Wi-Fi Authentication Demystified Companion Guide - Xirrus

Wi-Fi Authentication Demystified
Companion Guide
15
9
24
23
18
1
21
27
17
19
25
16
26
4
5 6 7
12 13 14
Across Down
2. EAP over LAN
6. Conveys data between points
8. Pipe diameter
9. Number of 802.11a nonoverlapping
channels
11. Receive/send radio signal
13. Extensible Authentication
Protocol
15. End of the link that responds
14
17. Amount of data sent in a
given time
18. Manages addressing and
protocol information
21 109 Hz
22. Only Wi-Fi Power Play
24. Supersedes WEP for 802.11
26. Contiguous frequencies
27. Opposite of transmitter
22
11
2
20
1. Highest performing access
device
3. Packet requesting information
4. Xirrus language
5. Circuitry to interpret and
execute
7. Path for signals
10. Fragment of data
12. Specification implementing
TKIP and AES
3
8
10
14. End of link initiating EAP
authentication
15. Type of medium in 802.11
16. Number of 802.11b/g nonoverlapping
channels
19. One-million cycles per second
20. Rate at which a repeating
event occurs
23. Standard for port-based
access control
25. Institute of engineers

2
Wi-Fi Authentication Demystified
Contents
Introduction .............................................................................3
The History of Authentication .....................................................4
Authentication Framework .........................................................5
Wireless Infrastructure ..............................................................7
Roaming and Authentication ......................................................9
Recommendations ..................................................................10
Leading Architecture ...............................................................11
About Xirrus ...........................................................................11
©2008 Xirrus, Inc. All Rights Reserved.

Companion Guide
Introduction
Authentication is a critical part of any network security policy. Authentication validates the identity of a user
or device, which is an important point as most people only look at authentication as authenticating the
client. When using a mutual authentication scheme, not only is the client authenticated, but so is the network
itself. This process allows the first device to authenticate the second and the second device to authenticate
the first. Initial wireless authentication used a wireless encryption method, known as WEP to provide the
authentication. The idea being that if both sides had a common encryption key it would serve as a way to
provide proper authentication. However, WEP was cracked and as a result it was no longer considered to be
sufficient as an authentication or encryption method.
The overall goal of Wi-Fi authentication is to ensure that an authorized device does not connect to unauthorized
access devices, such as a rogue AP. Rogue APs are unauthorized devices that have been detected in a
network. Rogues can be either benign, such as neighboring APs or newly added devices or a threat when
added to the network for malicious reasons. These rogue APs can create numerous issues for the network,
for example:
1.
2.
An attack called man-in-the-middle can occur in which the rogue insert themselves between authorized
devices and collect information and credentials from the user and the network.
An attack called replay-attack in which a valid data transmission is maliciously or fraudulently repeated
or delayed by the attacker. These attacks can be designed to steal information or effect the normal
operation, such as a denial of service attack.
Typical Wi-Fi Infrastructure
In a typical Wi-Fi
infrastructure, stations
associate to an Access
Point. The Access Point
is the Authenticator and
interfaces with the
Authentication Server to
validate the stations
identity and then allow
access to the network.
Wireless Stations
(Supplicant)
Ethernet
Switch
Authenticator Authenticator
©2008 Xirrus, Inc. All Rights Reserved. 3
Router
Authentication
Server

