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Reference Manual for the MR814 v3 Cable/DSL Wireless Router
D-10
Wireless Networking Basics
202-10039-01
How Does WPA Compare to IEEE 802.11i?
WPA will be forward compatible with the IEEE 802.11i security specification currently under
development. WPA is a subset of the current 802.11i draft and uses certain pieces of the 802.11i
draft that are ready to bring to market today, such as 802.1x and TKIP. The main pieces of the
802.11i draft that are not included in WPA are secure IBSS (Ad-Hoc mode), secure fast handoff
(for specialized 802.11 VoIP phones), as well as enhanced encryption protocols such as
AES-CCMP. These features are either not yet ready for market or will require hardware upgrades
to implement.
What are the Key Features of WPA Security?
The following security features are included in the WPA standard:
•
WPA Authentication
•
WPA Encryption Key Management
–
Temporal Key Integrity Protocol (TKIP)
–
Michael
message integrity code
(MIC)
–
AES Support
•
Support for a Mixture of WPA and WEP Wireless Clients
These features are discussed below.
WPA addresses most of the known WEP vulnerabilities and is primarily intended for wireless
infrastructure networks as found in the enterprise. This infrastructure includes stations, access
points, and authentication servers (typically RADIUS servers). The RADIUS server holds (or has
access to) user credentials (e.g., user names and passwords) and authenticates wireless users
before they gain access to the network.
The strength WPA comes from an integrated sequence of operations that encompass 802.1X/EAP
authentication and sophisticated key management and encryption techniques. Its major operations
include:
•
Network security capability determination. This occurs at the 802.11 level and is
communicated through WPA information elements in Beacon, Probe Response, and (Re)
Association Requests. Information in these elements includes the authentication method
(802.1X or Pre-shared key) and the preferred cipher suite (WEP, TKIP, or AES).
Page 147 / 162
Reference Manual for the MR814 v3 Cable/DSL Wireless Router
Wireless Networking Basics
D-11
202-10039-01
The primary information conveyed in the Beacon frames is the authentication method and the
cipher suite. Possible authentication methods include 802.1X and Pre-shared key. Pre-shared
key is an authentication method that uses a statically configured pass phrase on both the
stations and the access point. This obviates the need for an authentication server, which in
many home and small office environments will not be available nor desirable. Possible cipher
suites include: WEP, TKIP, and AES (Advanced Encryption Standard). We’ll talk more TKIP
and AES when addressing data privacy below.
•
Authentication. EAP over 802.1X is used for authentication. Mutual authentication is gained
by choosing an EAP type supporting this feature and is required by WPA. 802.1X port access
control prevents full access to the network until authentication completes. 802.1X
EAPOL-Key packets are used by WPA to distribute per-session keys to those stations
successfully authenticated.
The supplicant in the station uses the authentication and cipher suite information contained in
the information elements to decide which authentication method and cipher suite to use. For
example, if the access point is using the Pre-shared key method then the supplicant need not
authenticate using full-blown 802.1X. Rather, the supplicant must simply prove to the access
point that it is in possession of the pre-shared key. If the supplicant detects that the service set
does not contain a WPA information element then it knows it must use pre-WPA 802.1X
authentication and key management in order to access the network.
•
Key management. WPA features a robust key generation/management system that integrates
the authentication and data privacy functions. Keys are generated after successful
authentication and through a subsequent 4-way handshake between the station and Access
Point (AP).
•
Data Privacy (Encryption). Temporal Key Integrity Protocol (TKIP) is used to wrap WEP in
sophisticated cryptographic and security techniques to overcome most of its weaknesses.
•
Data integrity. TKIP includes a message integrity code (MIC) at the end of each plaintext
message to ensure messages are not being spoofed.
Page 148 / 162
Reference Manual for the MR814 v3 Cable/DSL Wireless Router
D-12
Wireless Networking Basics
202-10039-01
WPA Authentication: Enterprise-level User
Authentication via 802.1x/EAP and RADIUS
Figure D-1:
WPA Overview
IEEE 802.1x offers an effective framework for authenticating and controlling user traffic to a
protected network, as well as providing a vehicle for dynamically varying data encryption keys via
EAP from a RADIUS server, for example. This framework enables using a central authentication
server, which employs mutual authentication so that a rogue wireless user does not join the
network.
