802.11i secures the WLAN.

Wireless LAN has a checkered past. The initial attempt to provide privacy over the air using techniques native to 802.11 forced security-conscious users to turn to Layer 3 mechanisms, such as virtual private networks, to ensure the security of WLAN communications. Now, the 802.11i standard has been released, and over-the-air communications can finally be secured at Layer 2.

The IEEE's initial attempt at WLAN security was wired equivalent privacy (WEP), which was quickly shown to provide little of the privacy it advertised. In 802.11i, completely new techniques are used to secure communications.

First, a key called the pairwise master key (PMK) is established between the wireless station and the access point. This key is typically generated using 802.1X, that is, an EAP authentication of the user to a RADIUS or other authentication-authorization-accounting (AAA) server. Both station and AAA servers derive identical keys, and the AAA server returns that key to the access point. Alternatively, a pre-shared key may be configured in both station and access point as the PMK.

Next, the station and access point exchange a message sequence called the four-way handshake. In this exchange, the PMK and fresh (random) nonces (numbers only used once) from both station and access point are used to derive a new key, called the pairwise transient key. This key is subdivided into several keys: one to sign four-way handshake messages, one to encrypt a group key during the four-way handshake and one for security the data connection.

During the four-way handshake, the station and access point negotiate the type of encryption to be used for the data connection. Two encryption ciphers are negotiated: the pairwise cipher is used for unicast data between station and access point, and the group cipher is used for broadcast/ multicast traffic from the access point to multiple stations. The group cipher allows the access point to send one copy of each multicast packet to all stations, rather than to send a separately encrypted packet to each station.

While any encryption cipher may be negotiated, the cipher of choice for 802.11i is advanced encryption standard (AES), with a 128-bit key, in CCM mode (counter with CBC-MAC). AES is the U.S. federal government standard for encryption, and CCM has recently been approved as FIPS-compliant.

In an 802.11i-only environment, AES will normally be used both as the pairwise and group cipher. In a mixed environment, access points will typically use a lowest-common-denominator cipher as the group cipher, such as wired equivalent privacy or temporal key integrity protocol, to allow both 802.11i and pre-802.11i stations to decrypt multicast traffic.

802.11i also speeds roaming from one access point to the next. Previously, the station needed to perform a complete 802.1X authentication each time it associated with a new access point. With 802.11i, when the station returns to an access point it already authenticated with, it can reuse the PMK established with that access point to omit 802.1X authentication and perform only the four-way handshake. This speeds transitions between access points. Additionally, the station may "pre-authenticate" to a new access point it intends to roam to while still associated with the current access point; this allows the station to only perform a four-way handshake once it does roam.

Another fast-roaming technique made possible by 802.11i is informally called opportunistic key caching (also proactive key caching). If multiple access points are able to share PMKs among themselves, the station may be able to roam to a new access point it has not visited before and re-use a PMK established with the current access point. This allows the station to roam quickly to an access point it never authenticated to, without having to perform pre-authentication. This mechanism is commonly available on wireless switches, since the central switch can provide a store of PMKs across all the access point it controls. There are also centralized architectures using "fat" access points that can provide equivalent capability.

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This article was provided by Paul Funk, president of Funk Software, Cambridge, Mass.