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Appendix D Wireless LANs
VMG4380-B10A / VMG4325-B10A User’s Guide
336
IEEE 802.1x
In June 2001, the IEEE 802.1x standard was designed to extend the features of IEEE 802.11 to
support extended authentication as well as providing additional accounting and control features. It
is supported by Windows XP and a number of network devices. Some advantages of IEEE 802.1x
are:
•
User based identification that allows for roaming.
•
Support for RADIUS (Remote Authentication Dial In User Service, RFC 2138, 2139) for
centralized user profile and accounting management on a network RADIUS server.
•
Support for EAP (Extensible Authentication Protocol, RFC 2486) that allows additional
authentication methods to be deployed with no changes to the access point or the wireless
clients.
RADIUS
RADIUS is based on a client-server model that supports authentication, authorization and
accounting. The access point is the client and the server is the RADIUS server. The RADIUS server
handles the following tasks:
•
Authentication
Determines the identity of the users.
•
Authorization
Determines the network services available to authenticated users once they are connected to the
network.
•
Accounting
Keeps track of the client’s network activity.
RADIUS is a simple package exchange in which your AP acts as a message relay between the
wireless client and the network RADIUS server.
Types of RADIUS Messages
The following types of RADIUS messages are exchanged between the access point and the RADIUS
server for user authentication:
•
Access-Request
Sent by an access point requesting authentication.
•
Access-Reject
Sent by a RADIUS server rejecting access.
•
Access-Accept
Sent by a RADIUS server allowing access.
•
Access-Challenge
Sent by a RADIUS server requesting more information in order to allow access. The access point
sends a proper response from the user and then sends another Access-Request message.
The following types of RADIUS messages are exchanged between the access point and the RADIUS
server for user accounting:
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Appendix D Wireless LANs
VMG4380-B10A / VMG4325-B10A User’s Guide
337
•
Accounting-Request
Sent by the access point requesting accounting.
•
Accounting-Response
Sent by the RADIUS server to indicate that it has started or stopped accounting.
In order to ensure network security, the access point and the RADIUS server use a shared secret
key, which is a password, they both know. The key is not sent over the network. In addition to the
shared key, password information exchanged is also encrypted to protect the network from
unauthorized access.
Types of EAP Authentication
This section discusses some popular authentication types: EAP-MD5, EAP-TLS, EAP-TTLS, PEAP and
LEAP. Your wireless LAN device may not support all authentication types.
EAP (Extensible Authentication Protocol) is an authentication protocol that runs on top of the IEEE
802.1x transport mechanism in order to support multiple types of user authentication. By using EAP
to interact with an EAP-compatible RADIUS server, an access point helps a wireless station and a
RADIUS server perform authentication.
The type of authentication you use depends on the RADIUS server and an intermediary AP(s) that
supports IEEE 802.1x.
For EAP-TLS authentication type, you must first have a wired connection to the network and obtain
the certificate(s) from a certificate authority (CA). A certificate (also called digital IDs) can be used
to authenticate users and a CA issues certificates and guarantees the identity of each certificate
owner.
EAP-MD5 (Message-Digest Algorithm 5)
MD5 authentication is the simplest one-way authentication method. The authentication server
sends a challenge to the wireless client. The wireless client ‘proves’ that it knows the password by
encrypting the password with the challenge and sends back the information. Password is not sent in
plain text.
However, MD5 authentication has some weaknesses. Since the authentication server needs to get
the plaintext passwords, the passwords must be stored. Thus someone other than the
authentication server may access the password file. In addition, it is possible to impersonate an
authentication server as MD5 authentication method does not perform mutual authentication.
Finally, MD5 authentication method does not support data encryption with dynamic session key. You
must configure WEP encryption keys for data encryption.
EAP-TLS (Transport Layer Security)
With EAP-TLS, digital certifications are needed by both the server and the wireless clients for
mutual authentication. The server presents a certificate to the client. After validating the identity of
the server, the client sends a different certificate to the server. The exchange of certificates is done
in the open before a secured tunnel is created. This makes user identity vulnerable to passive
attacks. A digital certificate is an electronic ID card that authenticates the sender’s identity.
However, to implement EAP-TLS, you need a Certificate Authority (CA) to handle certificates, which
imposes a management overhead.
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Appendix D Wireless LANs
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338
EAP-TTLS (Tunneled Transport Layer Service)
EAP-TTLS is an extension of the EAP-TLS authentication that uses certificates for only the server-
side authentications to establish a secure connection. Client authentication is then done by sending
username and password through the secure connection, thus client identity is protected. For client
authentication, EAP-TTLS supports EAP methods and legacy authentication methods such as PAP,
CHAP, MS-CHAP and MS-CHAP v2.
PEAP (Protected EAP)
Like EAP-TTLS, server-side certificate authentication is used to establish a secure connection, then
use simple username and password methods through the secured connection to authenticate the
clients, thus hiding client identity. However, PEAP only supports EAP methods, such as EAP-MD5,
EAP-MSCHAPv2 and EAP-GTC (EAP-Generic Token Card), for client authentication. EAP-GTC is
implemented only by Cisco.
LEAP
LEAP (Lightweight Extensible Authentication Protocol) is a Cisco implementation of IEEE 802.1x.
Dynamic WEP Key Exchange
The AP maps a unique key that is generated with the RADIUS server. This key expires when the
wireless connection times out, disconnects or reauthentication times out. A new WEP key is
generated each time reauthentication is performed.
If this feature is enabled, it is not necessary to configure a default encryption key in the wireless
security configuration screen. You may still configure and store keys, but they will not be used while
dynamic WEP is enabled.
