iBet uBet web content aggregator. Adding the entire web to your favor.
iBet uBet web content aggregator. Adding the entire web to your favor.



Link to original content: https://web.archive.org/web/20071126054954/http://www.wifinetnews.com/archives/002452.html
Wi-Fi Networking News: Weakness in Passphrase Choice in WPA Interface
The Wayback Machine - https://web.archive.org/web/20071126054954/http://www.wifinetnews.com:80/archives/002452.html

Email Delivery

Receive new posts as email.

Email address

Syndicate WNN sites

Single feed for all sites

Syndicate this site

RSS 0.91 | RSS 2.0
RDF | Atom
Podcast only feed (RSS 2.0 format)
Get an RSS reader
Get a Podcast receiver

Contact

About This Site
Contact Us
Privacy Policy

Search

Google

Web this site

January 2007
Sun Mon Tues Wed Thurs Fri Sat
  1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31      

Stories by Category

Basics :: Basics
Casting :: Casting Listen In Podcasts Videocasts
Culture :: Culture Hacking
Future :: Future
Hardware :: Hardware Adapters Appliances Chips Consumer Electronics Gaming Home Entertainment Music Photography Video Gadgets Mesh Monitoring and Testing PDAs
Industry :: Industry Conferences Financial Deals Free Health Legal Research Vendor analysis
International :: International
Media :: Media IPTV Locally cached Streaming
Metro-Scale Networks :: Metro-Scale Networks Community Networking Municipal Public Safety
Network Types :: Network Types Broadband Wireless Cellular 2.5G and 3G 4G UMTS Power Line Satellite
News :: News Mainstream Media
Politics :: Politics Regulation Sock Puppets
Schedules :: Schedules
Security :: Security 802.1X
Site Specific :: Site Specific Administrative Detail April Fool's Blogging Book review Cluelessness Guest Commentary History Humor Self-Promotion Unique Who's Hot Today?
Software :: Software Open Source
Spectrum :: Spectrum
Standards :: Standards 802.11a 802.11e 802.11g 802.11n 802.20 Bluetooth MIMO UWB WiMAX ZigBee
Transportation and Lodging :: Transportation and Lodging Air Travel Aquatic Hotels Rails
Unclassified :: Unclassified
Vertical Markets :: Vertical Markets Academia Enterprise WLAN Switches Home Hot Spot Aggregators Hot Spot Advertising Road Warrior Roaming Libraries Location Medical Residential Rural SOHO Small-Medium Sized Business Universities Utilities wISP
Voice :: Voice

Archives

January 2007 | December 2006 | November 2006 | October 2006 | September 2006 | August 2006 | July 2006 | June 2006 | May 2006 | April 2006 | March 2006 | February 2006 | January 2006 | December 2005 | November 2005 | October 2005 | September 2005 | August 2005 | July 2005 | June 2005 | May 2005 | April 2005 | March 2005 | February 2005 | January 2005 | December 2004 | November 2004 | October 2004 | September 2004 | August 2004 | July 2004 | June 2004 | May 2004 | April 2004 | March 2004 | February 2004 | January 2004 | December 2003 | November 2003 | October 2003 | September 2003 | August 2003 | July 2003 | June 2003 | May 2003 | April 2003 | March 2003 | February 2003 | January 2003 | December 2002 | November 2002 | October 2002 | September 2002 | August 2002 | July 2002 | June 2002 | May 2002 | April 2002 | March 2002 | February 2002 | January 2002 | December 2001 | November 2001 | October 2001 | September 2001 | August 2001 | July 2001 | June 2001 | May 2001 | April 2001 |

Recent Entries

Wi-Fi Protected Setup Details Announced
Details on San Francisco/EarthLink Deal
San Francisco Reaches Deal with EarthLink, Google
Solid Coverage in Time of Muni Wi-Fi
NextWave Buys Go Networks
Surf, Sand, and Wi-Fi
Bluetooth Has Patent Woes
San Francisco! Slowly I Turned...Step by Step...Inch by Inch...
EarthLink CEO Garry Betty Dies
Rent-A-Cellular-Bridge from Avis

Site Philosophy

This site operates as an independent editorial operation. Advertising, sponsorships, and other non-editorial materials represent the opinions and messages of their respective origins, and not of the site operator or JiWire, Inc.

