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Security of 802.11

COMP4690: Advanced Topic

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Outline

Introduction of 802.11 Introduction of WEP Security of WEP References

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Example of Single-Cell Wireless LAN

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Example of Multi-Cell Wireless LAN

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IEEE 802.11 – infrastructure mode Smallest building block is Basic Service Set (BSS)

A number of stations executing the same MAC protocol Compete for access to same shared wireless medium

A BSS may be isolated, or may connect to a Backbone Distribution System (DS) through an Access Point (AP) AP functions as bridge

MAC protocol may be distributed or controlled by a central coordination function in AP

BSS generally corresponds to a “cell” DS can be a switch, wired network, or wireless

network

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IEEE 802.11 – infrastructure mode

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

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IEEE 802.11 Protocol Architecture

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Some history Cordless phones Analog cell phones Anybody with a scanner that operated at the right frequency

could easily listen to calls not intended for them. 802.11 tried to avoid, but the same cycle still played out… Wireless access points must announce themselves to the world:

by broadcasting 802.11 beacon frames. Wandering users with an 802.11 receiver can find out about

wireless networks in the area simply by putting up an antenna. War driving: driving around looking for access points

http://www.securityfocus.com/news/192 One quick solution: MAC-address filtering

But malicious attackers can alter addresses (with sufficient operating system privileges).

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Security of 802.11

Existing security consists of two subsystems A data encapsulation technique called Wired Equivalent Privacy (WEP)

An authentication algorithm called Shared Key Authentication

Defined in Section 8 “Authentication and privacy” in the IEEE Std 802.11 (in 1997).

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WEP Wired Equivalent Privacy

To protect authorized users of a wireless LAN from casual eavesdropping

To create the privacy achieved by a wired network (using a switch)

Because of the U.S. export regulations, WEP secret keys were limited to 40 bits. The rules were relaxed later.

WEP has been an option feature. But WEP itself was a flawed anchor point for

security. Even with “128-bit” version, WEP is still not secure!

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WEP

WEP relied on a secret key (40-bit) shared between the two peers (usually a mobile station and an access point).

Before transmission of a frame: Checksumming: compute an integrity checksum on the

message using CRC-32 algorithm, then concatenate the two into plaintext P

Encryption: encrypt the plaintext P using RC4. This requires an initialization vector (IV). IV has only 24 bits. It is inserted into the frame in plaintext (so that the peer can decrypt the ciphertext).

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WEP Encapsulation

802.11 Hdr Data

WEP Encapsulation Summary:

• Encryption Algorithm = RC4

• Per-packet encryption key = 24-bit IV concatenated to a pre-shared key

• WEP allows IV to be reused with any frame

• Data integrity provided by CRC-32 of the plaintext data (the “ICV”)

• Data and ICV are encrypted under the per-packet encryption key

802.11 Hdr DataIV ICV

Encapsulate Decapsulate

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WEP Encryption

IV: 24-bit, how often to change IV values?

Secret Key: 40-bit, be distributed by an external key management service

Seed: 64-bit

PRNG: Pseudo-Random Number Generator, using RC4 algorithm from RSA Data Security, Inc.

ICV: 32-bit, using CRC-32

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WEP Encryption

Message: M Initialization vector (IV): v Integrity checksum (ICV): c(M) After concatenation: P = M || c(M) Key sequence: RC4(v, k) Ciphertext: C = PRC4(v, k)

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WEP Decryption

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WEP Decryption

The receiver gets v from the received frame. The receiver calculates the key sequence using

RC4(v, k). The receiver decrypts the ciphertext using

P = CRC4(v, k) P is divided into two parts: M' and c' where c'

contains the last 32 bits of P. If c(M') = c', the receiver will think that M' is the message.

