Medium Access Control (MAC) and Physical Layer (PHY) - CISE

4-June-07 P1901_PRO_016_r0
NIST800-38A “Recommendations for Block Cipher Modes of Operation. Methods and Techniques”. By the use of
CTR IEEE P1901 achieves confidentiality for messages of arbitrary length.
Nevertheless it is still necessary to provide message integrity against tampering. For this purpose IEEE P1901 uses
the operation mode known as CCM (Counter with CBC-MAC (Message Authentication Check)) that avoids the use
of a separate mechanism for providing message integrity. CCM mode of operation combines CTR mode of
encryption with the CBC-MAC mode of authentication. CCM is defined in RFC 3610 and has been used and
studied for a long time and has well-understood cryptographic properties. The interesting point is that the same
encryption key can be used for both processes in conjunction with other parameters, thus leading, in effect, to two
separated keys.
CCM is only defined for 128-bit block ciphers and, though it is a generic mode applicable to any many ciphers, in
IETF RFC3610 it is defined for use with 128-bit AES.
CCM has two parameters (M and L). M indicates the length of the MIC (Message Integrity Check) and L indicates
the length of the message itself. For the selected algorithm M is equal to 4 (thus indicating that the Message
Integrity Check (MIC) produced is 4 bytes long) and L is equal to 2 (thus indicating that the length of the message
is at most 2^16 bytes as is exactly the maximum length of a burst).
Encryption and Integrity are both based on AES blocks and needs a different initialization variables and modes to
work. The information about the initial state for desencryption and integrity check is transported in the CCMP
Header.
Encryption and Integrity are applied when the burst is set as encrypted and over each one of the packets or packet
fragments inside the burst payload independently, but using the common initial state contained in the CCMP
Header, that is transmitted only once each burst at the beginning of the payload.
Each MIC is calculated over the Burst Header, CCMP Header, the Interpacket Header added by the LLC layer and
the packet or fragment of a packet including the padding, and then appended to this structure, althought Burst
Header and CCMP Header are only transmitted once at the beginning of the Burst.
The encryption algorithm is also applied in a fragment packet or packet basis. Burst Header, CCMP Header and
Interpacket Headers shall be sent in clear, and the encryption is applied over the fragment packet or packet
including its padding and the appended MIC.
10.1.2 Detailed encryption process
As has been described before, Encryption and Integrity algorithms are applied in a packet or fragment of packet
basis, but using common initial state information for each Burst contained in the CCMP Header.
Submission page 280 UPA-OPERA