Wi-Fi Security

Now that I have discussed the three ways to
secure your wireless connection, I’ll dive deep into
the details of Wi-Fi security. A secure network should (ideally) have
the following:

Authentication

This
is the process of verifying the identity of a user and making sure
that she is who she claims. When you log in to your Windows computer,
you are being authenticated via the username and
password. In a Wi-Fi network,
authentication comes into play when the access point has to determine
whether a machine can connect to it.

Authorization

This is the
process of allowing or denying access to a specific resource. You may
be authenticated as a user, but you may not be authorized to use
certain feature perhaps due to your user role (such as Guest, User,
Power User, Administrator). For example, suppose you are at a
wireless hotspot and have used up your allotted connection time: the
network knows who you are, but won’t authorize you
to access the Internet until you pay for more minutes.

Confidentiality

This ensures
the privacy of information that is being transmitted. Only an
authorized party (such as the recipient of an email message) can see
the information being transmitted. In a Wi-Fi network,
confidentiality is supported by protocols such as WEP, WPA, and
802.1X, which encrypt the data that moves through the air.

Integrity

This ensures that
the information that you have transmitted has not been tampered with
en route to its destination.

Authentication, authorization, confidentiality, and integrity are
also addressed by other systems on your network, just as they are on
a wired network:

• Passwords can be used to authenticate users when they log into a file
  server.

• User roles control which files a given user has access to.

• Web and email communications can be secured with SSL.

• Network traffic can be tunneled through a VPN.

In Wi-Fi, there are two main authentication schemes (see Figure 4-19):

• Noncryptographic

• Cryptographic

Figure 4-19. Authentication schemes

Under the noncryptographic scheme, you can authenticate in two ways:
one without an SSID and one with an SSID. If a
wireless network allows clients to connect to it without specifying
an SSID, it is known as Open System
Authentication
.

For Closed System
Authentication
, two methods are
possible: one using an SSID and one using a cryptographic key.

In an Open System Authentication scheme, there is no encryption
performed on the packets transmitted between the client and the
access point. The client does not need any SSID to join a network.
This is the simplest mode as the configuration is straightforward and
does not require any administration.

In the Closed System Authentication scheme, a client needs to specify
an SSID that is identical to that specified by the access point in
order to join the network. In addition, a shared key may also be used
to encrypt
the data packets
transmitted between the client and the access point. In 802.11, the
encryption method is known as Wired Equivalent Privacy (WEP), which
we discuss in greater length in Section 4.5.1, next.

To get
connected to a network in a closed
system, a client must fulfill one or several of the following
criteria:

1. The SSID of the client must match that of the access point. If a
   wireless access point has SSID broadcast turned on, your Windows XP
   computer should be able to detect its presence and allow you to
   connect to it. If the SSID broadcast is turned off, then the client
   must manually enter the SSID in order to associate with the access
   point. Getting associated with the access point is the first step in
   joining a network. Using an SSID to prevent people from accessing
   your network is not effective, since the SSID is often guessable and
   can be “sniffed” by network tools
   such as AiroPeek (more on this later).

   Tip

   There are actually two steps to gaining network access. The first is
   associating with the access
   point

   , which means that the access point is
   willing to talk to your machine. The second step is joining
   the network

   , which usually means that your machine has
   been assigned an IP address and can talk to other hosts on the
   network. Unless I need to specifically discuss one or the other of
   these steps, I’ll say “connected to
   the wireless network,” which means that the client
   has been associated to the access point and joined the network.

2. Some access points use MAC address filtering to prevent
   clients from associating with them. You can enter a list of MAC
   addresses that you would allow (or deny) association with the access
   point (this is usually done through a web-based configuration
   interface on the access point). Even if a client has the correct
   SSID, if its MAC address is not listed in the allow-list of the
   access point, it cannot be associated with the access point. Again,
   using MAC address filtering to prevent unauthorized access to the
   network is not foolproof — an unauthorized user can easily
   change his network card’s MAC address to that of an
   authorized client.

