Applications and the Future - Codes: The Guide to Secrecy from Ancient to Modern Times - page 348

Alice if mutual authentication is beingemployed; and only strong mutual
authentication is recommended for WLANs. This message interchange
will vary dependingon the authentication scheme employed.
4. Alice verifies the server's ID, if mutual authentication is beingused, then
sends her response to the ID challenge via the AP to the server.
5. The server either accepts or rejects the request by Alice.
6. If her request is accepted, the AP opens a port for her network access.
In the absence of an external authentication server, WPA is capable of exe-
cutingwhat is called a Pre-Shared Key mode (PSK) to verify ID, for Alice say,
both at her client station and the AP. This is accomplished via a password, or
some other ID, and she may gain access only if her password matches the AP's
password. This password also supplies the material for use by TKIP Temporal
Key Integrity Protocol (TKIP), which is part of the standard drafted by the
TGi, to generate an encryption key for each data packet.
Temporal Key Integrity Protocol
TKIP (also called WEP2), is a collection of algorithms to wrap around WEP
in order to patch the security holes, especially the use of static keys in WEP.
With WEP, encryption is optional, whereas with TKIP, it is mandatory. Indeed,
TKIP replaces WEP with a stronger encryption scheme usingcomputingpower
in existingwireless devices to execute the required operations. The following
are some features of TKIP.
1. MIC : This is a Message Integrity Code , (MIC) 9.21 employed to thwart forg-
eries; its code name is Michael . 9.22 The 8-byte MIC is placed after the data
portion of the TKIP frame and before the 4-byte ICV (Integrity Check
Value). (This fixes a WEP problem where Mallory can tamper with the
ICV before it is received, even though WEP enciphers it beforehand.) The
data, MIC, and ICV portion of the frame are WEP-encrypted. Michael
computes a keyed function of the message at the transmission site (as de-
scribed below), sends the resulting “tag” value together with the message
to the receiver. There the tagvalue is recalculated and compared with
the sent value. If the tags match, the message is accepted as authentic.
Otherwise it is rejected as a forgery.
9.21 We know this as a MAC (see page 136), but TGi has already used MAC to mean Media
Access Control , so we will conform with their acronym here, even though it conflicts directly
with the term we used as MIC on page 260, where we used it in reference to an un -keyed hash
function. With this warning of alternate usage for this discussion only , there should be no
confusion.
9.22 MIC was created by Niels Ferguson (see [82]), who is a cryptographic engineer and con-
sultant. His expertise lies in the design of cryptographic algorithms, protocols, and security
infrastructures, especially on a large scale. He worked with Bruce Schneier at Counterpane
Systems (see page 138), and coauthored a recent topic[85] with him.