4
For these reasons and more not listed, it helps to hide the users’ identity from being exposed from a sniffer or
other type of eavesdropper on the network. There are additional benefits to authentication, such as encryption
key management, which automatically expires user passwords and forces them to change credentials, like
user name and password on a regular basis. Authentication is critical for protecting corporate and personal
information, scaling and managing large groups of users at multiple locations normally requires the use
of dynamic authentication process. In addition to just authorizing access to the network it also provides
accounting and auditing information of every connection occurring in the network. All of this is extremely
important in proving compliance with regulations such as HIPPA and PCI. Many forms of Authentication also
allow for extended control over end-user access, such as time-of-day or restricted guest-access policies.
The History of Authentication
Most people are familiar with RADIUS, which stands for Remote Authentication Dial-In User Service and has
been around since the days of dial-up network access. The RADIUS server sits on the wired network and
completes the process of authentication. The RADIUS service has three components: The authentication
server, such as Microsoft’s IAS. The RADIUS client, in the wireless world this is the AP or the WLAN Switch and
the Supplicant. The supplicant is the Wi-Fi client to be authenticated. The supplicant forwards authentication
information to the RADIUS client, which in turns forwards this information to the RADIUS server. The server will
authorize or deny access to the network. In addition the RADIUS server may return configuration information
to the AP, such as placing the Wi-Fi user in a specific VLAN.
RADIUS
RADIUS (RFC 2138) defines the backend authentication process between the Authenticator and Authentication Server. RADIUS Attributes carry
specific authentication, authorization, information and configuration detail for the Access request and response types.
Code
(1 Byte)
Identifier
(1 Byte)
Length
(2 Bytes)
Value Description
0 Access-Request
2 Access-Accept
3 Access-Reject
4 Accounting-Request
5 Accounting-Response
11 Access-Challenge
12 Status-Server (experimental)
13 Status-Client (experimental)
255 Reserved
Authenticator
Field contains
challenge text
and MD5
hashed
responses
(passwords)
Authenticator
(16 Bytes)
Example Attributes include:
– User Name (Type Field = 1)
– Password (Type Field = 2)
Items such as which VLAN the user is to be
assigned to or what wireless user group policies
to use can be defined by the use of Vendor
Specific Attributes (VSAs) (Type Field = 26).
...
Attribute 1 Attribute ...N
Type
(1 Byte)
Values=
1 to 63
Length
(1 Byte)
Value
(1 or
more Bytes)
Attribute Field
A RADIUS server can also access things like an active directory service or other directory service on the back
end of the network to enforce policies. This allows RADIUS to be implemented without having to recreate
account information that may already exist in another directory.
©2008 Xirrus, Inc. All Rights Reserved.

In 1999, the 802.11standard was adopted which contained a couple of methods for basic authentication. One
was called “open authentication” which was not really authentication at all. Open authentication basically
allows Wi-Fi association to all 802.11 compliant devices. A second method was WEP and stood for Wired
Equivalent Privacy. This form of authentication, known as “shared key WEP authentication” allowed a shared
WEP key to be used for authenticating users to access the network. In May 2001, an IEEE Task Group known
as 802.11i began work on new enhanced security standards for 802.11. By August 2001, WEP was cracked
creating a large security breach and adversely impacting the adoption of Wi-Fi in the enterprise. At this point
WEP became known as Weak Encryption Protocol.
Needing improved security and not being able to wait for the developing IEEE standard, the Wi-Fi Alliance
announced in October 2002 a new security standard called WPA, which stands for Wi-Fi Protected Access.
It was a security enhancement based on the work being done by the IEEE 802.11i Task Group. WPA was
quickly put in place to correct the problems with WEP. This was accomplished via the implementation of an
authentication framework and stronger encryption modes, and the 802.11i addendum was finally ratified.
Authentication Framework
There were three basic building blocks that led up to 802.11i. First, there was EAP, which stands for Extensible
Authentication Protocol. EAP is a framework for authentication, allowing for a number of authentication
methods to be used.
EAP/EAPOL Frame Format
EAPOL (EAP Over LAN) is used by 802.1X to encapsulate the EAP protocol. The EAP protocol defines a number of methods for authentication.
EAPOL Packet
Destination MAC
(6 Bytes)
Value Description
1 Request
2 Response
3 Success
4 Failure
Source MAC
(6 Bytes)
Code
(1 Byte)
EAP Packet
Ethertype
Code
(2 Bytes)
0x888e
ID
(1 Byte)
Protocol
Version
(1 Byte)
1
Length
(2 Bytes)
# of Bytes
Packet
Type
(1 Byte)
Type
(1 Byte)
Body
Length
(2 Bytes)
# of Bytes
Data
Packet Body
Value Description
1 Identity
2 Notification
3 NAK
4 MD5 Challenge
Value Description
0 EAP Packet
1 EAPOL Start
2 EAPOL Logoff
3 EAPOL Key
4 EAPOL Alert
5 One Time Password
6 Generic Token Card
13 TLS
©2008 Xirrus, Inc. All Rights Reserved. 5

6
One of those methods is 802.1x, a port level authentication method originally designed for wired networks.
802.1x, EAP, and additional encryption modes TKIP and AES were all components of the 802.11i standard.
802.11i Security
802.11i is the official security standard for 802.11 Wireless LANs as ratified by the IEEE in 2004. Its operation consists of 4 primary phases
to establish secure communications. Phase 2 and portion of Phase 3 are addressed in this poster; Phase 4 and a portion of Phase 3 are
addressed in the companion Wi-Fi Encryption poster.
Station
Phase 1
Phase 2
Phase 3
Phase 4
Security Discovery/Negotiation
802.1X Authentication
Key Management RADIUS Key Distribution
Data Confidentiality and Integrity
Authenticator Authentication
Server
Additionally, mutual authentication and key exchange processes were added to the standard. All these
additions allowed the authentication process to scale and also provided for dynamic key creation and
updating, providing faster client authentication and roaming.
©2008 Xirrus, Inc. All Rights Reserved.