3. If WEP encryption is used on a wireless network, the client must
   specify the same WEP key as entered in the access point. Using a WEP
   key protects the data that is exchanged between the client and the
   access point. It also has the side effect of preventing unauthorized
   access to the network since a client needs the WEP key to encrypt and
   decrypt the packets exchanged. However, it has been proven that WEP
   is not secure and the WEP key can easily be recovered using freely
   available tools.

Wired Equivalent Privacy (WEP)

The main goal of WEP is to provide
confidentiality
of data packets. One secondary function of WEP is to provide
authorization to a
wireless network. This is, however, not the originally intended
design goal of WEP (but see the section on 802.1X later in this
chapter). Although WEP was initially designed to safeguard the
confidentiality of the data in a wireless network, it has been proven
to be insecure. To understand how WEP compromises your data in a
wireless network, let’s first understand how WEP
works.

WEP uses the RC4 stream cipher algorithm (search
the FAQ at http://www.rsasecurity.com/rsalabs/faq for
“RC4”). It takes in a key and generates a larger pseudorandom bit
sequence (the key stream
) that serves as an encryption key. The
key stream is then XOR’ed (a logical operation that
returns true if one, but not both, of two binary values are true)
with the original message (the
plaintext
)
to produce the ciphertext.

When the recipient receives the encrypted stream (the
ciphertext
),
it uses the shared key (to produce the same pseudorandom key stream)
and performs an XOR function to derive the original message.

Here is how WEP uses RC4 to encrypt network communications:

1. Before encryption, the packet is run through an
   integrity check algorithm to
   generate a checksum. This is to prevent the message from being
   tampered with.

2. The 40-bit WEP key is then combined with the 24-bit
   Initialization Vector (IV) to form a
   64-bit key.

3. RC4 then uses the 64-bit key to generate a keystream equal to the
   length of the plaintext to be encrypted (including the checksum
   generated by the integrity check algorithm in step 1).

4. The keystream is then XOR’ed with the plaintext to
   generate the encrypted packet. The IV is also appended in the header
   of the encrypted packet to create the ciphertext.

This encryption process is shown in Figure 4-20.

Figure 4-20. How WEP works

There are
many security concerns that have been raised with respect to WEP. The
first attack on WEP was identified by researchers Scott
Fluhrer,
Itsik Mantin,
and Adi Shamir.

Here are some of the more important security concerns regarding WEP:

• The use of a
  shared static key
  is a major concern as everyone uses the same static key to secure his
  communications. As soon as the key is made known, the network is no
  longer secure. Some access points use a passphrase to generate keys,
  which makes it easier to guess the key, since people tend to use
  familiar terms for passphrases.

Tip

Distributing WEP keys in a large network is not feasible. Imagine
trying to obtain a WEP key at the airport, or hassling a busy barista
at Starbucks for one.

• The IV is only 24 bits in length, which means the same IV is reused
  many times over. This is especially true in a busy access point. Most
  network cards reset the IV to 0 when it is initialized, and increment
  the IV by 1 for each subsequent packet. (Although there are over 16
  million different IVs, in practice you should begin to see the IVs
  repeat after more than 5000 packets are transmitted.) If an access
  point transmits packets of 1500 bytes in length, a 7 MB download
  would cause the same IV to be used again. It has been shown that when
  two eavesdroppers intercept two ciphertexts encrypted with the same
  keystream, it is possible to obtain the XOR of the two plaintexts.
  Over time, when more ciphertexts encrypted with the same keystream
  are collected, it is possible to recover the plaintext.

• If the keystream is recovered by an eavesdropper, forging a packet is
  easy, since you now have the keystream to generate the ciphertext.
  This can facilitate man-in-the-middle attacks, when a hacker may
  forge the identity of a legitimate user, and intercept and reroute
  the data transmitted.

• Due to the export regulations of the United States, the 802.11
  standard called for 40-bit WEP only. Most vendors introduced longer
  key length for their products, making their products proprietary and
  often not interoperable. Even so, since WEP is not a well-designed
  cryptographic system, having extra key length does not make your
  communications more secure.