Copyright

Entire site and all contents except otherwise noted © Copyright 2001-2006 by Glenn Fleishman. Some images ©2006 Jupiterimages Corporation. All rights reserved. Please contact us for reprint rights. Linking is, of course, free and encouraged.

Powered by
Movable Type

« WPA's Achilles Foot | Main | WPA's Little Secret »

November 4, 2003

Weakness in Passphrase Choice in WPA Interface

By Glenn Fleishman

By Robert Moskowitz
Senior Technical Director
ICSA Labs, a division of TruSecure Corp

Use of PSK as the key establishment method

WPA and 802.11i provide for a Pre-Shared Key (PSK) as an alternative to 802.1X based key establishment. A PSK is a 256 bit number or a passphrase 8 to 63 bytes long. Each station MAY have its own PSK, tied to its MAC address. To date, vendors are only providing for one PSK for an ESS, just as they do for WEP keying.

When a PSK is used instead of 802.1X, the PSK is the Pairwise Master Key (PMK) that is used to drive the 4-way handshake and the whole Pairwise Transient Key (PTK) keying hierarchy. There is a straightforward formula for converting a passphrase PSK to the 256-bit value needed for the PMK.

This paper will look into the risks of using a PSK and particularly the risk associated with a passphrase-based PSK.

How the PSK is used in WPA and 802.11i

The PSK provides an easily implemented alternative for the PMK as compared to using 802.1X to generate a PMK. A 256bit PSK is used directly as the PMK. When the PSK is a passphrase, the PMK is derived from the passphrase as follows:

PMK = PBKDF2(passphrase, ssid, ssidLength, 4096, 256)

Where the PBKDF2 method is from PKCS #5 v2.0: Password-based Cryptography Standard. This means that the concatenated string of the passphrase, SSID, and the SSIDlength is hashed 4096 times to generate a value of 256 bits. The lengths of the passphrase and the SSID have little impact on the speed of this operation.

The PTK is a keyed-HMAC function using the PMK on the two MAC addresses and the two nonces from the first two packets of the 4-Way Handshake. This is why the whole keying hierarchy falls into the hands of anyone possessing the PSK, as all the other information is knowable.

The Intra-PSK attack

The normal practice is to have a single PSK within an ESS. To generate any PTK, a device only needs to learn the two MAC addresses and nonces (and the selected ciphersuite). All of this is available in the initial exchange, from the ASSOCIATE through the 4-Way Handshake. Any device can passively listen for these frames and then generate the PTK. If the device missed these frames, it can send a DISASSOCIATE against the STA and force the STA to perform the ASSOCIATE through the 4-Way Handshake again.

Thus even though each unicast pairing in the ESS has unique keys (PTK) there is nothing private about these keys to any other device in the ESS.

The offline PSK dictionary attack

A station that does not know a passphrase-based PSK can attack it with an offline attack. This is effective for an outsider where there is a single PSK in the ESS, or an insider where there are unique PSKs.

The 802.11i standard points out that:

A passphrase typically has about 2.5 bits of security per character, so the passphrase of n bytes equates to a key with about 2.5n + 12 bits of security. Hence, it provides a relatively low level of security, with keys generated from short passwords subject to dictionary attack. Use of the key hash is recommended only where it is impractical to make use of a stronger form of user authentication. A key generated from a passphrase of less than about 20 characters is unlikely to deter attacks.

