Wimax standards and Security The Wimax

Backbone Hop-by-Hop Security

Data communications in 802.16 mesh networks are protected with hop-by- hop encryption of packets. Data can be encrypted with 56-bit DES or with the AES CCM algorithm. In each case, the encryption is accomplished with a shared, private, traffic encryption key (TEK) for the connection. TEKs are generated independently on the nodes with a pseudorandom algorithm spec- ified in Ref. 23. The PKM protocol specifies the mechanism for TEK exchange between nodes.
TEKs are exchanged between MAC layer neighbors. A node initiates the exchange by sending a key request to its neighbor. The key request mes- sage contains the sender’s X.509 certificate and a hash value calculated with the AK that the sender obtained from the base station during authorization.

If the neighbor node can authorize the request, by verifying the hash with its AK, it sends back a TEK encrypted with a KEK. The authentication of the packet verifies that both nodes are still authenticated with the base station. If one of the nodes is using an expired AK, the peer node will find out from the incorrect HMAC value for the packet.

The TEK is encrypted with one of three algorithms: 1024-bit RSA, 3-DES, or 128-bit AES. The key encryption method is assigned through the secu- rity association the connection is in. If 1024-bit RSA encryption is used for TEK encryption, the node sending the TEK uses the RSA public key that the requesting node sends in its X.509 certificate as the KEK. If 3-DES encryption is used for TEK encryption, the node sending the TEK uses the AK it obtained during the authorization from the base station to generate a private key. The private key is generated by first padding the AK with 0x63 repeated 64 times, taking the SHA-1 hash of the result and truncating it to 128 bits. If 128-bit AES encryption is used for the TEK, the KEK is obtained in the same way as for 3-DES encryption.

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