Tip

Many vendors have claimed that their products support longer
encryption keys such as 128 or 256 bits (which promises to be more
secure). This is not technically correct. Because the 128-bit or
256-bit designation is inclusive of the 24-bit IV, the effective key
lengths are 104 and 232 bits, respectively. However, some vendors do
have products that support 152 bits (128 bit keys + 24-bit IV).

Figure 4-21 shows how you can enable WEP in a
Linksys wireless router. For a
64-bit WEP key, only 40 bits (or 5 bytes) are specified by the user
(since 24 bits are used by the IV). So for a 64-bit key, you need to
enter 10 hexadecimal characters (since 2 hexadecimal characters make
up 1 byte). For a 128-bit WEP key, 26 hexadecimal characters are
needed.

Figure 4-21. Setting WEP keys in the Linksys access point

Figure 4-21 also shows the Linksys wireless access
point using a passphrase to generate four WEP keys. You can also
manually set any of the four keys as your WEP key without using a
passphrase. After you set the keys, click Apply.

Dynamic WEP Keys

Some vendors, like
Cisco and 3COM, have implemented a

dynamic WEP key in response to
WEP’s weaknesses. Per user session,
Cisco’s software generates a dynamic WEP key that is
not shared with other users.

However, you must use access points, wireless cards, and adapters
from the same vendor for dynamic WEP keys to work. For more
information on Cisco’s Dynamic WEP keys, visit
http://www.cisco.com/warp/public/cc/pd/witc/ao350ap/prodlit/1281_pp.pdf.

Tip

The wireless client enters one of the four keys specified in the
Linksys access point and not the passphrase itself. The passphrase is
used to simplify the task of generating
the WEP key.

802.11i

A longer-term solution to resolve WEP’s inadequacies
lies in the hands of the
IEEE workgroup TGi (http://grouper.ieee.org/groups/802/11/Reports/tgi_update.htm)
when they complete the 802.11i specifications at the end of
2003.

The 802.11i specifications will address the items in the list shown
next.

Use of 802.1X for authentication

The 802.1X specification is a framework for mutual
authentication between a client and
the access point. It may also use a RADIUS-based authentication
server and one of the
Extensible
Authentication Protocols (EAP) variations. 802.1X uses a new key for
each session; hence it replaces WEP’s static key.

Use of the Temporal Key Integrity Protocol (TKIP)

TKIP will be used as a
short-term solution to WEP’s flaws. It uses 128-bit
dynamic keys that are utilized by different clients. Because of the
changing keys, intruders would not have time to collect enough
packets to compromise the security scheme.

Use of Advanced Encryption Standard (AES)

The full implementation of
802.11i will utilize the AES encryption system for enhanced
encryption in access points. However, use of AES requires changes in
the chipsets used in wireless devices; thus, at the time of this
writing, no wireless device supports AES.

The 802.11i specification is tentatively called WPA2. See the next
section for more details.

Wi-Fi Protected Access (WPA)

While the industry is waiting for the 802.11i
specification to be ratified, the Wi-Fi Alliance has addressed the
present need for secure wireless communication by coming out with the
Wi-Fi Protected Access (WPA).

Tip

The WPA is also known as WPA1, while 802.11i is known as
WPA2.

WPA is a subset of the 802.11i standard and will be forward
compatible with it. The key components of WPA are:

802.1X

802.1X is a port-based authentication
mechanism
. See the
next section for a detailed discussion of 802.1X.

TKIP technologies

TKIP provides data encryption enhancements including a per-packet key
mixing function, a
Message Integrity Check (MIC)
called Michael, an extended Initialization Vector (IV) with
sequencing rules, and a re-keying mechanism. In a nutshell, TKIP
addresses WEP’s limitations by having dynamic keys
coupled with a much longer IV (which means that the chances of
reusing the same IV within a period of time are reduced).

So how does
WPA
differ from WEP? Table 4-1 shows the quick
comparison.

