Understanding Cipher Suites and WEP

Just as anyone within range of a radio station can tune to the station's frequency and listen to the signal, any wireless networking device within range of an access point can receive the access point's radio transmissions. Because WEP is the first line of defense against intruders, Cisco recommends that you use full encryption on your wireless network.

WEP encryption scrambles the communication between the access point and client devices to keep the communication private. Both the access point and client devices use the same WEP key to encrypt and unencrypt radio signals. WEP keys encrypt both unicast and multicast messages. Unicast messages are addressed to just one device on the network. Multicast messages are addressed to multiple devices on the network.

Extensible Authentication Protocol (EAP) authentication, also called 802.1x authentication, provides dynamic WEP keys to wireless users. Dynamic WEP keys are more secure than static, or unchanging, WEP keys. If an intruder passively receives enough packets encrypted by the same WEP key, the intruder can perform a calculation to learn the key and use it to join your network. Because they change frequently, dynamic WEP keys prevent intruders from performing the calculation and learning the key. See Chapter 11 "Configuring Authentication Types," for detailed information on EAP and other authentication types.

Cipher suites are sets of encryption and integrity algorithms designed to protect radio communication on your wireless LAN. Because cipher suites provide the protection of WEP while also allowing use of authenticated key management, Cisco recommends that you enable WEP by using the encryption mode cipher command in the CLI or by using the cipher drop-down menu in the web-browser interface. Cipher suites that contain TKIP provide the best security for your wireless LAN, and cipher suites that contain only WEP are the least secure.

These security features protect the data traffic on your wireless LAN:

•WEP (Wired Equivalent Privacy)—WEP is an 802.11 standard encryption algorithm originally designed to provide your wireless LAN with the same level of privacy available on a wired LAN. However, the basic WEP construction is flawed, and an attacker can compromise the privacy with reasonable effort.

•TKIP (Temporal Key Integrity Protocol)—TKIP is a suite of algorithms surrounding WEP that is designed to achieve the best possible security on legacy hardware built to run WEP. TKIP adds four enhancements to WEP:

–A per-packet key mixing function to defeat weak-key attacks

–A new IV sequencing discipline to detect replay attacks

–An extension of IV space, to virtually eliminate the need for re-keying

•Broadcast key rotation (also known as Group Key Update)—Broadcast key rotation allows the access point to generate the best possible random group key and update all key-management capable clients periodically.

Note Client devices using static WEP cannot use the access point when you enable broadcast key rotation. When you enable broadcast key rotation, only wireless client devices using 802.1x authentication (such as LEAP, EAP-TLS, or PEAP) can use the access point.

Creating WEP Keys

Note You need to configure static WEP keys only if your access point needs to support client devices that use static WEP. If all the client devices that associate to the access point use key management (WPA or 802.1x authentication) you do not need to configure static WEP keys.

Beginning in privileged EXEC mode, follow these steps to create a WEP key and set the key properties:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

interface dot11radio { 0 | 1 }

Enter interface configuration mode for the radio interface. The 2.4-GHz radio is radio 0, and the 5-GHz radio is radio 1.

Note Client devices using static WEP cannot use the access point when you enable broadcast key rotation. When you enable broadcast key rotation, only wireless client devices using 802.1x authentication (such as LEAP, EAP-TLS, or PEAP) can use the access point.

Example WEP Key Setup

Table 10-2 shows an example WEP key setup that would work for the access point and an associated device:

Table 10-2 WEP Key Setup Example

Key Slot

Access Point

Associated Device

Transmit?

Key Contents

Transmit?

Key Contents

1

x

12345678901234567890abcdef

-

12345678901234567890abcdef

2

-

09876543210987654321fedcba

x

09876543210987654321fedcba

3

-

not set

-

not set

4

-

not set

-

FEDCBA09876543211234567890

Because the access point's WEP key 1 is selected as the transmit key, WEP key 1 on the other device must have the same contents. WEP key 4 on the other device is set, but because it is not selected as the transmit key, WEP key 4 on the access point does not need to be set at all.

Enabling Cipher Suites and WEP

Beginning in privileged EXEC mode, follow these steps to enable a cipher suite:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

interface dot11radio { 0 | 1 }

Enter interface configuration mode for the radio interface. The 2.4-GHz radio is radio 0, and the 5-GHz radio is radio 1.

Step 3

encryption[vlan vlan-id]mode ciphers{[ tkip]} {[wep128 | wep40]}

Enable a cipher suite containing the WEP protection you need.

•(Optional) Select the VLAN for which you want to enable WEP and WEP features.

•Set the cipher options and WEP level. You can combine TKIP with 128-bit or 40-bit WEP.

Note If you enable a cipher suite with two elements (such as TKIP and 128-bit WEP), the second cipher becomes the group cipher.

Note If you configure ckip, cmic, or ckip-cmic, you must also enable Aironet extensions. The command to enable Aironet extensions is dot11 extension aironet.

Note You can also use the encryption mode wep command to set up static WEP. However, you should use encryption mode wep only if no clients that associate to the access point are capable of key management. See the Cisco IOS Command Reference for Cisco Access Points and Bridges for a detailed description of the encryption mode wep command.

Note When you configure the cipher TKIP (not TKIP + WEP 128 or TKIP + WEP 40) for an SSID, the SSID must use WPA or CCKM key management. Client authentication fails on an SSID that uses the cipher TKIP without enabling WPA or CCKM key management.

Step 4

end

Return to privileged EXEC mode.

Step 5

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no form of the encryption command to disable a cipher suite.

This example sets up a cipher suite for VLAN 22 that enables CKIP, CMIC, and 128-bit WEP.

ap1200# configure terminal

ap1200(config)# interface dot11radio 0

ap1200(config-if)# encryption vlan 22 mode ciphers ckip-cmic wep128

ap1200(config-if)# exit

Matching Cipher Suites with WPA

If you configure your access point to use WPA authenticated key management, you must select a cipher suite compatible with the authenticated key management type. Table 10-3 lists the cipher suites that are compatible with WPA.

Table 10-3 Cipher Suites Compatible with WPA

Authenticated Key Management Types

Compatible Cipher Suites

WPA

•encryption mode ciphers tkip

•encryption mode ciphers tkip wep128

•encryption mode ciphers tkip wep40

Note When you configure the cipher TKIP (not TKIP + WEP 128 or TKIP + WEP 40) for an SSID, the SSID must use WPA or CCKM key management. Client authentication fails on an SSID that uses the cipher TKIP without enabling WPA or CCKM key management.

Enabling and Disabling Broadcast Key Rotation

Broadcast key rotation is disabled by default.

Note Client devices using static WEP cannot use the access point when you enable broadcast key rotation. When you enable broadcast key rotation, only wireless client devices using 802.1x authentication (such as LEAP, EAP-TLS, or PEAP) can use the access point.

•Enter the number of seconds between each rotation of the broadcast key.

•(Optional) Enter a VLAN for which you want to enable broadcast key rotation.

•(Optional) If you enable WPA authenticated key management, you can enable additional circumstances under which the access point changes and distributes the WPA group key.

–Membership termination—the access point generates and distributes a new group key when any authenticated client device disassociates from the access point. This feature protects the privacy of the group key for associated clients. However, it might generate some overhead if clients on your network roam frequently.

–Capability change—the access point generates and distributes a dynamic group key when the last non-key management (static WEP) client disassociates, and it distributes the statically configured WEP key when the first non-key management (static WEP) client authenticates. In WPA migration mode, this feature significantly improves the security of key-management capable clients when there are no static-WEP clients associated to the access point.