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Computer Network 2026

networks) that the authentication server and the mobile device already have a shared
common secret before starting mutual authentication. In this step, the device and the
authentication server will use this shared secret along with nonces (to prevent relay
attacks) and cryptographic hashing (to ensure message integrity) in authenticating each
other. They will also derive the shared session key to be used by the mobile device and
the AP to encrypt frames transmitted over the 802.11 wireless link.
3. Shared symmetric session key distribution.
Since the symmetric encryption key is derived at the mobile device and the
authentication server, a protocol will be needed for the authentication server to inform
the AP of the shared symmetric session key. While this is rather straightforward, it still is
a necessary step.
4. Encrypted communication between mobile device and a remote host via the AP.
This, with the link-layer frames sent between the mobile device and the AP being
encrypted using the shared session key created and distributed by Steps 2 and 3.

AES symmetric key cryptography, which we covered Earlie, is typically used in practice for
encrypting/decrypting 802.11 frame data. Mutual Authentication and Shared Symmetric Session
Key Derivation The topics of mutual authentication and shared symmetric session key derivation
are the central components of 802.11 security.

Since it is here that security flaws in various earlier versions of 802.11 security have been
discovered, let’s tackle these challenges first. The issue of 802.11security has attracted
considerable attention in both technical circles and in the media.
While there has been considerable discussion, there has been little debate—there is universal
agreement that the original 802.11security specification known collectively as Wired Equivalent
Privacy (WEP) contained a number of serious security flaws [Fluhrer 2001; Stubblefield 2002].
Once these flaws were discovered, public domain software was soon available exploiting these
holes, making users of WEP-secured 802.11 WLANs as open to security attacks as users who used
no security features at all. Readers interested in learning about WEP can consult the references,
as well as earlier editions of this textbook, which covered WEP.

As always, retired material from this book is available on the Companion Website. Wi-Fi
Protected Access (WPA1) was developed in 2003 by the Wi-Fi Alli acne [Wi-Fi 2020] to overcome
WEP’s security flaws. The initial version of WPA1 improved on WEP by introducing message
integrity checks, and avoiding attacks that allowed a user to infer encryption keys after observing
the stream of encrypted messages for a period of time.

WPA1 soon gave way to WPA2, which mandated the use of AES symmetric key encryption. At
the heart of WPA is a four-way handshake protocol that performs both mutual authentication
and shared symmetric session-key derivation. The handshake protocol in simplified form. Note
that both the mobile device (M) and the authentication server (AS) begin knowing a shared secret
key KAS-M

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