Table 4-1. Comparing WPA to WEP

WPA

WEP

Key length

128-bit

40-bit to 232-bit

Key type

Dynamic key; per-user, per-session, per-packet keys

Static shared key; used by everyone in the network

Key distribution

Automatic key distribution

Each user must type in the key

Authentication

Uses 802.1X and EAP

Uses WEP key for authentication; flawed

WPA and Windows XP

Microsoft
has recently released the WPA Wireless
Security Update in Windows XP (http://support.microsoft.com/?kbid=815485).

The features of the security update are:

• 802.1X authentication

• WPA key management

• TKIP

• Michael

• AES support

However, before you start to use WPA, verify that your wireless card
and access point both support it. Check with your vendor to see
whether WPA can be supported via firmware upgrades.

802.1X Authentication

The 802.1X specification is a port-based network access control
mechanism: when a client is authenticated, the port is granted
access; if not, access to the port is denied. Although 802.1X was
originally designed for Ethernet networks, it can be applied to
wireless networks as well.

Tip

In a wireless LAN, a port is simply the connection between a client
and an access point.

This is how 802.1X works (see Figure 4-22):

1. The Supplicant (the

   client that wants to access a network
   resource) connects to the Authenticator (whose
   resource is needed).

2. The Authenticator asks for credentials from the Supplicant and passes
   the credentials to the Authenticating Server.

3. The Authenticating Server authenticates the
   Supplicant on behalf of the Authenticator.

4. If the Supplicant is authenticated, access is then granted.

Figure 4-22. Authenticating a Supplicant in 802.1X

Note that before the authentication is performed, all the
communications go through an uncontrolled
port

.
After authentication, the controlled port is
used.

For the Authentication Server to authenticate the Supplicant, the
Point-to-Point Protocol Extensible Authentication Protocol (EAP) is
used. EAP supports multiple authentication mechanisms and was
originally developed for PPP.

In a wireless network, a wireless client needs to connect to an
access point; in this case, the wireless access point is the
Authenticator. The Authenticator can maintain a database of users and
their respective
passwords. However, this is
a huge administrative task, especially in a large network. So an
access point can be connected to a
RADIUS (Remote Authentication
Dial-In User Service) server, which will maintain the database of
users and perform authentication on behalf of the access point. This
is as shown in Figure 4-23.

Figure 4-23. Using 802.1X authentication in a wireless network

Using a RADIUS server takes care of the authentication aspect of
security only. What about
confidentiality? Packets
traveling between the wireless clients and the access point must be
encrypted to ensure confidentiality.

When a client is validated at the RADIUS server, an authentication
key is transmitted to the access point. (This key is encrypted; only
the access point can decrypt it.) The access point then decrypts the
key and uses it to create a new key specific to that wireless client.
That key is sent to the wireless client, where it’s
used to encrypt the master global authentication key to the wireless
client. To address WEP’s shortcoming of a fixed key,
the access point will generate a new master authentication key at
regular intervals.

802.1X’s Support in Access Points

At the time of this writing, support of 802.1X is mostly limited to
enterprise-level
access points. However, if
you are lucky, your access point vendor may have firmware that allows
your access point to be upgraded to support 802.1X authentication.
The D-Link 900AP+ supports 802.1X after a
firmware upgrade. Best of all, it is a consumer access point that is
affordable (under $100 in the U.S.).

Types of EAP

There are many variants of
Extensible Authentication Protocol (EAP).
Here are some that you may come across in wireless security
literature:

EAP-MD5

EAP-MD5 uses the
challenge/response method to allow a server to authenticate a user
using a username and
password. MD5 does not provide mutual
authentication and is vulnerable to an offline dictionary attack.

EAP-TLS (EAP-Transport Layer Security)

EAP-TLS is based on X.509 (an ITU
standard specifying the contents of a digital certificate)
certificates. It is currently the most commonly used EAP type for
securing wireless networks. However, EAP-TLS requires the use of PKI
(Public Key Infrastructure), which is not feasible to be implemented
on small networks.

PEAP (Protected EAP)

To
counter the complexity of using EAP-TLS, PEAP was proposed as an
alternative. PEAP uses a server-side certificate to allow the
authentication of the server. It creates an EAP-TLS tunnel and then
uses other authentication methods over the tunnel. EAP methods such
as MD5, MS-CHAP, and MS-CHAP v2 are supported. PEAP was proposed as
an IETF standard by Microsoft, Cisco, and RSA.