It's important to note that 802.1x doesn't provide the actual authentication mechanisms. When
using 802.1x, the EAP type, such as Transport Layer Security (EAP-TLS) or EAP Tunneled
Transport Layer Security (EAP-TTLS) defines how the authentication takes place.
Note
: For environments with a Remote Authentication Dial-In User Service (RADIUS)
infrastructure, WPA supports Extensible Authentication Protocol (EAP). For environments
without a RADIUS infrastructure, WPA supports the use of a preshared key.
Together, these technologies provide a framework for strong user authentication.
Windows XP implements 802.1x natively, and several Netgear switch and wireless access point
products support 802.1x.
WPA
enabled
wireless
client with
“supplicant”
Certificate
Authority
(eg Win
Server,
VeriSign,
etc)
TCP/IP
Ports Closed
Until
RADIUS Server
Wired Network with
Optional
802.1x Port Based Network
Access Control
WPA enabled
Access Point
using
pre-shared key
or
802.1x
TCP/IP
Ports Opened
After
Authenticated
Wireless LAN
Login
Authentication
Page 149 / 162
Reference Manual for the MR814 v3 Cable/DSL Wireless Router
Wireless Networking Basics
D-13
202-10039-01
Figure D-2:
802.1x Authentication Sequence
The AP sends Beacon Frames with WPA information element to the stations in the service set.
Information elements include the required authentication method (802.1x or Pre-shared key) and
the preferred cipher suite (WEP, TKIP, or AES). Probe Responses (AP to station) and Association
Requests (station to AP) also contain WPA information elements.
1.
Initial 802.1x communications begin with an unauthenticated supplicant (i.e., client device)
attempting to connect with an authenticator (i.e., 802.11 access point). The client sends an
EAP-start message. This begins a series of message exchanges to authenticate the client.
2.
The access point replies with an EAP-request identity message.
1
2
3
4
5
6
7
Client with a WPA-
enabled wireless
adapter and supplicant
(Win XP, Funk,
Meetinghouse, etc.)
For example, a
WPA-enabled AP
For example, a
RADIUS server
Page 150 / 162
Reference Manual for the MR814 v3 Cable/DSL Wireless Router
D-14
Wireless Networking Basics
202-10039-01
3.
The client sends an EAP-response packet containing the identity to the authentication server.
The access point responds by enabling a port for passing only EAP packets from the client to
an authentication server located on the wired side of the access point. The access point blocks
all other traffic, such as HTTP, DHCP, and POP3 packets, until the access point can verify the
client's identity using an authentication server (e.g., RADIUS).
4.
The authentication server uses a specific authentication algorithm to verify the client's identity.
This could be through the use of digital certificates or some other EAP authentication type.
5.
The authentication server will either send an accept or reject message to the access point.
6.
The access point sends an EAP-success packet (or reject packet) to the client.
7.
If the authentication server accepts the client, then the access point will transition the client's
port to an authorized state and forward additional traffic.
The important part to know at this point is that the software supporting the specific EAP type
resides on the authentication server and within the operating system or application “supplicant”
software on the client devices. The access point acts as a “pass through” for 802.1x messages,
which means that you can specify any EAP type without needing to upgrade an 802.1x-compliant
access point. As a result, you can update the EAP authentication type to such devices as token
cards (Smart Cards), Kerberos, one-time passwords, certificates, and public key authentication or
as newer types become available and your requirements for security change.
WPA Data Encryption Key Management
With 802.1x, the rekeying of unicast encryption keys is optional. Additionally, 802.11 and 802.1x
provide no mechanism to change the global encryption key used for multicast and broadcast
traffic. With WPA, rekeying of both unicast and global encryption keys is required.
For the unicast encryption key, the Temporal Key Integrity Protocol (TKIP) changes the key for
every frame, and the change is synchronized between the wireless client and the wireless access
point (AP). For the global encryption key, WPA includes a facility (the Information Element) for
the wireless AP to advertise the changed key to the connected wireless clients.
If configured to implement dynamic key exchange, the 802.1x authentication server can return
session keys to the access point along with the accept message. The access point uses the session
keys to build, sign and encrypt an EAP key message that is sent to the client immediately after
sending the success message. The client can then use contents of the key message to define
applicable encryption keys. In typical 802.1x implementations, the client can automatically change
encryption keys as often as necessary to minimize the possibility of eavesdroppers having enough
time to crack the key in current use.

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