802.11i Packet Exchange
802.11i Packet Exchange describes the wireless authentication process, and begins with a supplicant
(the wireless station) associating to the access point and initiating an 802.1X exchange.
Authentication
Supplicant Authenticator
Server
Probe Request
Probe Response
Authentication Request
Authentication Response
Association Request
Association Response
EAPOL-Start (Start Process)
EAP-Request (Identity)
EAP-Response (Identity)
3. The users identity is passed to the
Authenticator and then forwarded to
the Authentication Server.
EAP-Request (Challenge)
4. An EAP packet with challenge text is
sent from the Authentication Server.
EAP-Response (Credentials)
5. An EAP packet with the encrypted
challenge text is sent back to
the Server.
EAP-Success
EAPOL Key 1
EAPOL Key 2
EAPOL Key 3
EAPOL Key 4
EAP-Logoff
Port Unauthorized
Port Authorized
1. The authentication process starts with
a virtual port in the Array set to
“unauthorized” such that only
authentication protocols are forwarded.
2. The station starts the authentication
process with an EAPOL Start message.
RADIUS Access Request
RADIUS Access Challenge
RADIUS Access Request
RADIUS Access Accept
6. If the station has the correct credentials,
a RADIUS Access Accept packet is returned,
which also includes a Master Key used by
WPA to generate unique per user encryption
keys (see Wi-Fi Encryption Poster).
7. 802.11i adds 4-way handshake to
generate and verify encryption keys for
the supplicant station (see Wi-Fi
Encryption Poster).
8. Upon successful authentication and key
exchange, the Access Point allows traffic to
be forwarded from the station to the network.
Port Unauthorized
Wireless Infrastructure
Now let’s talk about how authentication works in a
Wi-Fi network today. At a high level, the Wi-Fi client
associates to an AP, also known as the authenticator.
The station then sends an authentication request
to the authenticator. The authenticator is designed
so that prior to proper authentication all standard
packets are discarded. While in this state the AP will
only forward EAP packets. These packets are allowed
to transverse to the wired side in order to reach the
authentication server. Next, the clients and server use
the EAP packets to complete a four-way handshake.
The result of which is the authenticator and client
define session keys, and finally, the authenticator
moves its port into an authorized mode and normal
access to the network ensues.
As mentioned before, the 802.1x framework uses
EAP to exchange information; however there are
several types of EAP methods used today. Seven of
these types are approved for interoperability by the
Wi-Fi Alliance. The first is EAP-TLS, which requires a
server-site certificate and a client-site certificate for
credentials. The second most popular type is EAP-TTLS
whereby a user must have a server-site certificate, and
uses just a user name and password. Typically a thirdparty
supplicant is needed for this method.
Wireless Authentication
Framework
Wi-Fi Authentication (802.11i) is built on top of 802.1X and EAP.
EAP (RFC 3748)
Extensible
Authentication
Protocol
IEEE 802.1X
Wired port-based
authentication
uses EAP and EAPOL
as the underlying
authentication protocol
IEEE 802.11i
wireless authentication
extends 802.1X to a
wireless network and
generates a Master Key.
The Master Key is used by
the Access Point and station
to derive per session keys
©2008 Xirrus, Inc. All Rights Reserved. 7