The PTK is used in the 4-Way handshake to produce a hash of the frames. There is a long history of offline dictionary attacks against hashes. Any of these programs can be altered to use the information in the 4-Way Handshake as input to perform the offline attack. Just about any 8-character string a user may select will be in the dictionary. As the standard states, passphrases longer than 20 characters are needed to start deterring attacks. This is considerably longer than most people will be willing to use.

This offline attack should be easier to execute than the WEP attacks.

Using Random values for the PSK

The PSK MAY be a 256-bit (64 hexadecimal) random number. This is a large number for human entry; 20 character passphrases are considered too long for entry. Given the nature of the attack against the 4-Way Handshake, a PSK with only 128 bits of security is really sufficient, and in fact against current brute-strength attacks, 96 bits SHOULD be adequate. This is still larger than a large passphrase, but is unlikely to be in a dictionary attack. Using a relatively small random value represented in hexadecimal, and entering it as a passphrase will expand it to a proper 256-bit PSK.

Summary

Anyone with knowledge of the PSK can determine any PTK in the ESS through passive sniffing of the wireless network, listening for those all-important key exchange data frames. Also, if a weak passphrase is used, for example, a short passphrase, an offline dictionary attack can readily guess the PSK. Since the common usage will be a single PSK for the ESS, once this is learned by the attacker, the attacker is now a member of the ESS, and the whole ESS is compromised. The attacker can now read and forge any traffic in the ESS.

Pre-Shared Keying is provided in the standard to simplify deployments in small, low risk, networks. The risk of using PSKs against internal attacks is almost as bad as WEP. The risk of using passphrase based PSKs against external attacks is greater than using WEP. Thus the only value PSK has is if only truly random keys are used, or for deploy testing of basic WPA or 802.11i functions. PSK should ONLY be used if this is fully understood by the deployers.

Posted by Glennf at November 4, 2003 9:37 AM

Categories:

Trackback Pings

TrackBack URL for this entry:
https://db.isbn.nu/mt3/mt-tb.pl/1037

Listed below are links to weblogs that reference Weakness in Passphrase Choice in WPA Interface:

» Weakness in Passphrase Choice in WPA Interface from Liudvikas Bukys
Robert Moskowitz: Wi-Fi Networking News: Weakness in Passphrase Choice in WPA Interface [Read More]

Tracked on November 4, 2003 11:55 AM

» Weakness found in Wi-Fi security protocol from Digital Common Sense
November 06, InternetNews.com — Weakness found in Wi-Fi security protocol. Wireless security expert Robert Moskowitz has detected a glaring weakness in the interface design of a Wi-Fi Protected Access (WPA) protocol deployed in numerous Wireless ... [Read More]

Tracked on November 7, 2003 9:11 AM

» Problems with the WPA Security and Pre-Shared Keys from Bowulf Infosec & Weightloss Blog
Weakness in Passphrase Choice in WPA Interface Anyone with knowledge of the PSK can determine any PTK in the ESS through passive sniffing of the wireless network, listening for those all-important key exchange data frames. Also, if a weak passphrase... [Read More]

Tracked on November 10, 2003 2:18 PM

» Weaknesses found in new wireless standard from Lockergnome's Technology News
"A research paper released by a U.S.-based security expert last week highlighted the weaknesses of the new Wi-Fi Protected Access (WPA) standard compared to its predecessor, wired equivalent piracy (WEP) — the most commonly used wireless security stand... [Read More]

Tracked on November 11, 2003 12:25 PM

» http://doping.sics.se/linkflow/archives/2003_11.html#001635 from Emmanuel's Link Flow
Wi-Fi Networking News: Weakness in Passphrase Choice in WPA Interface... [Read More]

Tracked on November 19, 2003 12:24 AM

» Wifi van Arnhem naar Nijmegen: 29 hits. from Plein '44
[Read More]

Tracked on November 29, 2004 5:10 PM

Comments

Part of the JiWire Network

Secure your Wi-Fi with
JiWire Hotspot Helper™

Directory tags: Wi-Fi | WiFi | DMOZ Wireless | Yahoo Wi-Fi