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Wep

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Wep

Security of the WEP algorithm

This is some information about our analysis of the Wired Equivalent Privacy (WEP) algorithm, which is part of the 802.11 standard. This work was performed jointly by Nikita Borisov, Ian Goldberg, and David Wagner. If you have any questions, please contact us at wep@isaac.cs.berkeley.edu.

Executive Summary

We have discovered a number of flaws in the WEP algorithm, which seriously undermine the security claims of the system. In particular, we found the following types of attacks:

Passive attacks to decrypt traffic based on statistical analysis.

Active attack to inject new traffic from unauthorized mobile stations, based on known plaintext.

Active attacks to decrypt traffic, based on tricking the access point.

Dictionary-building attack that, after analysis of about a day's worth of traffic, allows real-time automated decryption of all traffic.

Our analysis suggests that all of these attacks are practical to mount using only inexpensive off-the-shelf equipment. We recommend that anyone using an 802.11 wireless network not rely on WEP for security, and employ other security measures to protect their wireless network.

Note that our attacks apply to both 40-bit and the so-called 128-bit versions of WEP equally well. They also apply to networks that use 802.11b standard (802.11b is an extension to 802.11 to support higher data rates; it leaves the WEP algorithm unchanged).

WEP setup

The 802.11 standard describes the communication that occurs in wireless local area networks (LANs). The Wired Equivalent Privacy (WEP) algorithm is used to protect wireless communication from eavesdropping. A secondary function of WEP is to prevent unauthorized access to a wireless network; this function is not an explicit goal in the 802.11 standard, but it is frequently considered to be a feature of WEP.

WEP relies on a secret key that is shared between a mobile station (eg. a laptop with a wireless ethernet card) and an access point (ie. a base station). The secret key is used to encrypt packets before they are transmitted, and an integrity check is used to ensure that packets are not modified in transit. The standard does not discuss how the shared key is established. In practice, most installations use a single key that is shared between all mobile stations and access points. More sophisticated key management techniques can be used to help defend from the attacks we describe; however, no commercial system we are aware of has mechanisms to support such techniques.

The following two sections describe the problems in the algorithm and the technical details of our attacks; they assume some background understanding of cryptographic protocols. You may wish to skip to the following section, which discusses the practicality of the attacks.

Problems

WEP uses the RC4 encryption algorithm, which is known as a stream cipher. A stream cipher operates by expanding a short key into an infinite pseudo-random key stream. The sender XORs the key stream with the plaintext to produce ciphertext. The receiver has a copy of the same key, and uses it to generate identical key stream. XORing the key stream with the ciphertext yields the original plaintext.

This mode of operation makes stream ciphers vulnerable to several attacks. If an attacker flips a bit in the ciphertext, then upon decryption, the corresponding bit in the plaintext will be flipped. Also, if an eavesdropper intercepts two ciphertexts encrypted with the same key stream, it is possible to obtain the XOR of the two plaintexts. Knowledge of this XOR can enable statistical attacks to recover the plaintexts. The statistical attacks become increasingly practical as more ciphertexts that use the same key stream are known. Once one of the plaintexts becomes known, it is trivial to recover all of the others.

WEP has defenses against both of these attacks. To ensure that a packet has not been modified in transit, it uses an Integrity Check (IC) field in the packet. To avoid encrypting two ciphertexts with the same key stream, an Initialization Vector (IV) is used to augment the shared secret key and produce a different RC4 key for each packet. The IV is also included in the packet. However, both of these measures are implemented incorrectly, resulting in poor security.

The integrity check field is implemented as a CRC-32 checksum, which is part of the encrypted payload of the packet. However, CRC-32 is linear, which means that it is possible to compute the bit difference of two CRCs based on the bit

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