EAP-TTLS (EAP Tunneled TLS)

EAP-TTLS is similar to PEAP. It creates a
tunnel between the user and the RADIUS server. It supports EAP
methods such as MD5, MS-CHAP, and MS-CHAP v2.

LEAP (Lightweight EAP)

LEAP
is Cisco’s proprietary version of EAP, which works
mostly with Cisco’s wireless cards, RADIUS servers,
and access points.

MS-CHAP v2 (Microsoft Challenge-Handshake Authentication Protocol Version 2)

Originally
designed by Microsoft as a PPP authentication protocol, MS-CHAP v2 is
a password-based, challenge-response, mutual authentication protocol
that uses the Message Digest 4 (MD4) and Data Encryption Standard
(DES) algorithms to encrypt responses. MS-CHAP v2 is now an EAP type
in Windows XP.

Windows XP (Service Pack 1) supports PEAP and EAP-TLS. Prior to
Service Pack 1, EAP-TLS and EAP-MD5 are supported. Figure 4-24 shows the various layers of EAP and their
relationships to
802.1X.

Figure 4-24. The variants of EAP and their relationships to 802.1X and 802.11

Using 802.1X in Windows XP

This section
explains how to implement 802.1X
authentication using PEAP and MS-CHAP v2 authentication methods in
Windows XP. Using PEAP with MS-CHAP v2 authentication allows users to
be authenticated using a username and
password. This is much
easier to administer than PEAP with EAP-TLS, which requires
certificates to be installed on a user’s computer.
Also, PEAP with MS-CHAP v2 requires only a server certificate to be
installed.

Tip

If you are only interested in knowing how to log in to an 802.1X
protected wireless network, you can skip to Section 4.5.4.4, later in this
chapter.

Configuring the RADIUS server (IAS)

For
this
section, I assume you have the following:

• Windows 2000 Server SP3 or greater acting as a Domain controller
  using Active Directory.

• Microsoft 802.1X Authentication Client for Windows 2000 (http://support.microsoft.com/default.aspx?scid=kb;en-us;313664)
  installed on your computer. Although the word
  “client” appears in the name, this
  is a required component for the server.

Tip

The Microsoft 802.1X Authentication Client for Windows 2000 contains
support for PEAP, which is required if you specify the use of PEAP on
the client side (this is supported in Windows XP if you install the
“Windows XP Support Patch for Wireless Protected
Access”).

Also, PEAP requires Certificate Services to be installed on your
Windows 2000. You can install Certificate Services by going to the
Control Panel and using Add/Remove Programs, then selecting
Add/Remove Windows Components.

First, you must configure the RADIUS server
(Internet Authentication Server) on
the Windows 2000 Domain Controller:

1. Launch Internet Authentication Service (IAS) by clicking on Start
   → Programs → Administrative Tools →
   Internet Authentication Service (see Figure 4-25).

   

   Figure 4-25. Registering IAS with Active Directory

2. Right-click on Internet Authentication Service (local) and select
   “Register Service in Active
   Directory”.

3. Right-click on Clients and select “New
   Client”.

4. Give a name to your client, say
   “AP” (which is your access point,
   the Authenticator).

5. Enter the IP address of the access point and check
   “Client must always send the signature attribute in
   the request”. Enter a secret key to be known by both
   the access point and IAS.

6. Right-click on Remote Access Policies and select
   “New Remote Access Policy”.

7. Give a name to your new policy, such as “Wireless
   access”. Click Next.

8. Add an attribute. Select the
   “Day-And-Time-Restriction”
   attribute and click Add.

9. Choose the time that you want the user to be allowed access to the
   network. Select all available time slots, click Permitted, and click
   OK. Click Next.

10. Select the “Grant remote access
    permission” option to allow the remote user to log
    on if he is authenticated. Click Next.