8
The next type, and probably most commonly deployed is EAP-PEAP, which stands for Protected EAP. In this
method, the server-site certificate is required, the client-site certificate is optional, and a standard user name
and password is used. Advantage of PEAP is it can leverage user name and passwords already defined in
Windows Active Directory. Another type commonly seen is EAP-PEAP-GTC, which stands for Generic Token
Card. It is a physical token that is used in the authentication process. Likewise, EAP-SIM uses a SIM card, a
Subscriber Identity Module, for a GSM mobile handset. The last two types are Cisco authored and proprietary
protocols. One is LEAP (Lightweight Extensible Authentication Protocols), which was widely used early on,
but is not recommended anymore due to a dictionary attack that can be easily harnessed against it. LEAP
did not require certificates on both sides of the link as only a password was needed. To fix LEAP, fast-EAP
was deployed. It is still password based and also does not require certificates on either side of the link.
EAP Types
Server Side Client Side User Credentials User Database Security
EAP Type Description Certificate Certificate Used Access Issues
EAP-PEAP Protected EAP Required Optional Windows XP, 2000, CE, Windows Domains,
(widely used) Username/Passwords and Active Directory
other 3rd party Supplicants
EAP-TLS EAP with Transport Layer Security Required Required Certificate Windows Domains, User Identity
Active Directory, Exposed
Novel NDS OTP
EAP-TTLS EAP with Tunneled Transport Required None Password Windows Domains,
Layer Security Active Directory
EAP-PEAP-GTC Protected EAP with Generic Required None Windows, Novell NDS,
Token Card One Time Password Token
EAP-SIM EAP – Subscriber Identity Module Required None Subscriber Identity Module
(SIM). Uses SIM card found in (SIM Card)
GSM mobile phone handsets
LEAP Lightweight EAP. Not recommended None None Password Windows Domains, Dictionary Attack
due to dictionary attacks Active Directory User Identity Exposed
Fast EAP Cisco EAP based on PEAP None None Password Windows Domains,
Active Directory
RADIUS also has the ability to use what are called VSA, or vendor specific attributes. By using the VSA’s
information can be passed from the RADIUS server to the authenticator, or AP. This information can be used
for assigning a used group or VLAN assignment so policies can be applied to the end user connecting to the
network.
Another type of authentication available is web-based authentication, typically used to allow temporary users
or guests to gain restricted access to the network. This process is used in many places, but commonly seen
in hotels. When a user opens their web browser, they are redirected to a webpage where they can enter a
user name and password. Authentication is then granted normally from RADIUS server and the session can
continue or their connection may be redirected back to another webpage. Web pages can be hosted directly
in the Xirrus Wi-Fi Array where it can be implemented on a per SSID-basis.
©2008 Xirrus, Inc. All Rights Reserved.

Web-based Authentication
Web-Based Authentication eliminates need to configure client software but requires manual entry of username/password. It is not used to
configure an encrypted wireless link.
1. A user associates to an open Wi-Fi network
2. User’s web session is captured and redirected to a
landing page in the Access Point
3. The user is prompted for a username and password
4. The Access Point uses these credentials to
authenticate the user with the Authentication Server
5. Access is granted and the user’s original URL
is reloaded
Roaming and Authentication
Authenticator Authentication
Server
Captive Portal Original URL
Clients using 802.11i can pre-authenticate with multiple access points at the same time providing for faster
roaming methods across the network. The security association generates something called the Pairwise Master
Key (or PMK) which is the result of the four-way hand shake discussed before. The PMK can be cached by the
client and network AP anticipating the fact that the client will roam from one AP to another. When a client
attempts to roam to another AP, they can request the PMK ID they were using before or that they have in their
cache. As a result of this cached key the full 802.1x exchange is not required, thus saving considerable amounts
of time. This feature is fully supported by Xirrus Wi-Fi Arrays and is crucial for things like voice roaming time
needs to be as short as possible.
802.11i Fast Roaming
Supplicant
Stations can
pre-authenticate
with new Access Point
prior to roaming
Pre-
Authenticate
then
Roam
PMK Caching
Authenticator
Authentication
Server
©2008 Xirrus, Inc. All Rights Reserved. 9
Ethernet
Switch
Access Points can share Pairwise Master Keys (PMK)
in advance of stations roaming to them
Stations can use existing PMK when roaming to a new
Access Point that has pre-shared it with prior Access Point
If Access Point has PMK, only the 4-way handshake needs
to take place, otherwise full 802.11X exchange takes place

10
Recommendations
Our recommendation for authentication is as follows:
Use 802.11i and WPA-2 for the strongest security that’s available today, as well as PEAP with MS-chap
for easiest administration where no client site certificates are needed. It uses the built-in Windows user
name and password that the user is already assigned for the domain.
Use an authentication server to enforce access policies like time-of-day access. It also notifies what
resources VLAN users have access to on the wired network. Web-based authentication is a great way to
allow easy access on to the Wi-Fi network.
Lastly, replication and availability of your authentication server is important. RADIUS servers need to be
capable of handling the peak loading in terms of the number of users that authenticate to it at the same
time. Also the location of your RADIUS server should not be located near a slow wan link or a remote site
where it might take time and latency before the authentication process completes.
©2008 Xirrus, Inc. All Rights Reserved.

