wireless network security: wep and beyond

Wireless Network Security:WEP And Beyond

Heidi ParsayeJason DeVriesRoxanne Ilse

Heidi Parsaye - Jason DeVries - Roxanne Ilse

Outline

• Wireless networking basics– Attempts at making wireless networking secure

• Wired Equivalent Privacy– Why it’s no longer private– Brief overview of how to crack

• Beyond WEP – WiFi Protected Access (WPA)


Wireless Broadband

• How Does Wireless Broadband Work?

• Benefits of Wireless Broadband

• Disadvantage of Wireless Broadband


Wireless Network Security

• IEEE 802.11 WI-FI

• Wired Equivalent Privacy (WEP)

• TKIP (Temporal Key Integrity Protocol)

• MAC address filtering

• Wi-Fi Protected Access (WPA and WPA2) Heidi Parsaye - Jason DeVries - Roxanne

Ilse

Encryption Of WEP Data


Decryption Of WEP Data


Important Details About WEP Frames

• All 802.11 WEP frames contain a plaintext header followed by encrypted data.

• The Initialization Vector is included in the plaintext.

• There is no CRC on the plaintext header. We can easily spoof the BSSID to get around MAC address filtering.

• No attempt is made to hide packet lengths.


Important Details About WEP Frames

• The RC4 Initialization Vector must be sent in plaintext. The recipient needs to be combine it with the secret key to re-create the state array used for decryption.


The Problem With WEP

• It’s actually a problem with RSA RC4 which was designed in 1987 by Ron Rivest (the R in RSA).

• In 2001, Scott Fluhrer, Itsik Mantin, and Adi Shamir (the S in RSA) discovered that the first few bytes of the RC4 data are non-random and leak information about the key.


The Problem With RC4

• The “Secret Key” used by KSA is actually the Initialization Vector (3 bytes) plus the Secret Key (5 or 13 bytes).

• Since we know the first three values, we know the output for the first three iterations of KSA.


The Problem With RC4

• If we can get the state array, we can now start plugging data into PRGA. More specifically, we can start running it in reverse to give us a hint about the secret key.


Another Weakness

• The 3-byte LLC Header is always the same on every frame, starting with 0xAA, indicating that SNAP is next.

• In fact, with a certain message we’ll cover later, we know the values for 16 of the encrypted bytes.

• Knowing some of the encrypted plaintext makes the job even easier.


Getting The Secret Key

• What we really need to see is the exact same plaintext message encrypted thousands of times using different Initialization Vectors.

• If we get enough unique Initialization Vectors, we can crack the secret key.

• But how do we get a WEP network to encrypt and transmit the exact same message thousands of times?– The answer: Ask the network the same question… get

the same answer thousands of times!


We Have Ways Of Making You Talk• Ok, so what question can we ask the network

thousands of times and get the same answer?– Hey network… what’s my IP address? This is known

as an ARP request.• Since we don’t have the secret key, we can’t

encrypt our own ARP request.• That means we need to steal a legitimate ARP

request from the network. Once we get one, we’ll replay it thousands of times. We’ll force the network to talk to us as it replies to these requests… generating messages for us.


ARP Requests

• But if the data is encrypted, how could we find and read an ARP request?– The answer: We don’t need to read it or decrypt its

content. We just need to recognize it as what we need.

• Two facts about ARP requests help us:– They’re always the same fixed length. We can look

for that.– It will be sent to a broadcast address. Remember, the

destination MAC address is sent as plaintext in the 802.11 header so we can read that part.


Retransmitting ARP Requests• Look at the 802.11 frame again.

Once we steal a legitimate ARP request, there’s absolutely nothing to keep us from spoofing our BSSID and retransmitting the exact same request as many times as we want.

• We don’t know the values of the encrypted bytes we’re transmitting, but that’s ok. We don’t care.

• We also won’t be able to read the ARP reply sent by the network. We don’t care about the contents. The important part is that they are the same every time.


Recent Work

• In 2005, Andreas Klein extended the 2001 work of Fluhrer, Mantin, and Shamir. He found additional correlations between the encrypted data and the secret key. However, his method still relied on educated guesses to compute all bytes of the secret key sequentially.– If while computing the 10th byte it turns out you

made an incorrect guess on the 4th byte, you have to throw out all computations done from the 4th byte onward and start again.


Recent Work

• In 2007, Erik Tews, Ralf-Philipp Weinmann, and Andrei Pyshkin optimized Klein’s 2005 attack for usage against WEP.– Most notably, they modified the attack such that it

is possible to compute the secret key bytes independently, instead of sequentially… much more efficient, less wasted computations.

– Working at 802.11g data rates, they showed they could crack 128-bit WEP with just 85,000 packets, a success rate of 95%... in less than 60 seconds.


Using AirCrack


Beyond WEP – WPA2

• Implements mandatory elements of 802.11i• Available in personal (SOHO) and enterprise

mode• Uses AES (Advanced Encryption Standards)


WPA2 Components

• WPA2 Wi-Fi certified client devices; may require software/hardware upgrades

• Client supplicant, such as Microsoft or Funk Odyssey

• EAP Authentication Types• WPA2-Enterprise Wi-Fi Certified APs; may

require firmware or hardware upgrade• Authentication Server (RADIUS)/Database

(SQL, LDAP or AD)Heidi Parsaye - Jason DeVries - Roxanne

Ilse

How WPA2 Works• Initiated when user associates with an AP• User must authenticate first before AP will allow access to

network• Authentication process enabled by IEEE 802.1X/EAP

framework• Client & authentication server mutually authenticate with

each other via the AP• Once authenticated, the authentication server & client

simultaneously generate a “Pairwise Master Key” (PMK)• 4-way handshake between client and AP to complete

authentication and establish AES encryption keys to encrypt data exchanged between client and AP


wireless network security: wep and beyond

Documents