Wimax standards and Security The Wimax

FIGURE 11.9
DES-CBC encryption.

2. 
   AES
   

The IEEE 802.16e standard added the use of AES to provide stronger encryp- tion of data. It defines the use of AES in four modes: CBC, counter encryption (CTR), CTR with CBC message authentication code (CCM), and ECB. CTR mode is considered better than CBC mode due to its ability to perform paral- lel processing of data, preprocessing of encryption blocks, and is simpler to implement. CCM mode adds the ability to determine the authenticity of an encrypted message to CTR mode. ECB mode is used to encrypt TEKs.

1. 
   AES in CCM Mode
   

AES-CCM requires that the transmitter constructs a unique nonce, which is a per-packet encryption randomizer. IEEE 802.16e defines a 13-byte nonce, as shown in Figure 11.10. Bytes 0–4 are constructed from the first 5 bytes of the GMH. Bytes 5–8 are reserved and are all set to 0. Bytes 9–12 are set to the packet number (PN). The PN is associated with an SA and set to 1 when the SA is established and when a new TEK is installed. Since the nonce is dependent on the GMH, modifications to the GMH can be detected by the receiver [7,8].

0 4 5 8 9 12

First 5 bytes of GMH

0x00000000

Packet number

FIGURE 11.10
CCM nonce.


0 1 13 14 15

Flag (0x19)

Nonce

Length of payload