Friday, August 8, 2008

Wired Equivalent Privacy

Wired Equivalent Privacy (WEP) is a deprecated algorithm to secure IEEE 802.11 wireless networks. Wireless networks broadcast messages using radio and are thus more susceptible to eavesdropping than wired networks. When introduced in 1999, WEP was intended to provide confidentiality comparable to that of a traditional wired network.

Beginning in 2001, several serious weaknesses were identified by cryptanalysts with the result that today a WEP connection can be cracked with readily available software within minutes. Within a few months the IEEE created a new 802.11i task force to counteract the problems. By 2003, the Wi-Fi Alliance announced that WEP had been superseded by Wi-Fi Protected Access (WPA), which was a subset of then upcoming 802.11i amendment. Finally in 2004, with the ratification of the full 802.11i standard, the IEEE declared that both WEP-40 and WEP-104 "have been deprecated as they fail to meet their security goals". Despite its weaknesses, WEP is still widely in use. WEP is often the first security choice presented to users by router configuration tools even though it provides a level of security that deters only unintentional use, leaving the network vulnerable to deliberate compromise.

WEP is sometimes inaccurately referred to as Wireless Encryption Protocol.

  • Encryption details

WEP was included as the privacy of the original IEEE 802.11 standard ratified in September 1999.[5] WEP uses the stream cipher RC4 for confidentiality,[6] and the CRC-32 checksum for integrity. It was deprecated as a wireless privacy mechanism in 2004, but for legacy purposes is still documented in the current standard.

Basic WEP encryption: RC4 keystream XORed with plaintext
Basic WEP encryption: RC4 keystream XORed with plaintext

Basic WEP encryption: RC4 keystream XORed with plaintext

Standard 64-bit WEP uses a 40 bit key (also known as WEP-40), which is concatenated with a 24-bit initialization vector (IV) to form the RC4 traffic key. At the time that the original WEP standard was being drafted, U.S. Government export restrictions on cryptographic technology limited the key size. Once the restrictions were lifted, all of the major manufacturers eventually implemented an extended 128-bit WEP protocol using a 104-bit key size (WEP-104).

A 128-bit WEP key is almost always entered by users as a string of 26 Hexadecimal (Hex) characters (0-9 and A-F). Each character represents 4 bits of the key. 4 × 26 = 104 bits; adding the 24-bit IV brings us what we call a "128-bit WEP key". A 256-bit WEP system is available from some vendors, and as with the above-mentioned system, 24 bits of that is for the I.V., leaving 232 actual bits for protection. This is typically entered as 58 Hexadecimal characters.
(58 × 4 = 232 bits) + 24 I.V. bits = 256 bits of WEP protection.

Key size is not the only major security limitation in WEP. Cracking a longer key requires interception of more packets, but there are active attacks that stimulate the necessary traffic. There are other weaknesses in WEP, including the possibility of IV collisions and altered packets, that are not helped at all by a longer key.

  • Authentication

Two methods of authentication can be used with WEP: Open System authentication and Shared Key authentication.

For the sake of clarity, we discuss WEP authentication in the Infrastructure mode (ie, between a WLAN client and an Access Point), but the discussion applies to the Ad-Hoc mode too.

In Open System authentication, the WLAN client need not provide its credentials to the Access Point during authentication. Thus, any client, regardless of its WEP keys, can authenticate itself with the Access Point and then attempt to associate. In effect, no authentication (in the true sense of the term) occurs. After the authentication and association, WEP can be used for encrypting the data frames. At this point, the client needs to have the right keys.

In Shared Key authentication, WEP is used for authentication. A four-way challenge-response handshake is used:

I) The client station sends an authentication request to the Access Point.

II) The Access Point sends back a clear-text challenge.

III) The client has to encrypt the challenge text using the configured WEP key, and send it back in another authentication request.

IV) The Access Point decrypts the material, and compares it with the clear-text it had sent. Depending on the success of this comparison, the Access Point sends back a positive or negative response. After the authentication and association, WEP can be used for encrypting the data frames.

At first glance, it might seem as though Shared Key authentication is more secure than Open System authentication, since the latter offers no real authentication. However, it is quite the reverse. It is possible to derive the keystream used for the handshake by capturing the challenge frames in Shared Key authentication. Hence, it is advisable to use Open System authentication for WEP authentication, rather than Shared Key authentication. (Note that both authentication mechanisms are weak).

  • Remedies

Use of encrypted tunneling protocols (e.g. IPSec, Secure Shell) can provide secure data transmission over an insecure network. However, replacements for WEP have been developed with the goal of restoring security to the wireless network itself.

  • 802.11i (WPA and WPA2)

The recommended solution to WEP security problems is to switch to WPA2 or the less resource intensive WPA. Either is much more secure than WEP. To add support for WPA or WPA2, some old Wi-Fi access points might need to be replaced or have their firmware upgraded. WPA was designed as an interim software solution for WEP; it runs on the same hardware that WEP does.

Implemented non-standard fixes

  • WEP2

This stopgap enhancement to WEP was present in some of the early 802.11i drafts. It was implementable on some (not all) hardware not able to handle WPA or WPA2, and extended both the IV and the key values to 128 bits. It was hoped to eliminate the duplicate IV deficiency as well as stop brute force key attacks.

After it became clear that the overall WEP algorithm was deficient however (and not just the IV and key sizes) and would require even more fixes, both the WEP2 name and original algorithm were dropped. The two extended key lengths remained in what eventually became WPA's TKIP.

  • WEPplus

Also known as WEP+. A proprietary enhancement to WEP by Agere Systems (formerly a subsidiary of Lucent Technologies) that enhances WEP security by avoiding "weak IVs". It is only completely effective when WEPplus is used at both ends of the wireless connection. As this cannot easily be enforced, it remains a serious limitation. It is possible that successful attacks against WEPplus will eventually be found. It also does not necessarily prevent replay attacks.

  • Dynamic WEP

Change WEP keys dynamically. Vendor-specific feature provided by several vendors such as 3Com.

The dynamic change idea made it into 802.11i as part of TKIP, but not for the actual WEP algorithm.


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