Lecture-07-security in 802.11-wep

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Security in 802.11 – WEP
Dr. Nguyen Tuan Nam
[email protected]
Why Is Security Important for a
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Perimeter of University
Block illegal access/attack
Computer Server
Access Firewall
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Security of Wireless Networks
• All vulnerabilities exist in conventional wired
network apply to wireless technologies
• Malicious entities may gain unauthorized access
to an agency’s computer network through
wireless connections, bypassing any firewall
• Airwave, the underlying communication medium,
is open to intruders (vs. wired network)
• Sensitive information that is not encrypted may
be intercepted and disclosed (shared medium)
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Security of Wireless Networks
• DoS attacks may be directed at wireless
connections or devices (shared medium)
• Malicious entities may be able to violate
the privacy of legitimate users
– Tracking movement
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Maintaining a Secure Wireless
• Maintain full understanding of the
topology of wireless network
• Create backups of data frequently
• Perform periodic security testing and
assessment of the wireless network
• Apply patches and security enhancements
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WEP (Wired Equivalent Privacy)
• Encryption used in access point mode
• Data Link Layer & Physical Layer
• 64 bits to 128 bits (including 24 bit IVs)
• Supposed to be secured and encrypted all
traffic between AP and wireless devices
• However, not secured
• Provide end-end security?
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Quiz – WEP
Node D Node A FTP to Server C: Node D and B can see
Account: root the password???
Password: p@sswd
Node A
Node B
FTP connection
TCP header p@sswd
Server C
IP header TCP header p@sswd
MAC hdr IP header TCP header p@sswd
MAC hdr IP header TCP header encrypted
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Basic Encryption
• Ceasar’s cipher
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• Authentication
– Are only authorized persons allowed to gain
access to my network?
• Confidentiality
– Are only authorized persons allowed to view
my data?
• Integrity
– Is the data coming into or exiting the network
trustworthy—has it been tampered with?
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• Two means to validate wireless users
– Open system authentication
– Shared-key authentication
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Shared Key Authentication
Node A Shared key
Generate random numbers
Encrypt Challenge e
Using shared key
ted te
x t Decrypt the answer
r m S u ccess Authenticate whom?
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Privacy of 802.11
• Use of cryptographic techniques for
wireless interface
• Use RC4 symmetric-key, stream cipher
algorithm to generate a pseudo-random
data sequence
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• A long random (or pseudo-random) string used to encrypt a
message with a simple ⊕
(XOR) operation à one-time pad
• A stream cipher generates a one-time pad and applies it to a stream
of plaintext with ⊕
• RC 4
– Stream cipher designed by Ron Rivest
– Was a trade secret until 1994
• A description of it anonymously posted to the Cypherpunks mailing list
– Considered secure as long as discarding the first few (256) octets of the
generated pad
– Extremely simple and fast generator of pseudo-random streams of
– Passes all usual randomness tests
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Privacy of 802.11 – Stream
What is the problem with this mechanism?
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Same Key
• Key K, Plaintext P1, P2:
– C1 = P1 XOR RC4(K)
– C2 = P2 XOR RC4(K)
àC1 XOR C2 = P1 XOR P2
• Frequency analysis techniques:
– Can decrypt information about P1, P2
– If P1 is known à RC4(K) can be calculated à
P2 can be calculated
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Encryption with IVs
Changed per
packet (3
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IVs Shared Key Per Packet Key
001 1001111110 0011001111110
010 1001111110 0101001111110
011 1001111110 0111001111110
100 1001111110 1001001111110
101 1001111110 1011001111110
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WEP Privacy Using RC4
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• Provide data integrity for message transmitted
between wireless clients and access points
• Reject any messages that had been changed by
an active adversary in the middle
• Use simple encrypted CRC approach
– CRC is computed and appended to the payload
– Integrity-sealed packet is encrypted using RC4
– After received packet, receiver decrypts packet
– Compute the CRC and compare with the original
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