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Properties of Vernam Ciphers

The WEP encryption algorithm RC4 is a Vernam Cipher:

Pseudo-random number

generator

Encryption Key K

Plaintext data byte p

Random byte b

Ciphertext data byte p

Decryption works the same way: p = c b

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Properties of Vernam Ciphers

Thought experiment 1: what happens when p1 and p2 are encrypted under the same “random” byte b?

c1 = p1 b c2 = p2 b

Then:

Conclusion: it is a very bad idea to encrypt any two messages using the same key sequence.

The ciphertexts can be sniffered easily. So once p1 is know, you can know p2, and vice versa.

c1 c2 = (p1 b) (p2 b) = p1 p2

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Keystream Reuse

Keystream (or key sequence): RC(v, k) Usually k is fixed. How about v?

It’s better to have different values of v for different packets: per-packet IV.

However, “IV collision” is difficult to avoid: For some implementations, IV is reset to 0 when

initialized, and incremented by one for each packet. IV has only 24 bits, too short!!!

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How to Read WEP Encrypted Traffic (1)

•If IV is randomly chosen, by Birthday Paradox, probability Pn that two packets will share same IV after n packets is P2 = 1/224 after two frames and Pn = Pn–1 + (n–1)(1–Pn–1)/ 224 for n > 2.

• 50% chance of a collision exists already after only 4823 packets!!!

•Once two encrypted packets that use the same IV are discovered, various methods of attack can be applied to recover the plaintext. If the plaintext of one of the messages is knows, it is easy to derive the contents of the other one; and also other messages that have been encrypted using the same IV can be directly decrypted.

802.11 Hdr DataIV ICV

24 bits Encrypted under Key +IV using RC4

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How to Read WEP Encrypted Traffic (2)

Ways to accelerate the process: Send spam into the network: no pattern

recognition required! Get the victim to send e-mail to you

The AP creates the plaintext for you! Decrypt packets from one Station to another

via an Access PointIf you know the plaintext on one leg of the

journey, you can recover the key stream immediately on the other

Etc., etc., etc.

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Data Integrity

CRC checksum is insufficient to ensure data integrity. It is designed to detect random errors, not to stop

malicious attackers! It is easy for attackers to modify the message

and to inject messages without detection! Mainly because CRC is a linear function of the

message: c(xy) = c(x) c(y)

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Message Modification

Assume C is the ciphertext of message M C = RC4(v, k) (M||c(M))

For an attacker, he can easily capture C and v. But he doesn’t know k.

The main purpose of the attacker is to replay a different message C' to the receiver without being detected. In another word, the receiver will get a wrong message M'.

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Message Modification

Given any E, the attacker can calculate its checksum c(E). Then the attacker generates C' as: C' = C (E||c(E))

Once C' is received by the receiver, it will decrypt as follows:

Therefore the receiver gets a wrong message M' = ME.

4( , )

( || ( )) 4( , )

4( , ) ( || ( )) ( || ( )) 4( , )

( || ( )) ( || ( ))

( ) || ( )

|| ( )

C RC v k

C E c E RC v k

RC v k M c M E c E RC v k

M c M E c E

M E c M E

M c M

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Final answer?

WPA: Wi-Fi Protected Access Intended as an intermediate measure to take the place of WEP

while 802.11i was prepared 802.11i

Ratified in June 2004. Supported by Intel, IBM, etc. AES-CCMP (Counter-Mode/CBC-MAC Protocol) WRAP (Wireless Robust Authenticated Protocol) 802.1x

WPA2 WPA2 is the certified form of 802.11i tested by the Wi-Fi Alliance. WPA2 implements the mandatory elements of 802.11i .

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References

ISO/IEEE Std 802.11, Section 8, “MAC and PHY Specifications -- Authentication and Privacy”, 1999 Edition.

Nikita Borisov, et al, “Intercepting Mobile Communications: the Insecurity of 802.11”, 2001.

W. A. Arbaugh, et al, “Your 802.11 Wireless Network has No Clothes”, 2001.

Matthew Gast, “Wireless LAN Security: a Short History”, 2002.