Section 5.2: Wireless LAN Security
This section covers the following topics:
Security as defined in the 1997 802.11 specification- Why was Wired Equivalent Privacy (WEP) selected as the encryption algorithm and Open and Shared Key selected as the authentication algorithms?
Security vulnerabilities in the 1997 802.11 standard- Why is WEP ineffective for encryption and Open and Shared Key ineffective for authentication?
Next generation wireless LAN (WLAN) security enhancements- What are the new technologies being used to secure 802.11 WLANs?
For developers who have implemented or are currently implementing 802.11 wireless, an alphabet soup of acronyms describe enhanced 802.11 WLAN security. Abbreviations such as 802.1X, EAP, LEAP, PEAP, EAP-TLS, WEP, TKIP, WPA, and AES are common terms in describing 802.11 security. For the network administrator accustomed to dealing with IP and connectivity-oriented technologies, these new security-focused protocols can prove confusing.
Imagine extending a long Ethernet cable from your internal network outside your office and laying it on the ground in the parking lot. Anyone who wants to use your network can simply plug into that network cable. Connecting unsecured WLANs to your internal network has the
potential to offer the same opportunity. 802.11-based devices communicate with one another using radio frequencies (RFs) as the carrier signal for data. The data is broadcast from the sender in the hopes that the receiver is within RF range. The drawback to this mechanism is that any other station within range of the RF also receives the data. Without a security mechanism of some sort, any 802.11 station can process the data sent on a WLAN, as long as that receiver is in RF range. To provide a minimum level of security in a WLAN, you need two components:
A means to decide who or what can use a WLAN- This requirement is satisfied by authentication mechanisms for LAN access control.
A means to provide privacy for the wireless data- The requirement is satisfied by encryption algorithms.
5.2.1: 802.11 Encryption
The 802.11 specification provides data privacy with the WEP algorithm. WEP is based on the RC4 symmetric stream cipher. The symmetric nature of RC4 requires that matching WEP keys, either 40 or 104 bits in length, must be statically configured on client devices and access points (APs). WEP was chosen primarily because of its low computational overhead. Although 802.11-enabled PCs are common today, this situation was not the case back in 1997. The majority of WLAN devices were application-specific devices (ASDs). Examples of ASDs include barcode scanners, tablet PCs, and 802.11-based phones. The applications that run on ASDs generally do not require much computational power, so as a result, ASDs have meager CPUs. WEP is a simple-to-implement algorithm that you can write in as few as 30 lines of code, in some cases. The low overhead incurred by WEP made it an ideal encryption algorithm to use on ASDs.
To avoid the ECB mode of encryption, WEP uses a 24-bit IV, which is concatenated to the key before being processed by the RC4 cipher. Figure 4-5 shows a WEP-encrypted frame, including the IV.
The 802.11 specification requires that matching WEP keys be statically configured on both client and infrastructure devices. You can define up to four keys on a device, but you can use only one at a time for encrypting outbound frames. Figure 5-1 shows a Cisco Aironet client configuration screen for WEP configuration.
WEP encryption is used only on data frames and during Shared Key authentication. WEP encrypts the following fields of an 802.11 data frame:
The data or payload
The integrity check value (ICV)
All other fields are transmitted without encryption. The IV must be sent unencrypted within the frame so that the receiving station can use it to properly decrypt the payload and ICV.
5.2.2: 802.11 Authentication
The authentication framework in 802.11 is the 802.11 authentication management frame. The authentication frame facilitates Open and Shared Key authentication algorithms, yet the frame itself does not possess the ability to authenticate a client. Because the shortcomings of 802.11 authentication have already been highlighted, it is important to understand what is needed to provide secure authentication in a WLAN.
802.11 is missing some key components to provide effective authentication:
Centralized, user-based authentication Dynamic encryption keys Encryption key management Mutual authentication
User-based authentication is critical for network security. Device-based authentication, such as Open or Shared Key authentication, does not prevent unauthorized users from using authorized devices. Also, logistical issues, such as lost or stolen devices and employee termination, can force network administrators to manually rekey all 802.11 APs and clients.
Centralized, user-based management via an authentication, authorization, and accounting (AAA) server, such as a RADIUS, lets you allow or disallow specific users, regardless of the specific devices they use. The requirement for user-based authentication has a positive side effect: user-specific encryption keys. Authentication types that support the creation of dynamic encryption keys fit well into the WLAN security and management model. Per user, dynamic keys relieve the network administrator from having to statically manage keys. Encryption keys are dynamically derived and discarded as the user authenticates and disconnects from the network. Should you need to remove a user from the network, you only need to disable her account to prevent her access.
Mutual authentication is two-way authentication. The "two-way" nature comes from not only the network authenticating the client, but also the client authenticating the network. In Open and Shared Key authentication, the AP or network authenticates the client. The client does not know for sure that the AP or network is valid because no mechanism is defined in the
802.11 specification to allow the client to authenticate the network. As a result, a rogue AP or rogue client station can pose as a valid AP and subvert the data on the client's machine.
Figure 5-2 diagrams one-way authentication versus mutual authentication.
Figure 5-2. One-Way Authentication Versus Mutual Authentication
802.11 WLAN vendors and the IEEE understand the need to augment and replace existing security mechanisms, both in authentication and encryption. Work is currently underway in task group I of the
802.11 working group, and after the changes are complete, the security specifications will be ratified as the 802.11i specification.
The IEEE has addressed the shortcomings of 802.11 authentication by incorporating the 802.1X authentication framework. 802.1X itself is an IEEE standard that provides all 802 link layer topologies with extensible authentication, normally seen in higher layers. 802.1X is based on a Point-to-Point Protocol (PPP) authentication framework known as the Extensible Authentication Protocol (EAP). In oversimplified terms, 802.1X encapsulates EAP messages for use at Layer 2. 802.11i incorporates the 802.1X authentication framework requiring its use for user-based authentication. Figure 4-18 illustrates 802.1X with respect to authentication algorithms and 802 link layer topologies.
Figure 5-3. 802.1X in Contrast to 802 Link Layer Topologies
EAP (RFC 2284) and 802.1X do not mandate the use of any specific authentication algorithm. The network administrator can use any EAP-compliant authentication type for either 802.1X or EAP authentication. The only requirement is that both the 802.11 client (known as the supplicant) and the authentication server support the EAP authentication algorithm. This open and extensible architecture lets you use one authentication framework in differing environments, where each environment may use a different authentication type. Examples of EAP authentication types include the following:
EAP-Transport Layer Security (EAP-PEAP)- Operates similar to Secure Sockets Layer (SSL) at the link layer. Mutual authentication is accomplished via server-side digital certificates used to create a SSL tunnel for the client to securely authenticate to the network.
EAP-Message Digest 5 (EAP-MD5)- Similar to the Challenge Handshake Authentication Protocol (CHAP), EAP-MD5 provides a password based, one-way authentication algorithm.
EAP-Cisco- Also known as LEAP, EAP-Cisco was the first EAP type defined specifically for use in WLANs. EAP-Cisco is a password-based, mutually authenticating algorithm. 802.1X authentication requires three entities:
The supplicant- Resides on the WLAN client
The authenticator- Resides on the AP
The authentication server- Resides on the RADIUS server
These entities are logical software components on network devices. With respect to 802.11, the authenticator creates a logical port per client device, based on the client's association ID (AID). This logical port has two data paths: uncontrolled and controlled. The uncontrolled data path allows all 802.1X authentication traffic through to the network.