11. Click on Edit Profile… to choose the authentication type.

12. Click the Authentication tab and select the options as shown in Figure 4-26. Click OK.

    

    Figure 4-26. Selecting the EAP type to be used in IAS

13. When prompted to read the help files on the EAP types checked, click
    No.

14. Click Finish to complete the setup.

Configuring Active Directory

Once the RADIUS server is configured, you need to give access
permission to users in Active Directory.

1. Click on Start → Programs → Administrative Tools
   → Active Directory Users and Computers.

2. Click the Users item and double-click on the username that you want
   to grant wireless access to.

3. Select the Dial-in tab and select the Allow access option. Click OK.

Configuring the access point

Most consumer
access points available today do not support 802.1X authentication.
You would need to buy enterprise-level access points in order to use
802.1X authentication. Fortunately, a few consumer access points,
such as the D-Link 900AP+, support 802.1X via a
firmware download. If you own the D-Link 900AP+ access point, be sure
to check out D-Link’s web site to download the
latest firmware. (If you purchased your access point after mid-2003,
it may already have the firmware with 802.1X support).

You can administer the DWL-900AP+ access point using either a
web-based utility or the provided Access Point Manager (see Figure 4-27). You can access the Access Point Manager by
clicking Start → Programs → D-Link Airplus Access
Point → D-Link AirPlus Manager.

Figure 4-27. The Access Point Manager

To configure the DWL-900AP+ for 802.1X authentication, click on the
802.1X Setting link in the Access Point Manager (see Figure 4-27).

1. Turn on the 802.1X Function checkbox.

2. Select the length of the key and enter the information for the RADIUS
   server that you set up in the previous section.

Tip

You need to enable WEP in order for 802.1X authentication to work.

1. The port number for the
   RADIUS server is 1812. You can
   specify up to two RADIUS servers. Also, enter the shared secret key
   that you entered in IAS.

Configuring the client

The last
stage is configuring the client. This is also the stage for readers
who are trying to log on to a network that uses 802.1X
authentication.

Tip

The test computer that I used for this book was updated with the
Windows XP Support Patch for Wireless Protected Access (see
http://support.microsoft.com/?kbid=815485).
If you have not installed the patch, some of the screen elements may
differ slightly.

1. Right-click on the Wireless Network Connection icon located in the
   Tray and select “View Available Wireless
   Networks” (see Figure 4-28).

   

   Figure 4-28. Viewing the available wireless networks

2. Select the SSID of the network that you wish to connect to. In my
   case, default is the network that implements
   802.1X authentication. Select default and enter the network (WEP) key
   for this network. Turn on the checkbox “Enable IEEE
   802.1X authentication for this network” (see Figure 4-29).

   

   Figure 4-29. Selecting the default wireless network

3. Click on Advanced… to configure the settings for the selected
   network (see Figure 4-30).

   

   Figure 4-30. Configuring the default wireless network for 802.1X authentication

4. Under the Available networks section, select default and click
   Configure.

5. Click the Authentication tab (see Figure 4-31).

   

   Figure 4-31. Choosing PEAP as the EAP type

6. For EAP type, select Protected EAP (PEAP) and click Properties.

7. Select Secured password (EAP-MSCHAP v2) as the authentication method
   (see Figure 4-32). Click Configure….

Figure 4-32. Choosing the authentication method

Tip

Fast Reconnect allows PEAP to quickly resume a TLS session. It
minimizes the connection delay in wireless networks when a wireless
device roams from one access point to another.

1. Turn off the checkbox “Automatically use my Windows
   logon name and password (and domain if any)” (see
   Figure 4-33). Click OK three times to complete the
   settings.

   

   Figure 4-33. Disabling automatic Windows logon

2. Finally, double-click the Wireless Network Connection icon located in
   the Tray again and connect to the default
   network. You will be asked to supply your credentials to log on to
   the network (see Figure 4-34).

   

   Figure 4-34. Prompting for the user credentials

3. Enter your username and password; if you are a valid user (see Figure 4-35), you will be connected to the network.

Figure 4-35. Logging in to the RADIUS server

To confirm that you are connected to the network, launch your web
browser and see if you can connect to the Internet. You may also use
the ipconfig /all
command to see whether you are
assigned
an IP address.