This section describes how Wi-Fi LAN packets are encrypted using CCMP. The first important point is that CCMP encrypts data at the MPDU level. The difference between MPDU and MSDU is discussed in Chapter 11; but to recap, the MPDU corresponds to the frames that actually get transmitted over the radio link. There is one MPDU for each frame transmitted, and the MPDU itself might be the result of fragmenting larger packets passed from a higher layer, called MSDUs.
Steps in Encrypting a Transmission
Figure 12.3 shows the flow of data from MSDU to MPDU and eventually out to the radio link.
Figure 12.3. Flow of Frames Through CCMP
The data arrives as an MSDU and may be broken into fragments. Each fragment is formed into an MPDU and assigned its own IEEE 802.11 header containing source and destination addresses and other information. At this point, each MPDU is processed by the CCMP algorithm to generate a new encrypted MPDU. Only the data part is encrypted, not the header. However, CCMP does more than just encrypt portions of the MPDU. It also inserts extra fields, causing the resulting encrypted MPDU to be 16 bytes longer than the original.
An overview of the steps in encrypting an MPDU are shown in Figure 12.4 and described below:
Figure 12.4. Steps in Processing an MPDU
The encrypted MPDUs are placed on a queue prior to transmission. There might be several queues waiting their turn based on some priority policy. This allows for later extension to accommodate different traffic classes under IEEE 802.11e. Immediately prior to transmission, some of the fields of the IEEE 802.11 header are updated to meet transmission rules. Those fields that are subject to such changes are called mutable fields and are excluded from the MIC computation.
The CCMP header must be prepended to the encrypted data and transmitted in the clear (that is unencrypted). The CCMP header has two purposes. First, it provides the 48-bit packet number (PN) that provides replay protection and enables the receiver to derive the value of the nonce that was used in the encryption. Second, in the case of multicasts, it tells the receiver which group key has been used (see Chapter 10). The format of the CCMP header is very similar to that used for the TKIP header. This is intentional to simplify implementation for access points that need to receive transmissions from a mixed group of TKIP and CCMP mobile devices. The format is shown in Figure 12.5.
Figure 12.5. CCMP Header
Six bytes are used for the 48-bit PN value, 1 byte is reserved, and the remaining byte contains the KeyID bits, the function of which is described in Chapter 10. Note that the bit next to the KeyID bits is set to a value of 1, corresponding to the Extended IV bit in TKIP. This value indicates that the frame format is RSN rather than the earlier WEP format.
Overview of Implementation
Implementation of the CCMP block can be viewed as a single process with inputs and outputs, as shown in Figure 12.6.
Figure 12.6. Encryption and Decryption with CCMP
Note that the decryption phase has the same inputs as the encryption phase (except that the input MPDU is encrypted). This is because the header information, including the CCMP header, is transmitted across the link in the clear and can therefore be extracted by the receiver prior to decryption.
The implementation of CCMP (shown in Figure 12.4 as a "block") must keep a sequence counter called the packet number (PN), which it increments for each packet processed. This prevents an attacker trying to reuse a packet that has previously been sent. The PN is 48 bits long, large enough to ensure it never overflows. There will never be two packets sent with the same sequence value. Of course if you power down the device and restart, the PN will be reset, but this will be with a different key value and hence does not create a threat.
Implementation of the CCMP encryption block is shown in Figure 12.7.
Figure 12.7. CCMP Encryption Block
Note how the computation occurs in two stages: first, the MIC is calculated and appended to the MPDU, and then the entire MPDU (including MIC) is encrypted to produce the result. Let's look in more detail at each step.
An encrypted MPDU contains two more fields than an unencrypted MPDU. It has the CCMP header and the MIC value. The MIC field is 8 octets (64 bits). Note that the MIC is only half the size of the AES block but is still long enough to reduce the chance of a successful MIC forgery to less than 1 in 1019.
The order of fields in the encrypted MPDU is shown in Figure 12.8.
Figure 12.8. MPDU Encrypted under CCMP (CH = CCMP Header)
Steps in Encrypting an MPDU
Before starting the encryption process, it is useful to prepare all the pieces of the MPDU in the order they will eventually appear. We start off with three pieces: the MAC header, the CCMP header, and the plaintext data, as shown in Figure 12.8a. The mutable fields of the MAC header are masked out by setting them to 0. The CCMP header is filled in with the PN and KeyID bits. Note that the PN is incremented by one for each MPDU prior to being used. The data portion can be filled in with plaintext data.
The MAC header and CCMP headers will not be encrypted but need to be protected by the MIC. These two items are grouped together to form the authenticated data, as shown in Figure 12.8b. The first job after assembling the pieces is to compute the MIC.
Computing the MIC
Computation of the MIC is done using CBC-MAC, which encrypts a starting block and then successively XORs subsequent blocks and encrypts the result. The final MIC is one 128-bit block, but we only need a 64-bit MIC so, for CCMP, we discard the lower 64 bits of the result.
In CCMP the first block of the CBC-MAC computation is not taken directly from our MPDU but is formed in a special way using a nonce value. The format of the first block is shown in Figure 12.9 comprising a nonce and two other fields: Flag and DLen.
Figure 12.9. Format of the First Block for CBC-MAC
The nonce guarantees freshness by ensuring that each encryption uses data that has never been used before (under a given key). You might think that we could just use the packet number (PN) for the nonce because it increments for each MPDU and hence never repeats. However, remember that the key is shared between at least two communicating parties (more for the group key) and these parties may, each at some point, use a PN that has already been used by another party, violating the "use once per key" rule. To avoid this problem, the nonce is formed by combining the PN with the MAC address of the sender.
