Appendix A WiMAX Security
User’s Guide 259
the network. In addition to the shared key, password information exchanged is
also encrypted to protect the network from unauthorized access.

Diameter

Diameter (RFC 3588) is a type of AAA server that provides several improvements
over RADIUS in efficiency, security, and support for roaming.
Security Association
The set of information about user authentication and data encryption between two
computers is known as a security association (SA). In a WiMAX network, the
process of security association has three stages.
• Authorization request and reply
The MS/SS presents its public certificate to the base station. The base station
verifies the certificate and sends an authentication key (AK) to the MS/SS.
• Key request and reply
The MS/SS requests a transport encryption key (TEK) which the base station
generates and encrypts using the authentication key.
• Encrypted traffic
The MS/SS decrypts the TEK (using the authentication key). Both stations can
now securely encrypt and decrypt the data flow.

CCMP

All traffic in a WiMAX network is encrypted using CCMP (Counter Mode with Cipher
Block Chaining Message Authentication Protocol). CCMP is based on the 128-bit
Advanced Encryption Standard (AES) algorithm.
‘Counter mode’ refers to the encryption of each block of plain text with an
arbitrary number, known as the counter. This number changes each time a block
of plain text is encrypted. Counter mode avoids the security weakness of repeated
identical blocks of encrypted text that makes encrypted data vulnerable to
pattern-spotting.
‘Cipher Block Chaining Message Authentication’ (also known as CBC-MAC) ensures
message integrity by encrypting each block of plain text in such a way that its
encryption is dependent on the block before it. This series of ‘chained’ blocks
creates a message authentication code (MAC or CMAC) that ensures the encrypted
data has not been tampered with.