**Advanced Encryption Standard** (**AES**) is the new data encryption standard adopted by the National Institute of Standards as part of Federal Information Processing Standard (FIPS) and is specified as a link-layer encryption method to be used in WiMAX Technology. Advanced Encryption Standard (AES) is based on the Rijndael algorithm, which is a block ciphering method believed to have strong cryptographic properties. Besides offering strong encryption, Advanced Encryption Standard (AES) is fast, easy to implement in hardware or software, and requires less memory than do other comparable encryption schemes. The computational efficiency of Advanced Encryption Standard (AES) has been a key reason for its rapid widespread adoption. The Advanced Encryption Standard (AES) algorithm operates on a 128-bit block size of data, organized in a 4 x 4 array of bytes called a state. The encryption key sizes could be 128, 192, or 256 bits long; WiMAX Technology specifies the use of 128-bit keys.

In order to use a block cipher, such as **Advanced Encryption Standard** (**AES**), a reversible mechanism is needed to convert an arbitrary length message into a sequence of fixed-size blocks prior to encryption. The method to convert between messages and blocks is referred to as the cipher’s mode of operation, several of which are proposed for Advanced Encryption Standard (AES). The mode of operation needs to be carefully chosen so that is does not create any security holes and with implementation considerations in mind. The mode used in WiMAX Technology is called the counter mode. In counter mode, instead of directly encrypting the plain text, an arbitrary block, called the counter, is encrypted using the Advanced Encryption Standard (AES) algorithm, and the results are XORed with the plain text to produce the ciphertext. The arbitrary block is called the counter because it is generally incremented by 1 for each successive block processed ciphertext is never the same for two identical inputs, thereby providing protection from an onlooker observing patterns of repetition in the ciphertext. *(Arkoudi-Vafea Aikaterini, 2006)*

In addition to providing this additional protection, the counter mode has the remarkable property of making the decryption process exactly the same as encryption, since XORing the same value twice produces the original value, making the implementation easier. Counter mode is also suitable for parallel encryption of several blocks. Further, if the message doesn’t break into an exact number of blocks, this mode allows you to take the last short block and XOR it with the encrypted block and simply send the required number of bits from the output. These interesting properties make counter mode a popular choice for **Advanced Encryption Standard** (**AES**) implementation. Both Wi-Fi and WiMAX systems specify the use of AES in counter mode with Cipher Block Chaining Message Authentication Code (CBC-MAC). CBC-MAC, a protocol defined in RFC 3610, uses the same encryption key for deriving a message integrity check value. *(Arkoudi-Vafea Aikaterini, 2006)*