Leading Architecture
Xirrus planned for the success of Wi-Fi by developing
an award-winning Wi-Fi architecture powerful
enough to handle high-bandwidth applications today
and modular enough to be upgraded for future
enhancements.
With the Wi-Fi Array, Xirrus delivers the only ‘Power
Play’ architecture in Wi-Fi networking with the
most bandwidth and coverage per cable drop in
the industry. Xirrus Wi-Fi Arrays deliver up to 8x
the bandwidth of a single access point and are
compact, easy-to-install, ceiling-mounted devices.
No other current-generation Wi-Fi technology can
deliver the bandwidth or throughput of Xirrus Arrays
because they are limited to 2 radios producing only
108Mbps of shared bandwidth.
Xirrus Wi-Fi Array
Multiple Wi-Fi
Radios Produce
864Mbps of
Bandwidth
Redundant Gigabit Ethernet Uplinks
High Gain
Directional
Antennas
Increase
Range
©2008 Xirrus, Inc. All Rights Reserved. 11
Sectored
Antenna
Sectored
Antenna
Sectored
Antenna
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Sectored
Antenna
Sectored
Antenna
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi Controller
Sectored
Antenna
Ethernet
Switch
Sectored
Antenna
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Sectored
Antenna
Sectored
Antenna
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Wi-Fi
Radio
Sectored
Antenna
Sectored
Antenna
Sectored
Antenna
50% Sector
Overlap
No other current-generation Wi-Fi technology can deliver
the bandwidth or throughput of Xirrus Wi-Fi Arrays.
By integrating these key components: the Wi-Fi controller, Gigabit Ethernet Switch, Gigabit uplinks, multiple
access points, sectored antenna system, Wi-Fi stateful firewall and Wi-Fi threat sensor into a single device,
Xirrus Arrays are able to provide a centrally-managed platform that delivers unparalleled range, client capacity
and performance, along with better RF management and roaming for voice, video and data applications — all
in a single device that is fully upgradeable to 802.11n.
About Xirrus
Xirrus, Inc. is a privately held firm headquartered in Westlake Village, California. Founded by the same team
that created Xircom (acquired by Intel in 2001), Xirrus has developed the next generation in enterprise
wireless LAN architectures centered around the award-winning Array.
Backed by leading venture capital firms U.S. Venture Partners and August Capital, Xirrus brings a proven
management team and patented approach to delivering the performance, scalability and security needed to
deploy a true wireless extension of the wired Ethernet network capable of delivering Triple Play (voice, video,
data) enablement.

15
S
H
A
R
E
D
M
E
D
I
U
M
9
T
U
W
W
P
A
24
8
Wi-Fi Authentication Demystified
Crossword Puzzle—Answer Key
23
E
P
8
0
2
1
X
C
O
N
T
R
O
L
L
E
R
2
18
T
I
F
1
1
A
R
R
A
Y
21
27
C
R
G
1
R
-
I
E
E
E
T
C
S
H
A N T
H
T H R
E
A M
H
E
H Z
17
19
25
16
26
S
E
4
X
I
R
R
I
A
N
5 6 7
12 13 14
Across Down
2. EAP over LAN
6. Conveys data between points
8. Pipe diameter
9. Number of 802.11a non-overlapping
channels
11. Receive/send radio signal
13. Extensible Authentication Protocol
15. End of the link that responds
17. Amount of data sent in a given time
18. Manages addressing and protocol
information
21 109 Hz
22. Only Wi-Fi Power Play
24. Supersedes WEP for 802.11
26. Contiguous frequencies
27. Opposite of transmitter
14
O
P
I
G
E
U
E
V
22
N
E
E
G
X
C
E
11
A
A
A
U
T
H
E
N
T
I
C
A
T
O
R
2
E
L
N
P
P
R
R
20
A
T
U
F
R
E
Q
U
E
N
C
Y
3
P
R
O
B
E
R
E
Q
U
E
S
T
8
U
M
O
A
N
S
L
N
N
10
D
P
A
C
K
E
T
W I D T
1. Highest performing access device
3. Packet requesting information
4. Xirrus language
5. Circuitry to interpret and execute
7. Path for signals
10. Fragment of data
12. Specification implementing TKIP
and AES
14. End of link initiating EAP
authentication
15. Type of medium in 802.11
16. Number of 802.11b/g
non-overlapping channels
19. One-million cycles per second
20. Rate at which a repeating event
occurs
23. Standard for port-based access
control
25. Institute of engineers
C
H
A
N
N
E
L
Xirrus, Inc.
www.xirrus.com
sales@xirrus.com
370 North Westlake Blvd. Suite 200
Westlake Village, California 91362, USA
1.800. 947.7871 Toll Free in the US
+1. 805.497.0955 Corporate Office
+1. 805.497.7871 Sales
+1. 805.449.1180 Fax
Copyright© 2008, Xirrus, Inc. All Rights Reserved. Xirrus and the Xirrus logo
are trademarks of Xirrus, Inc. All other trademarks belong to their respective
owners. Protected by patent #US D526,973 S. Other patents pending.