This document discusses wireless security and protocols such as WEP, WPA, and 802.11i. It describes weaknesses in WEP such as vulnerabilities in the RC4 encryption algorithm that allow attacks like dictionary attacks. It introduces WPA as an improvement over WEP that uses stronger encryption keys, protocols like TKIP that change keys dynamically, and AES encryption in 802.11i as stronger alternatives. It also discusses authentication methods like 802.1X that distribute unique keys to each user to address issues with shared keys in WEP.

1. COEN 350
Mobile Security

2. Wireless Security
 Wireless offers additional challenges:
 Physical media can easily be sniffed.
 War Driving

Legal?
 U.S. federal computer crime statute, Title 18 U.S.C. 1030,

Crime to knowingly access a computer used in interstate or
foreign communication "without authorization" and obtain any
information from the computer.

Crime to access a computer without authorization with "intent
to defraud" to obtain "anything of value."

But not if "the object of the fraud and the thing obtained
consists only of the use of the computer and the value of
such use is not more than $ 5,000 in any 1-year period."

3. Wireless Security
 Wireless offers additional challenges:
 Physical media can easily be sniffed.
 Mobile computing needs to preserve
battery power.

Calculations cost more on a mobile platform.

Especially important for sensor networks

4. Wireless Security:
Attackers Perspective
 Knowing the Threat
 Targets of opportunity

Goal is
 Internet access.
 Easy pickings.
 Targeted attacks

Targets assets valuable enough.
 Internal attackers

Most Dangerous

Can open an unintentional security hole

5. COEN 351 E-Commerce
Security
 E-Commerce Security Course Homepage
 Lecture Notes

6. IEEE 802.11
 Wired Equivalent Privacy (WEP)
Protocol
 Based on a shared secret k.

Distributed out of band.
 Uses CRC for internal integrity protection.
 Uses RC4 to encrypt network traffic.

7. WEP Protocol

8. WEP Protocol
 Confidentiality
 Original packet is first check-summed.
 Checksum and data form the payload.
 Transmitting device creates a 24-bit
random initialization vector IV.
 IV and shared key are used to encrypt with
RC4

9. WEP Protocol
 RC4
 Generates a pseudo-random stream of
bytes (keystream)

Based on a secret internal state
 Permutation S of all 256 possible bytes
 Two index pointers
 Plaintext is XORed with keystream

10. WEP Protocol
 RC4
 Key Scheduling Algorithm (KSA)

Initializes S based on a key
for i from 0 to 255
S[i] := i
j := 0
for i from 0 to 255
j := (j + S[i] + key[i mod keylength]) mod 256
swap(S[i],S[j])

11. WEP Protocol
 RC4
 Pseudo-Random Generation Algorithm
(PRGA)

Generates pseudo-random byte stream
i := 0
j := 0
while GeneratingOutput:
i := (i + 1) mod 256
j := (j + S[i]) mod 256
swap(S[i],S[j])
output S[(S[i] + S[j]) mod 256]

12. WEP Protocol
 RC4
 Known weaknesses

Keystream slightly biased
 Fluhrer & McGrew attack can distinguish keystream
from random stream given a GB of input.
 Fluhrer, Mantin, Shamir: statistics for output of the
first few bytes of output keystream are non-random,
leaking information about key.

13. WEP Protocol
 Authentication
 Station associating with access point
needs to authenticate itself.
 Both exchange the type of authentication
that is accepted.

Open: Just identification between station and
AP

Shared Secret: Participants send nonces to
each other, encrypt the nonce using WEP (and
the shared secret key), and verify the other’s
response.

14. WEP has no key management
 Everyone allowed to have access to a
wireless network has the same key.
 Anyone with the key can read ALL
traffic.

15. WEP: RC4
 RC4 uses the key and the IV to produce
a stream of pseudo-random bytes.
 Calculates cipher text from plaintext by
XORing the pseudo-random stream
with the plain-text.

16. WEP: RC4

17. WEP: Attacks on RC4
 Dictionary Attack

Build database:

224
different IVs

Build a database of 224
streams of MTU bytes
(2,312 B) for each different IV.

Takes < 40 GB storage.
 XOR two entries with the same IV.

Result are the two plaintexts XORed.

Natural language text has enough redundancy
to decrypt the XOR of two text streams.

