Wireless Network Security

CSIS 5857: Encoding and Encryption

Wireless LAN Terminology• Station: Device capable of IEEE 802.11 wireless

connectivity (wireless laptop, etc.)

• Distribution System: Backbone system for long-distance communication (lines/satellite/etc.)

• Access point: Bridge between station and distribution system (or between stations in same BSS)

• Basic Service Set (BSS): Set of stations in range of access point

Wireless LAN Structure

Wireless LAN Structure• Differences from LAN security:

• Any transmission may be listened to by any device in range of access point

All transmissions must be secured

• Other wireless devices in range of AP can attempt to connect to it

Any device wishing to communicate through access point must be authenticated

802.11i RSN Services• Access control:

– Preventing access toaccess point until authentication

• Authentication:– Mutual authentication between stations and access points,

and key generation• Privacy:

– Encrypting/hashing/etc. of messages to insure confidentiality/information integrity/etc.

Authentication Server• Authentication server

– Contacted by AP when station requests access– Uses authentication protocol to establish station identity– AP blocks communication with rest of network until

authentication complete

IEEE 802.11i Operation Phases

Discovery Phase• Station contacts AP

– Challenge-response can be used to authenticate AP• Station passes security suite to AP (like SSL)• AP responds

with chosen algorithms

Authentication Phase• Station sends request to AP for connection to

authentication server• Extensible authentication

protocol (EAP) used for mutual authentication

• AS generates keys for secure informationexchange

Authentication Phase• Extensible Authentication Protocol:

– Based on challenge-response– Station and AS share secret information (such as

symmetric key)– AS sends challenge to station (via AP)– Station sends response– May repeat with multiple challenges (10 – 20) for

added protection against replay attacks

Key Management Phase• Possible methods:

– Station and AP already share pre-shared key (PSK) previously installed on station

– AS generates and sends master session key (MSK) to station as part of EAP challenge response

• Used to generate pairwise master key

Key Management Phase• HMAC-SHA1 used to generate Pairwise transient key from

– Pairwise master key– MAC address of station and AP (prevents replay attacks)

• PTK consists of– Key used for confirmation of other keys– Key used to encrypt other keys– Actual temporal key used to

securely transmit data

Key Management Phase• Handshake between station and AP used to

establish and confirm keys used for secure transmission