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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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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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Is WPA Perfect?
WPA is not without its vulnerabilities. Specifically, it is susceptible to denial of service (DoS)
attacks. If the access point receives two data packets that fail the Message Integrity Code (MIC)
check within 60 seconds of each other then the network is under an active attack, and as a result,
the access point employs counter measures, which includes disassociating each station using the
access point. This prevents an attacker from gleaning information about the encryption key and
alerts administrators, but it also causes users to lose network connectivity for 60 seconds. More
than anything else, this may just prove that no single security tactic is completely invulnerable.
WPA is a definite step forward in WLAN security over WEP and has to be thought of as a single
part of an end-to-end network security strategy.
Product Support for WPA
Starting in August, 2003, NETGEAR, Inc. wireless Wi-Fi certified products will support the WPA
standard. NETGEAR, Inc. wireless products that had their Wi-Fi certification approved before
August, 2003 will have one year to add WPA so as to maintain their Wi-Fi certification.
WPA requires software changes to the following:
Wireless access points
Wireless network adapters
Wireless client programs
Supporting a Mixture of WPA and WEP Wireless Clients
To support the gradual transition of WEP-based wireless networks to WPA, a wireless AP can
support both WEP and WPA clients at the same time. During the association, the wireless AP
determines which clients use WEP and which clients use WPA. The disadvantage to supporting a
mixture of WEP and WPA clients is that the global encryption key is not dynamic. This is because
WEP-based clients cannot support it. All other benefits to the WPA clients, such as integrity, are
maintained.
However, a mixed mode supporting WPA and non-WPA clients would offer network security that
is no better than that obtained with a non-WPA network, and thus this mode of operation is
discouraged.
Changes to Wireless Access Points
Wireless access points must have their firmware updated to support the following:
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The new WPA information element
To advertise their support of WPA, wireless APs send the beacon frame with a new 802.11
WPA information element that contains the wireless AP's security configuration (encryption
algorithms and wireless security configuration information).
The WPA two-phase authentication
Open system, then 802.1x (EAP with RADIUS or preshared key).
TKIP
Michael
AES
(optional)
To upgrade your wireless access points to support WPA, obtain a WPA firmware update from your
wireless AP vendor and upload it to your wireless AP.
Changes to Wireless Network Adapters
Wireless network adapters must have their firmware updated to support the following:
The new WPA information element
Wireless clients must be able to process the WPA information element and respond with a
specific security configuration.
The WPA two-phase authentication
Open system, then 802.1x (EAP or preshared key).
TKIP
Michael
AES
(optional)
To upgrade your wireless network adapters to support WPA, obtain a WPA update from your
wireless network adapter vendor and update the wireless network adapter driver.
For Windows wireless clients, you must obtain an updated network adapter driver that supports
WPA. For wireless network adapter drivers that are compatible with Windows XP (Service Pack 1)
and Windows Server 2003, the updated network adapter driver must be able to pass the adapter's
WPA capabilities and security configuration to the Wireless Zero Configuration service.
Microsoft has worked with many wireless vendors to embed the WPA firmware update in the
wireless adapter driver. So, to update you Windows wireless client, all you have to do is obtain the
new WPA-compatible driver and install the driver. The firmware is automatically updated when
the wireless network adapter driver is loaded in Windows.
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Changes to Wireless Client Programs
Wireless client programs must be updated to permit the configuration of WPA authentication (and
preshared key) and the new WPA encryption algorithms (TKIP and the optional AES component).
To obtain the Microsoft WPA client program, visit the following Microsoft Web site.

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