Connectivity and Data Protection
FIGURE 3.9
An Encrypted Data Frame Decode
Figure 3.10 shows where the IV (not encrypted), data (encrypted), and
ICV (encrypted) are positioned within the MAC frame’s body.¹ The
MPDU frame body has a maximum size of 2312 bytes, 2304 bytes of
which may be used to carry upper layer protocols.² When WEP is in
use, the MPDU frame body is expanded by 8 bytes to 2312 bytes.³
These extra 8 bytes are used for the IV (4 octets) and ICV (4 octets).
The encrypted data is exactly the same length as the plain text data was
because that is how stream ciphers such as RC4 work. Compare the
position of each item in Figure 3.9 with those shown in Figure 3.10 and
notice that wireless protocol analyzers show frame contents in order
from first to last.
1
IEEE 802.11 – 1999 (R2003) – Section 8.2.3
2
IEEE 802.11 – 1999 (R2003) – Section 6.2.1.1.2
3
IEEE 802.11 – 1999 (R2003) – Section 7.1.3.1.9 & 8.2.5
