Securing The Airwaves

Since the ratification of the IEEE 802.11b standard in 1999, wireless LANs (WLAN)

have become more prevalent. Today, wireless WLANs are widely deployed in places

such as corporate office conference rooms, industrial warehouses, Internet-ready

classrooms and even coffee houses.

These IEEE 802.11-based WLANs present new challenges for network

administrators and information security administrators alike. Unlike the

relative simplicity of wired Ethernet deployments, 802.11-based WLANs broadcast

radio-frequency (RF) data for the client stations to hear. This presents new and

complex security issues that involve augmenting the 802.11 standard.

Securing 802.11 WLANs



Some mistakenly believe WEP to be the only component to WLAN security, but

wireless security actually consists of three components:

• The
  
  authentication framework
• The authentication algorithm
• The data privacy or encryption algorithm

802.1x authentication framework



The IEEE 802.1x standard provides a framework for many authentication types

and the link layer. The 802.1x authentication framework is included in the draft

for 802.11 MAC layer security enhancements currently being developed by the IEEE

802.11 Task Group i (TGi). The 802.1x framework provides the link layer with

extensible authentication, normally seen in higher layers.

802.1x requires three entities:

• The supplicant-resides on the WLAN client

• The authenticator-resides on the access point

• The authentication server-resides on the RADIUS server

These

entities are logical entities on the network devices. The authenticator creates

a logical port per client, based on the client's association ID (AID). This

logical port has two data paths. The uncontrolled data path allows network

traffic through the network. The controlled data path requires successful

authentication to allow network traffic through.

The supplicant becomes active on the medium and associates to

the access point. The authenticator detects the client association and enables

the supplicant's port. It forces the port into an unauthorized state so that

only 802.1x traffic is forwarded. All other traffic is blocked.

802.1x provides the means for a WLAN client to communicate

with an authentication server to validate the client credentials. 802.1x is

extensible and allows a variety of authentication algorithms to operate over it.

Extensible Authentication Protocol (EAP) Algorithm



The EAP supports centralized, user-based authentication with the ability to

generate dynamic WEP keys.

The EAP authentication is designed to function on top of the

802.1x authentication framework.

Mutual Authentication: Many authentication algorithms exist,

each with an ideal use. In the world of WLANs, the client needs to be certain

that it is communicating with the intended network device. The lack of physical

connectivity between the client and the network requires the client to

authenticate the network as well as to be authenticated by the network.

User-based Authentication: 802.11 authentication is

device-based. The user of the device is invisible to the authenticator, and so

unauthorized users can access the network simply by gaining access to an

authorized device. Notebooks with 802.11 NICs using static WEP with 802.11

authentications create network vulnerability if the notebook is stolen or lost.

Such an event would require the network administrator to rapidly re-key the

wireless network and all clients.

The scenario is all too common and is a major barrier to

deployment for WLANs. Hence, authenticating the user rather than the WLAN device

makes the process of authentication tighter.

Dynamic WEP Keys: User-based mutual authentication provides

an easy-to-administer and secure authentication scheme, yet a mechanism is still

needed to manage WEP keys efficiently. This need has driven the requirement for

the authentication algorithm to generate keying material for dynamic WEP keys. A

user-based mechanism to generate unique keying material for each client relieves

network administrators from the burden of managing static keys and manually

re-keying as needed.

802.1x session timeouts force the client to re-authenticate

to maintain network connectivity. Although re-authentication is transparent to

the client, the process of re-authentication in an algorithm that supports

dynamic WEP will generate new WEP keys at every re-authentication interval. This

is an important feature in mitigating statistical key derivation attacks.

Temporal Key Integrity Protocol (TKIP)



TKIP is a draft standard with Task Group i of the IEEE 802.11 working group.
Although TKIP is not a ratified standard, it provides three major enhancements

to WEP.

Message Integrity Check (MIC)-The MIC function provides

effective frame authenticity to mitigate man-in-the-middle vulnerabilities. The

MIC augments the ineffective integrity check function (ICV) of the 802.11

standard.

Per-Packet Keying: It provides every frame with a new and

unique WEP key that mitigates WEP key derivation attacks.

Broadcast Key Rotation: Dynamic key rotation for broadcast

and multicast traffic.

802.1x authentication types that support user-based WEP keys

provide WEP keys for unicast traffic only. To provide encryption for broadcast

and multicast traffic, the following need to be done:

• Employ a static broadcast key configured on the access
  
  point

• Enable broadcast key rotation for dynamic broadcast key
  
  generation

A static broadcast key must be configured on an access point

for 802.1x clients to receive broadcast and multicast messages. In wireless LAN

deployments where a static broadcast key will go through the per-packet keying

process. This reduces the opportunity for statistical key derivation attacks,

but because the base broadcast key remains static, the IV space will recycle,

causing key streams to be reused. Statistical attacks may take much longer to

execute, but they are still possible.

Static broadcast key deployments might be required in some

instances. Broadcast keys are sent from the access point to the client,

encrypted with the client's unicast WEP key. Because the broadcast keys are

distributed after authentication, access points do not have to be configured

with the same broadcast key. The access point generates broadcast WEP keys using

a seeded pseudorandom number generator (PRNG). The access point rotates the

broadcast key after a configured broadcast WEP key timer expires. This process

should generally be in sync with the timeouts configured on the RADIUS servers

for user reauthentication.

Broadcast key rotation is designed for 802.1x-enabled access

point deployments. In mixed static WEP/802.1x deployments, broadcast key

rotation may cause connectivity problems in static WEP clients. Therefore, it is

recommended that broadcast key rotation be enabled when the access point

services an 802.1x exclusive wireless LAN.

Devendra Kamtekar,

Principal Consultant, Cisco Systems, India & SAARC

What Lies Ahead

For many enterprises, managing a wireless network has felt

like riding a runaway horse without a bridle, however, enterprises can focus on

their businesses without worrying of security. In addition, the IEEE is

enhancing WEP with TKIP and providing robust authentication options with 802.1x

to make 802.11-based WLANs more secure. At the same time, the IEEE is looking to

stronger encryption mechanisms. The IEEE has also adopted the use of the

Advanced Encryption Standard (AES), the next-generation encryption function,

approved by the National Institute of Standards and Technology (NIST) to the

data-privacy section of the proposed 802.11i standard.