18. WEP: Attacks on RC4
 Dictionary Attack
 Many packages can be completely or
partially guessed.
 XORing guessed plaintext and captured
cipher gives pseudo-random byte stream
for a given IV.
 Some implementations reset IVs poorly.
 This simplifies dictionary attacks.

19. WEP: Attacks on RC4
 Injection Attack
 Attacker creates packets on the wireless
connection.
 Attacker XORs plaintext and cipher.

Builds Pseudo-Random Stream database
indexed by IV.

20. RC4
Fluhrer, Mantin, Shamir Attack
 First few bits of several thousand
messages reveals key.
 Based on an analysis of the RC4 code.

Originally kept secret, but later leaked on the
internet.

21. RC4
Fluhrer, Mantin, Shamir Attack
 Key Scheduling Algorithm
 Sets up RC4 state array S
 S is a permutation of 0, 1, … 255
 Output generator uses S to create a
pseudo-random sequence.
 First byte of output is given by
S[S[1]+S[S[1]]].

First byte depends on
 {S[1], S[S[1], S[S[1]+S[S[1]]}

22. RC4
Fluhrer, Mantin, Shamir Attack
 Key Scheduling Algorithm
 First byte of plain text package is part of the SNAP header

0xAA for IP and ARP packages

0xFF or 0xE0 for IPX

Guessing the first byte is trivial
 Some IVs are vulnerable: “resolved”

(KeyByte+3, 0xFF, *)

Plus some more
 Easy to test whether an IV is vulnerable.
 Search for vulnerable IVs.
 They leak key bytes probabilistically.
 Large number of packets does it.

23. RC4
Fluhrer, Mantin, Shamir Attack
 Optimization needs about 5,000,000 to
1,000,000 packages.
 Counter-measures:
 Change key frequently.
 Change IV counters to avoid bad IVs.

24. WEP Message Modification
 WEP uses CRC code to ascertain integrity of
messages.
 CRC code is linear:
 CRC(x ⊕ y) = CRC(x) ⊕ CRC(y).
 Attacker knows plaintext M and desired modification
∆ for target plaintext M’ = M ⊕ ∆.
 Attacker want to substitute X = P⊕(M,CRC(M)) for
P⊕(M’,CRC(M’)).
 Attacker sends
X⊕(∆,CRC(∆)) = P⊕(M,CRC(M)) ⊕(∆,CRC(∆))
= P⊕(M’,CRC(M’))

25. Wireless Insecurity Problems
 WiFi card software allows users to
change the MAC address.

26. Wireless Security
 Casual user, low yield traffic
 WEP is good enough.
 Enterprise, Commercial
 Combine WEP with higher order security

SSH

VPN

IPSec

27. WPA
 Created by WiFi Alliance
 Certification started April 2003
 Uses 802.1X authentication server

Distributed different keys to each user.
 Can also be used in “pre-shared key”
(PSK) mode

Every user uses the same passphrase.

Called WPA Personal

28. IEEE 802.1X
http://www.linux.com/howtos/8021X-HOWTO/index.shtml
 Standard for port-
based authentication.
 Uses a third-party
authentication server
such as Radius

29. WPA
 Protocol changes over WEP
 CRC is replaced by “Michael” MIC.

MIC now includes a frame counter, preventing replay
attacks.

Payload bit flipping is now impossible.
 Data encryption still uses RC4, but now

Prevents key recovery attacks on WEP by using
 128b Key
 48b Initialization vector
 Temporal Key Integrity Protocol (TKIP) changes key
dynamically.

30. TKIP
 Temporal Key Integrity Protocol
 Ensures that every data packet has its own
encryption key.

31. 802.11i
 Uses AES instead of RC4.
 Subset published as WPA2
 Uses 802.1X authentication

32. Protocol Layers
 WEP
 Privacy only.
 Very elementary security.
 WPA
 Temporal Key Exchange Protocol

Fixes WEP that scrambles keys between packages and adds a secure
message check.
 AES: Advanced Encryption Standard
 802.11i
 Military grade encryption, replaces DES
 802.1X
 General purpose and extensible framework for authentication users
and generating / distributing keys.
 Simple Secure Network (SSN)
 Recipe for authentication based on 802.1X

33. COEN 351 E-Commerce
Security
 E-Commerce Security Course Homepage
 Lecture Notes