The third field included in the nonce is the Priority field. The Priority field is a placeholder for future capability when there are different traffic streams with different characteristics (audio, video, and so on). In such a case, it might be useful to have a separate PN for each type of data. The three fields combine to create the 104-bit nonce, as shown in Figure 12.10.
Figure 12.10. Constructing the First Block for CBC-MAC
The other two fields that, with the nonce, create the first block for CBC-MAC are also shown in Figure 12.10. The flag field has a fixed value of 01011001 and indicates, among other things, that the MIC is 64 bits. In other (non-RSN) applications of CCM, the Flags field might be different, but this does not concern us here. The last field, DLen, indicates the length of the plaintext data.
Once the first block has been prepared, the MIC is computed one block at a time by incorporating the authenticated data and then incorporating the plaintext data. One of the characteristics of CBC-MAC is that it works only for an exact number of blocks. If the data doesn't divide into an exact number, it must be padded. For the purposes of the MIC computation, CCMP requires that both the authenticated data and the plaintext data be padded to an exact number of blocks. In IEEE 802.11, it is likely that neither the authenticated data nor the plaintext data will be a suitable length, so each is padded with zeros to meet this requirement, as shown in Figure 12.8C. The MIC is computed across the combination of the special first block, the authenticated data, and the plaintext data, including the zero pad bytes. Note that the pad bytes are only inserted for the MIC computation and are not actually inserted in the MPDU, as illustrated by Figure 12.8d.
Encrypting the MPDU
Once the MIC has been computed and appended to the plaintext data, we are ready to start encrypting the MPDU. The encryption occurs using counter mode and starting with the data immediately following the CCMP header in the template. Note that because of the padding during the MIC computation, we are guaranteed that the blocks to be encrypted will be aligned with the blocks included in the MIC computation. The encrypted data replaces the original data for the entire data portion and the MIC value, resulting in a complete encrypted MPDU ready to be queued for transmission, as shown in Figure 12.8E. It is not necessary to use padding for the encryption stage because counter mode allows any excess bits in the last block to be discarded.
An essential step in counter mode is to initialize the counter in a way that avoids ever using the same start value twice. Therefore the counter is constructed from a nonce in an almost identical way to that for the MIC. In fact the nonce value used is identical to that of the MIC and includes the sequence counter, source MAC address, and priority fields. This value is then joined with two fields: Flag and Counter ("Ctr"), as shown in Figure 12.11.
Figure 12.11. Constructing the Counter for CCMP AES Counter Mode
The ctr value starts at 1 and counts up as counter mode proceeds. Because the nonce is a unique value and the ctr field is 16 bits long, you are guaranteed to have unique counter values for any message with fewer than 65536 blocks. This easily accommodates the largest MPDUs allowed in IEEE 802.11.
Well, almost ready. First we need to put back all the fields in the MAC header that were masked out for the MIC computation. Although these fields are not used for the MIC, they may still be important.
Once the counter is initialized, encryption can proceed as described in the previous section "Counter Mode" in this chapter. Each successive value of the counter is encrypted using the secret key and XORed with the template data to produce the encrypted data.
When the encrypted MPDU is delivered to the receiver, the first job is to get the right key for decryption. The correct pairwise keys are selected based on the source MAC address in the MAC header. There are a number of steps the receiver must take to extract and check the validity of the received data. Decryption is only one step and this process is more generally called decapsulation.
The packet number (PN) is sent unencrypted in the CCMP header. The first thing the receiver does is to read the PN and compare to the last frame received. If the sequence number is lower or equal to the last one, it should be discarded as a replay of an old message. In this case the receiver goes no further with the MPDU.
Assuming the PN matches, the next step is to prepare for decryption using AES/counter mode. This requires the computation of the starting value for the counter, which must match that value used in encryption. All the information is available in the received frame. The sequence number can be combined with the source MAC address and priority to create the nonce. This is then combined with the known flag value and the start ctr value (also 1) to create the initial counter. Note there is no secret here: Any attacker can compute the same value. However, it is of no use unless the secret key is also known. Decryption proceeds as for encryption. Successive values of the counter are encrypted and XORed with the received MPDU to restore the unencrypted data and the MIC value.
The next stage is to verify that the MIC value is correct. The MIC value is recalculated across the same data (and padding) as the original MPDU at the sender. The mutable fields in the header are masked out and the computation performed over the whole MPDU, excluding the MIC. Of course, if the data is unaltered from when it was sent, and we have the right secret key, the same result will be obtained. This can be compared to the MIC value sent with the frame: A match means the frame is valid. A mismatch is most likely evidence of an attack and the frame will be discarded.
Interestingly, with CCMP the process of decryption is almost identical to that for encryption, leading to a nice simplification for implementation. Once the MPDU is decoded, the MIC and CCMP header can be removed, and is the remaining data is passed up for reassembly with other received fragments to reform the MSDU. You can see how the CCMP process gives protection against forgery, eavesdropping, and copy/replay attacks. It is very strong.
As we said at the start of the chapter, the most advanced security protocol is of no use if the underlying cipher mechanism (in this case, AES) has a flaw. AES has no known flaws that might compromise security. If you are interested, Appendix A describes how AES works. This appendix includes some mathematics, which may be unfamiliar. It you are prepared to accept AES as a "black box" that encrypts blocks of data, then feel free to skip the appendix!