Note: EAP-MD5 cannot be used with Dynamic WEP Key Exchange
For added security, certificate-based authentications (EAP-TLS, EAP-TTLS and PEAP) use dynamic
keys for data encryption. They are often deployed in corporate environments, but for public
deployment, a simple user name and password pair is more practical. The following table is a
comparison of the features of authentication types.
Table 127
Comparison of EAP Authentication Types
EAP-MD5
EAP-TLS
EAP-TTLS
PEAP
LEAP
Mutual Authentication
No
Yes
Yes
Yes
Yes
Certificate – Client
No
Yes
Optional
Optional
No
Certificate – Server
No
Yes
Yes
Yes
No
Dynamic Key Exchange
No
Yes
Yes
Yes
Yes
Credential Integrity
None
Strong
Strong
Strong
Moderate
Deployment Difficulty
Easy
Hard
Moderate
Moderate
Moderate
Client Identity Protection
No
No
Yes
Yes
No
Page 339 / 367
Appendix D Wireless LANs
VMG4380-B10A / VMG4325-B10A User’s Guide
339
WPA and WPA2
Wi-Fi Protected Access (WPA) is a subset of the IEEE 802.11i standard. WPA2 (IEEE 802.11i) is a
wireless security standard that defines stronger encryption, authentication and key management
than WPA.
Key differences between WPA or WPA2 and WEP are improved data encryption and user
authentication.
If both an AP and the wireless clients support WPA2 and you have an external RADIUS server, use
WPA2 for stronger data encryption. If you don't have an external RADIUS server, you should use
WPA2-PSK (WPA2-Pre-Shared Key) that only requires a single (identical) password entered into
each access point, wireless gateway and wireless client. As long as the passwords match, a wireless
client will be granted access to a WLAN.
If the AP or the wireless clients do not support WPA2, just use WPA or WPA-PSK depending on
whether you have an external RADIUS server or not.
Select WEP only when the AP and/or wireless clients do not support WPA or WPA2. WEP is less
secure than WPA or WPA2.
Encryption
WPA improves data encryption by using Temporal Key Integrity Protocol (TKIP), Message Integrity
Check (MIC) and IEEE 802.1x. WPA2 also uses TKIP when required for compatibility reasons, but
offers stronger encryption than TKIP with Advanced Encryption Standard (AES) in the Counter
mode with Cipher block chaining Message authentication code Protocol (CCMP).
TKIP uses 128-bit keys that are dynamically generated and distributed by the authentication server.
AES (Advanced Encryption Standard) is a block cipher that uses a 256-bit mathematical algorithm
called Rijndael. They both include 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.
WPA and WPA2 regularly change and rotate the encryption keys so that the same encryption key is
never used twice.
The RADIUS server distributes a Pairwise Master Key (PMK) key to the AP that then sets up a key
hierarchy and management system, using the PMK to dynamically generate unique data encryption
keys to encrypt every data packet that is wirelessly communicated between the AP and the wireless
clients. This all happens in the background automatically.
The Message Integrity Check (MIC) is designed to prevent an attacker from capturing data packets,
altering them and resending them. The MIC provides a strong mathematical function in which the
receiver and the transmitter each compute and then compare the MIC. If they do not match, it is
assumed that the data has been tampered with and the packet is dropped.
By generating unique data encryption keys for every data packet and by creating an integrity
checking mechanism (MIC), with TKIP and AES it is more difficult to decrypt data on a Wi-Fi
network than WEP and difficult for an intruder to break into the network.
The encryption mechanisms used for WPA(2) and WPA(2)-PSK are the same. The only difference
between the two is that WPA(2)-PSK uses a simple common password, instead of user-specific
credentials. The common-password approach makes WPA(2)-PSK susceptible to brute-force
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Appendix D Wireless LANs
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340
password-guessing attacks but it’s still an improvement over WEP as it employs a consistent,
single, alphanumeric password to derive a PMK which is used to generate unique temporal
encryption keys. This prevent all wireless devices sharing the same encryption keys. (a weakness of
WEP)
User Authentication
WPA and WPA2 apply IEEE 802.1x and Extensible Authentication Protocol (EAP) to authenticate
wireless clients using an external RADIUS database. WPA2 reduces the number of key exchange
messages from six to four (CCMP 4-way handshake) and shortens the time required to connect to a
network. Other WPA2 authentication features that are different from WPA include key caching and
pre-authentication. These two features are optional and may not be supported in all wireless
devices.
Key caching allows a wireless client to store the PMK it derived through a successful authentication
with an AP. The wireless client uses the PMK when it tries to connect to the same AP and does not
need to go with the authentication process again.
Pre-authentication enables fast roaming by allowing the wireless client (already connecting to an
AP) to perform IEEE 802.1x authentication with another AP before connecting to it.
Wireless Client WPA Supplicants
A wireless client supplicant is the software that runs on an operating system instructing the wireless
client how to use WPA. At the time of writing, the most widely available supplicant is the
WPA patch
for Windows XP, Funk Software's Odyssey client.
The Windows XP patch is a free download that adds WPA capability to Windows XP's built-in "Zero
Configuration" wireless client. However, you must run Windows XP to use it.
WPA(2) with RADIUS Application Example
To set up WPA(2), you need the IP address of the RADIUS server, its port number (default is 1812),
and the RADIUS shared secret. A WPA(2) application example with an external RADIUS server
looks as follows. "A" is the RADIUS server. "DS" is the distribution system.
1
The AP passes the wireless client's authentication request to the RADIUS server.
2
The RADIUS server then checks the user's identification against its database and grants or denies
network access accordingly.
3
A 256-bit Pairwise Master Key (PMK) is derived from the authentication process by the RADIUS
server and the client.

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