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Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Wireless Networking Basics
D-11
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 passphrase on both the
stations and the access point. This removes 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. The 802.1X
port access control prevents full access to the network until authentication completes. The
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 four-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 plain text
message to ensure messages are not being spoofed.
Page 112 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
D-12
Wireless Networking Basics
WPA Authentication: Enterprise-level User
Authentication via 802.1x/EAP and RADIUS
Figure D-3:
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 113 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Wireless Networking Basics
D-13
Figure D-4:
802.1x Authentication Sequence
The access point (AP) sends Beacon Frames with WPA information elements 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 114 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
D-14
Wireless Networking Basics
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.
Page 115 / 122
Reference Manual for the Wireless Cable Modem Gateway CG814WG v2
Wireless Networking Basics
D-15
Temporal Key Integrity Protocol (TKIP)
WPA uses TKIP to provide important data encryption enhancements including a per-packet key
mixing function, a message integrity check (MIC) named Michael, an extended initialization
vector (IV) with sequencing rules, and a re-keying mechanism. TKIP also provides for the
following:
•
The verification of the security configuration after the encryption keys are determined.
•
The synchronized changing of the unicast encryption key for each frame.
•
The determination of a unique starting unicast encryption key for each preshared key
authentication.
Michael
With 802.11 and WEP, data integrity is provided by a 32-bit
integrity check value
(ICV) that is
appended to the 802.11 payload and encrypted with WEP. Although the ICV is encrypted, you can
use cryptanalysis to change bits in the encrypted payload and update the encrypted ICV without
being detected by the receiver.
With WPA, a method known as
Michael
specifies a new algorithm that calculates an 8-byte
message integrity code
(MIC) using the calculation facilities available on existing wireless
devices. The MIC is placed between the data portion of the IEEE 802.11 frame and the 4-byte ICV.
The MIC field is encrypted together with the frame data and the ICV.
Michael also provides replay protection. A new frame counter in the IEEE 802.11 frame is used to
prevent replay attacks.
AES Support
One of the encryption methods supported by WPA beside TKIP is the advanced encryption
standard (AES), although AES support will not be required initially for Wi-Fi certification. This is
viewed as the optimal choice for security conscience organizations, but the problem with AES is
that it requires a fundamental redesign of the NIC’s hardware in both the station and the access
point. TKIP was a pragmatic compromise that allows organizations to deploy better security while
AES capable equipment is being designed, manufactured, and incrementally deployed.

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