Security in Wireless LAN 802.11

1
Security in Wireless LAN 802.11

• Layla Pezeshkmehr
• CS 265
• Fall 2003-SJSU
• Dr.Mark Stamp

2
5 basic threats to WLAN

• Sniffing - eavesdropping
• Invasion steal valid STAs access to gain
  access to network
• Traffic redirection change in ARP table
• Denial of service (DOS)
• Flood the network
• Disrupt connection between machines
• Prevent a STA from connecting to WLAN
• Rogue networks and station redirection Man- in-
  the- middle attacks.

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IEEE 802.11 Authentication Open Key

• Uses null authentication, Simple
• Is the default authentication
• 2 steps
• A sends a request authentication to B
• B sends the result back to A
• If dot11 Authentication Type at B is set to
  "Open System" ? Returns "success" ? A is
  mutually authenticated
• Otherwise A is not authenticated

4
Shared key Authentication

• Provides a better degree of authentication.
• Station must implements WEP
• (Wired Equivalent Privacy)
• 4 steps
• Request sends an Authentication frame to AP.
• AP replies with a random challenge text generated
  by the WEP engine( 128 bit).
• STA copy the challenge text, encrypt it with a
  shared key then send the frame to the AP.
• AP decrypt the received frame, then verifies the
  32- bits CRC ICV, and that the challenge text
  matches the one it sends earlier to the station.
• Successful/negative authentication if
  match/mismatch

5
Identity Problems

• Open System authentication
• Null authentication.
• Messages sent in clear.
• Any one can impersonate either the station or the
  access point.
• Shared key authentication
• Only station authenticates itself.
• No mechanism for AP to prove its identity to the
  station therefore malicious AP.
• Only the station is authenticated not the user of
  the station.

6
Shared key vulnerabilities (cont)

• Exchanging both challenge and response occurs
  over the wireless link and is vulnerable to a
  man-in-the-middle attack.

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IEEE 802.11 Wired Equivalent Privacy (WEP)
Protocol

• The goal is to provide data privacy to the level
  of a wired network.
• (WEP) algorithm is used to prevent eavesdropping.
• An encapsulation of 802.11 data frame.
• 64- bits key (40-bit secret key,24-bit "init"
  vector).
• Symmetric algorithm because the same key is used
  for cipher and decipher.
• Data integrity checked with CRC-32.

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WEP Encryption

• A key shared among members of the BSS.
• Sender calculates CRC of the frame's data.
• WEP appends a new generated 24-bit initialization
  vector (IV) to the shared key.
• WEP PRNG (RC4) is used to generate a key stream.
• XORs key stream against (payload CRC) to
  produce ciphertext.
• The sender also inserts the IV into frame header,
  and sets the WEP encrypted packet bit indicator.

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WEP Decryption

• Receiver extracts IV from the frame
• appends IV to the BSS shared key, and generates
  the "per- packet" RC4 key sequence
• ciphertext is XORed against the key steam to
  extract plaintext.
• Verification performs integrity check on
  plaintext
• Compares ICV1 result with the ICV transmitted.

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WEP Decryption
11
ICV Weakness

• How is the attacker able to modify ICV to match
  the bit-flipped changes to the frame?

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WEP Problems-with RC4

• flip a bit in the ciphertext (C) ? the
  corresponding bit in the plaintext will be
  flipped.
• Eavesdropper intercepts 2 ciphertext encrypted
  with the same key stream ? possible to obtain the
  XOR of the 2 plaintexts.
• c1 p1 ? b c2 p2 ? b
• ? c1 ? c2 (p1 ? b) ? (p2 ? b) p1 ? p2

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WEP Problems-with IV

• IV is 24 bits cleartext, part of a message.
• A small space of initialization vectors
  guarantees the reuse of the same key stream.
• AP constantly send 1500 byte pkt at 11 Mbps will
  exhaust the space of IV after
• 1500 8/(11 10 6 ) 2 24 18000s
  5h
• When the same key is used by all mobile stations
  more chances of IV collision.

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Cisco enhancements to 802.11 WEP to increase
security

• Mutual authentication instead of one-way
  authentication
• Secure key derivation using one way hash function
• Dynamic WEP keys instead of static WEP keys
• Initialization Vector changes

15
Today future control

• Service Set Identifier (SSID)
• Each AP has an SSID of the AP to identify itself.
  STA have to know the SSID of the AP to which it
  wants to connect. SSID keeps a STA from
  accidentally connecting to neighboring AP.
• This does not solve other security issues and
  does not keep an attacker from setting up a
  "rogue" AP that uses the same SSID as the valid AP

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Today future control (cont.)

• MAC filters
• AP check MAC addresses of STAs before being
  connected to the network
• AP keep a list of MAC addresses in long- term
  memory.
• AP may send a RADIUS request with the MAC address
  as the userID (and a null password ) to a central
  RADIUS server to check the list for an address.

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The End