ed Encrypted Ciphertext data block data block data block data block data block data block data block Figure 8-8: Counter Mode of Operation of a Block Cipher (AES) Wireless LAN Security Message Data block Data block Data block Data block Data block Data block Data block Encryption Encrypted Encrypted Encrypted Encrypted Encrypted Encrypted Encrypted Ciphertext data block data block data block data block data block data block data block Figure 8-9: Electronic Code Book (ECB) Mode of Operation of a Block Cipher (AES) message, SO that patterns in the input data are preserved as patterns in the encrypted data, simplifying the task of discovering the encryption key. Counter mode removes this correspondence and eliminates the risk of key discovery as a result of such patterns. Wireless robust authenticated protocol (WRAP) is a further security mechanism supported by RSN, and is based on the so-called offset codebook (OCB) mode of operation of a block cipher. This scheme has a number of advantages over AES-CCMP, including being computationally more efficient, as it performs message integrity checking, authentication and encryption in a single calculation. However, WRAP was not adopted by TGi as the base security protocol for RSN because OCB is a patented scheme, which would have introduced licensing complications into the standard. Cipher Block Chaining Message Authentication Code (CBC-MAC) CBC-MAC is a message authentication and integrity method that can be used with block ciphers such as AES. The acronym MIC will be used instead of MAC for the Message Authentication Code, to avoid confusion with MAC as in Media Access Control. Cipher block chaining (the CBC part) is a mode of operation of a block cipher in which the ciphertext of one block becomes part of the encryption algorithm for the next block, and SO on. The CBC-MAC message authentication code is generated as shown in Figure 8-10, with the following steps; Step (1) A 104-bit nonce is created by combining an 8-bit priority field with the 48-bit source MAC address 