has two phases: handshake and data transfer. The
handshake initiates the SSL session, during which data is securely
transmitted between a client and server. During the handshake, the
following occurs:
1. The client and server establish the SSL version they are to use.
2. The client and server negotiate the cipher suite for the session, which
includes encryption, authentication, and key exchange algorithms.
3. The symmetrical key is exchanged.
4. The client authenticates the server (optionally, the server
authenticates the client).
SSL messages are encapsulated by the Record Layer before being passed
to TCP for transmission. Four types of SSL messages exist, they are:
❑ Handshake
❑ Change Cipher Spec
❑ Alert
❑ Application data (HTTP, FTP or NNTP)
As discussed previously, the Handshake message initiates the SSL
session.
The Change Cipher Spec message informs the receiving party that all
subsequent messages are encrypted using previously negotiated
security options. The parties use the strongest cryptographic systems
that they both support.