[Intro. to Computer Security Course Note] Ch 24

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Ch24. Wireless Network Security

Wireless Security

  • Key factors: contributing to higher security risks (v.s. wired networks)
    • Channel
      • Vulnerabilities in communication protocols
    • Mobility
    • Resources
      • Smartphones/tablets: sophisticated OS but limited memory and processing resources
    • Accessibility
      • Sensors/robots: may be left unattended in remote and/or hostile locations

Wireless Network Threats

Wireless Security Measures

  • Securing wireless transmissions
    • Principal threats: eavesdropping, altering, inserting, messages, disruption
    • eaves (屋簷)
  • Countermeasures for eavesdropping
    • Signal-hiding techniques
    • Encryption
  • For altering and inserting
    • Encryption and authentication protocols
  • Securing wireless networks
    • Wireless APs: unauthorized access to the network
    • Principal approach: IEEE 802.1X (port-based network access control)
      • Prevent rogue APs and other authorized devices

Wireless Network Security Techniques

Mobile Device Security

  • An organization’s networks must accommodate
    • Growing use of new devices
    • Cloud-based apps
    • De-perimeterization
      • A multitude of network perimeters around devices, apps, user, and data
    • External business requirements

Security Threats

Mobile Device Security Strategy

  • Device security
  • Traffic security
    • Based on encryption and authentication
    • via VPN
  • Barrier security
    • Firewall

IEEE 802.11 Wireless LAN Overview

Terminology & Services

  • Access point (AP)
    • Any entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations
  • Basic service set (BSS)
    • A set of stations controlled by a single coordination function
  • Coordination function
    • The logical function that determines when a station operating within a BSS is permitted to transmit and may be able to receive PDUs
  • Distribution system (DS)
    • A system used to interconnect a set of BSSs and integrated LANs to create an ESS
  • Extended service set (ESS)
    • A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to the LLC layer at any station associated with one of these BSS
  • MAC protocol data unit (MPDU)
    • The unit of data exchanged between two peer MAC entities using the services of the physical layer
  • MAC service data unit (MSDU)
    • Information that is delivered as a unit between MAC users
  • Station
    • Any device that contains an IEEE 802.11 conferment MAC and physical layer

Distribution of Messages within a DS

  • Two services involved
    • Distribution: exchange MPDUs between two BSS
    • Integration: data transfer between a Wi-Fi station and a LAN station on an integrated IEEE 802x LAN
  • Transition types, based on mobility:
    • No transition
    • BSS transition
    • ESS transition

Distribution Service

  • Association
    • Establishes an initial association between a station and an AP
  • Reassociation
    • Enables an established association to be transferred from one AP to another
  • Disassociation

Wireless Fidelity (Wi-Fi) Alliance

  • 802.11b
    • First 802.11 standard to gain broad industry acceptance
  • Wireless Ethernet Compatibility Alliance (WECA)
    • Industry consortium(合夥) formed in 1999: interoperation between vendors
  • Term used for certified 802.11b products: Wi-Fi
  • Wi-Fi Protected Access (WPA)
    • Certification procedures for IEEE 802.11 security standards
    • Most recent version: WPA2

IEEE 802.11 Protocol Stack

General IEEE 802 MPDU Format

IEEE 802.11i Wireless LAN Overview

  • Wired Equivalent Privacy (WEP) algorithm
    • 802.11 privacy
  • Wi-Fi Protected Access (WPA)
    • Set of security mechanisms: eliminates most 802.11 security issues
    • Based in the current state of the 802.11i standard
  • Robust Security Network (RSN)
  • Wi-Fi Alliance certifies vendors in compliance with the full 802.11i (WPA2)

Five Phases of Operation for an RSN

  • Phase 1: Discovery Phase
    • Security capabilities
      • Confidentiality and MPDU integrity protocols
      • Authentication method
      • Cryptography key management approach
    • Cipher suite (Confidentiality/Integrity)
      • WEP, TKIP, CCMP
    • AKM: Authentication and Key Management
      • IEEE 802.11x, pre-shared key
    • 802.1X Access Control
  • Phase 2: Authentication Phase
    • IEEE 802.11x AC
      • 802.1X Control channel is unblocked
      • 802.11 data channel is blocked
    • Three phases
      • Connect to AS
      • EAP exchange
      • Secure key delivery
    • EAP Authentication Protocol
      • Initiated by the server (authenticator)
      • Authentication is mutual between the client and authentication server
    • Popular EAP Methods
      • Cisco LEAP (Lightweight EAP)
        • complex passwords are required
      • EAP-FAST (EAP-Flexible Authentication via Secure Tunneling)
        • No need of strong password or any certificate
        • Using a PAC (Protected Access Credential) to establish a TLS tunnel
      • EAP-TLS
        • Using PKI: both client and AS need a certificate (X509 certificates)
        • One of the most secure EAP standards available
      • PEAP (Protected EAP)
        • Encapsulating EAP within a potentially encrypted and authenticated TLS tunnel
        • Only the server authentication is performed using PKI certificate
        • Client is authenticated using either EAP-GTC or EAP-MSCHAPv2 within the tunnel
          • EAP-GTC (Generic Token Card)
          • EAP-MSCHAPv2 (Microsoft’s Challenge Handshake Authentication Protocol)
  • Phase 3: Key Management Phase
    • Pairwise keys: for communication between a STA and an AP
      • Pre-shared key (PSK)
      • Master session key (MSK)
      • Pairwise master key (PMK)
        • PSK or derived from MSK
      • Pairwise transient key (PTK)
      • Hierarchy
    • Group keys: for multicast communication
      • Group master key (GMK)
      • Group temporal key (GTK)
      • Hierarchy
    • IEEE 802.11i Phases of Operation
      • 4-way Handshake
      • Group Key Handshake
  • Phase 4: Protected Data Transfer Phase
    • Temporal Key Integrity Protocol (TKIP)
      • Only software changes are required to old WEP
      • Message integrity
        • Message Integrity Code (MIC)
        • 64-bit: source/destination MAC address + data filed + key material
      • Data confidentiality
        • Encrypting MPDU + MIC using RC4
      • 256-bit TK
        • Two 64-bit keys for MIC: one key for each direction
        • 128 bits: generate the RC4 key for encryption
        • TKIP sequence counter (TSC): monotonically increasing
          • Included with each MPCU and protected by MIC -> against replay attack
          • Combined with session TK -> dynamic encryption key
    • Counter Mode-CBC MAC Protocol (CCMP)
      • 需要硬體支援
      • Message integrity
        • Cipher block chaining message authentication code
      • Data confidentiality
        • CTR block cipher mode of operation with AES
      • Same 128-bit AES key for both
      • A 48-bit packet number: a nonce(暫時) to prevent replay attack
    • IEEE 802.11i Pseudorandom Function (PRF)
      • A pseudorandom bit stream: HMAC-SHA-1
        • A message and a key (at least 160 bits) -> 160-bit hash value
        • SHA-1 property: change of a single bit -> a new hash value with no apparent connect
        • PTK:
          • K = PMK
          • A = “Pairwise key expansion”
          • B = Concatenation of two MAC address and two nonces
          • Len = 384 bits