Cryptography Reference
In-Depth Information
Alice
Bob
Generate
IV
Compute
ICV
IV
||
E
IV
||
K
( data ||
ICV
)
Form
IV
||
K
and decrypt
Verify
ICV
Figure 12.3.
WEP encryption process
Alice:
1. generates a 24-bit pseudorandom initialisation vector
IV
and appends the
WEP key
K
to
IV
to form the key:
K
=
IV
||
K
;
2. computes a 32-bit CRC checksum
ICV
of the data, and appends this to the
data;
3. encrypts the data plus
ICV
using key
K
;
4. sends
IV
(in the clear) and the resulting ciphertext to Bob.
Bob then:
1. appends the WEP key
K
to the received
IV
to form the key
K
;
2. decrypts the ciphertext using
K
and extracts the checksum
ICV
;
3. verifies that the checksum
ICV
is correct.
If the verification of
ICV
is successful then Bob accepts the data packet.
ENTITY AUTHENTICATION IN WEP
The WEP entity authentication technique is very simple. It is based on the same
challenge-response principle that we first discussed for dynamic passwords in
Section 8.5 and that was used in several of the protocols analysed in Section 9.4.
If Alice (a device) wants to identify herself to Bob (a wireless access point):
1. Alice sends a request to authenticate to Bob;
2. Bob sends a nonce
r
B
to Alice;