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Weakness in Passphrase Choice in WPA Interface







Weakness in Passphrase Choice in WPA
Interface

Weakness in Passphrase Choice in WPA
Interface
11/04/2003 02:32 PM

By Robert MoskowitzSenior Technical DirectorICSA 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...




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In my recent post on Blog-Hosted Conversations, I threw out the following Question as a possible first Conversation topic:

How could we overcome the huge disconnect that exists today between the people who have great ideas and the people who have the money and other resources to realize those ideas?

The question provoked almost as much response as Jeremy Heigh's idea for the Conversations did, so I thought it might be worth exploring further how and why this disconnect exists.

In the post I mentioned that I belong to these informal communities and networks, with about 1500 members in total:
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  • technophiles/social networkers,
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Some of those people are useful 'connectors' that give me access to other communities and networks: For example, some of the bloggers in my business, progressive, Salon and Canadian blogger networks also happen to be published authors, journalists, publishers, economists, venture capitalists, teachers or professors, and know others in these fields. Some of them are true Tipping Point 'connectors< /a>', who pride themselves on hooking people up with others they would probably never find on their own. The logic of LinkedIn and eCademy is based on the presumption that if your networks are substantial and well-managed you should be able to get access to virtually anyone and anything you need through "the strength of weak ties (SWT)".

This may work fine in the application that SWT theory was originally developed around -- finding prospective employees. The employer is often looking for the best possible candidate, and wants to cast as wide a net as possible. The onus is on the job-seeker to navigate his/her way through the weak ties and win the job. The employer incurs virtually no cost in casting the wide net (especially today with the cost of posting electronically virtually zero).

This is especially true if the recruiter delegates pre-screening duties to HR staff or a 'head-hunter' to create a tiny short-list of candidates to interview. And today, with big corporations being net destroyers, not creators, of jobs, where searches for well-paying jobs are increasingly scarce, a wide net can attract some extraordinary people, enough to pay for the head-hunter in spades. When you're a big corporation with lots of resources at your disposal, it's a buyer's market.

But in most situations -- the search for business partners, marriage partners, jobs or investors for example, or the search for experts or employees if you're an entrepreneur with modest resources -- the value of casting a wide net in the search is limited by two constraints:
  1. There is a significant cost to the searcher of each potential candidate to be considered. That cost can be mental or physical energy, or time, or money, or all of the above. If you open yourself up to candidates outside your immediate network, you can attract a flood of candidates, many of whom will be inappropriate, annoying, dangerous or even fraudulent. 
    • If you're looking for a business partner you'll likely attract unskilled unemployed people who would really rather just have a job.
    • If you're looking for a marriage partner you may attract hookers, golddiggers, economic refugees, and their respective pimps.
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Unlike the large corporation executive, you can't afford to hire someone to separate the wheat from the chaff (or, more likely, find the needle in the haystack). And even if you could, it's quite possible the agent you hire will accept kickbacks from one of the candidates to give them the inside track. Bottom line: Better not cast a wide net. Go to your strong personal contacts one at a time and ask them for one candidate. Repeat until you find the right person.
  1. The human need for trust in all important relationships means that you will tend to prefer a fair candidate you know and trust well, over a good candidate that someone you trust trusts but whom you don't know well enough to trust. And you'll prefer either of these over a sensational candidate you don't know from Adam. Trust takes time, shared experiences, and usually face-to-face contact. Bottom line: We usually go with who we know.
Now consider this from the perspective of the person you're seeking -- the prospective business or marriage partner, expert, investor or employer. They're getting overwhelmed by twice or thrice-removed referrals for connections. Most of the 'callers' are looking to get something that the recipient is unable or disinclined to give (especially to a stranger), or may not have even thought about. What do they do? They tell their close contacts not to refer anyone to them. They unsubscribe from social network lists. They get unlisted phone numbers and unlisted e-mail addresses. They get agents and intermediaries to handle communications for them and shield them from 'weak ties'.

tipping_pointSo instead of the idealized networks of the Tipping Point, shown at right, where connectors, mavens and salesmen work to connect people and ideas virally, we end up with the constricted, broken networks shown in the diagrams above: Outgoing connections are constricted by the high cost of extending too wide a net, and the lack of trust the further away the connection is, to the point the 'ideal' connection is rarely made. And returning connections are likewise constricted by the sequential disconnects of connectors, agents & intermediaries, and filters, to the point the people you most want to connect with are often the least likely to 'return your call'. This has always been so, and insofar as information is concerned, the Internet is much less constricted than previous information channels. But insofar as people are concerned, I would argue that the disconnects are as great as they have always been. The rich, the famous, the powerful, the most-wanted and the ideal matches are no more accessible and available for relationships than they ever were. Even those who are not still on the wrong side of the digital divide have mostly reintermediated themselves so the technologically possible connection between everyone and everyone else is kept humanly impossible.

Such is the weakness of weak ties. When it comes to human connection, the network is still broken.

I think this is the reason for the disconnect between people with great ideas and people with the money and resources to realize them -- the reason so many great ideas go nowhere.

So now we need a Blog-Hosted Conversation to discuss what to do about it -- how to work around these disconnects. I suspect that part of the answer is permissioning and permission marketing. We need to give something away to establish trust and differentiate ourselves from the 'inauthentic' and 'unqualified' callers, and to make ideal connections.

My first Blog-Hosted Conversation will take place at the end of the month. Stay tuned.

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Weakness in Passphrase Choice in WPA Interface

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