Wifi, how does it work?

[eTX]

Hi,

I have always thought that wifi is interesting and I have searched for information of how they work and how can you break them, but I always find information about programs such as aircrack and so one and I would like to know, which is the real way of cracking WEP, WPA etc. Not only using Aircrack and those programs.

Thank you for your attention.

by

[Marc-Andre Leger]

There was an excellent artile in an IEEE magazine a few months ago that explains very well how WiFi works from a technical point of view, but put in a simple language. I have accee to the magazine as I subscribe to it, but you can get it via their website or through a university or school:

Wi-Fi--The Nimble Musician in Your Laptop by Leeper, David G.

This paper appears in: Computer, April 2007, Volume: 40, Issue: 4 On page(s): 108-110

Abstract
It turns out that modern Wi-Fi radios are nimble musicians too, except that they generate radio waves rather than audio waves, and they are much, much faster. The integration of computer engineering and wireless technology, Wi-Fi gives every laptop its own "nimble musician" to send and receive data. A Wi-Fi radio applies computer science to the task by mathematically computing what the waveform would look like if we had actually built all those precision oscillators. The computer technology is known as digital signal processing, and the computation algorithm is known as the fast Fourier transform

As for WEP cracking, you can look here: http://docs.lucidinteractive.ca/index.php/Cracking_WEP_and_WPA_Wire...

It's not that youi really need Aircrack, but you need data... Just google "korek chop chop attack" This is interesting: http://www.security-assessment.com/files/presentations/sa-practical...

[Wifi Ninja]

Science behind Cracking WEP
Old-School :) Came out before WEP was soooo Bad, but has on How it Works

http://www.lava.net/~newsham/wlan/WEP_password_cracker.pdf


Some good Links
WEP Dead Again Part 1
http://www.securityfocus.com/infocus/1814
WEP Dead Again Part 2
http://www.securityfocus.com/infocus/1824



WPA Science Behind it

Good How to and write up
http://www.smallnetbuilder.com/content/view/30278/98/


Wi-Fi Protected Access (WPA) was created to solve the gaping security flaws that plagued WEP. Perhaps the most predominant flaw in WEP is that the key is not hashed, but concatenated to the IV, allowing completely passive compromise of the network. With WEP, you can literally sit in your car listening for packets on a network. Once you have captured enough of them, you can extract the key and connect to the network.

WPA solves this problem by rotating the key on a per-packet basis, which renders the above method useless. However, nothing is perfectly secure, and WPA-PSK is particularly vulnerable during client association, during which the hashed network key is exchanged and validated in a "four-way handshake".

The Wi-Fi Alliance, creators of WPA, were aware of this vulnerability and took precautions accordingly. Instead of concatenating the key in the IV (the weakness of WEP), WPA hashes they key using the wireless access point's SSID as a salt. The benefits of this are two-fold.

First, this prevents the statistical key grabbing techniques that broke WEP by transmitting the key as a hash (cyphertext). It also makes hash precomputation via a technique similar to Rainbow Tables more difficult because the SSID is used as a salt for the hash. WPA-PSK even imposes a eight character minimum on PSK passphrases, making bruteforce attacks less feasible.

So, like virtually all security modalities, the weakness comes down to the passphrase. WPA-PSK is particularly susceptible to dictionary attacks against weak passphrases. In this How To, we'll show you how to crack weak WPA-PSK implementations and give you some tips for setting up a secure WPA-PSK AP for your SOHO.

In a passive attack, all we need to do is listen on a specific channel and wait for a client to authenticate. Kismet is the weapon of choice here, although airodump-ng works too. Kismet gives you much more control and information than airodump-ng, but unfortunately doesn't provide notification to alert you of a successful WPA-PSK association four-way handshake. Airodump-ng does, but gives you less dynamic control of the capture card's behavior and very little information (compared to Kismet).

Active Attacks we can target associated clients of a certain AP with forged deauthentication packets, which should cause the client to disassociate from the AP. We then listen for the reassociation and subsequent authentication. This is a little trickier and also detectable, since we're sending out packets. But it's much quicker than waiting for a genuine association (in most cases).

After identifying our target AP with associated clients, we need to set up the wireless hardware for packet injection.

The Wi-Fi Alliance was wise to implement an eight character minimum for WPA-PSK. Making the key that long essentially renders brute force methods useless. This is because the number of possible typeable character combinations for keys of an eight character length is just above six quadrillion (that's 948 or about 6 x 1015).

My poor little laptop can only crunch about 35 hashes a second, so it would take me about five-and-a-half million years (I'm not kidding here either, I did the math!) to create a hash table for an eight character hash table or to test all possible combinations when brute-forcing a key.

And what's more, since the hash is salted with the SSID of the AP, that hash table I just spent five million years creating, would be good only against APs with that exact SSID. So, clearly we're not going to be brute-forcing any WPA keys anytime soon.

What we can do, however, is limit the list of possible passphrases by making educated guesses, compute the hashes of those guesses and check them against our captured key. This technique is referred to as a dictionary attack.

BackTrack v2 comes bundled with a good offering of simple wordlists, as well as four lists of passwords common in the '90s, reverse-sorted by occurrence (more common passwords are at the top, less common passwords are at the bottom). The lists seem to be missing from Backtrack v3, but there are plenty of wordlists around the 'net.

Using the wordlists in Backtrack version 2, we can mount a dictionary attack on our captured WPA handshake using either aircrack-ng or coWPAtty. Aircrack-ng runs much faster on my attacking system (testing 3740 keys took 35 seconds), and has native optimization for multiple processors. coWPAtty, on the other hand, runs much slower (testing the same 3740 keys took almost 2 minutes) and can accept hash files precomputed by genpmk.

The obvious limitation of these techniques is the existence of the key within the dictionary file used for the attack. I hope that I never see WPA keys like "dinosaur" or "dictionary", which will be easily cracked by coWPAtty or aircrack-ng.

But something like "dinosaur52" or "D1cti0nary" would seem pretty secure at this point, right? They would at least be missed by a plain-jane sweep through the dictionary and would take a couple million years to straight brute force.

To extend the list of possible keys, we can use the legendary *NIX password cracking tool John the Ripper's wordlist mangling rules to generate permutations and common password additions from a simple dictionary file. These are then fed into either coWPAtty or aircrack-ng on the fly.

When using dictionary attacks, we don't need to worry about short passphrases making it through; coWPAtty and aircrack-ng are both smart enough to drop passphrases shorter than eight characters. In fact, it's smart to leave them in the dictionary file in case they become long enough as a result of John's word mangling rules

So, how long and cryptic does a passphrase really need to be? The straight answer is: as long and as cryptic as possible! With John's word mangling rules, we're systematically and intelligently attacking passphrases by incorporating common human substitutions and combinations with dictionary lists. Of course, there are some word-based passphrases that could slip through John's mangling rules. but all it takes is a combination of simple rules to catch that those as well.

The plane-jane wordlist that comes with coWPAtty contains 10,201 words. After default mangling with John, that number blossoms to 498,989. Adding our rules from above and that number climbs to 45,720,022. The more rules we add, the more the passphrase search space keeps expanding.

This is still a far-cry from the six quadrillion possible combinations out there. But what makes this dangerous is that we started with distinct set of possible passphrases and used a semi-human approach to making them more cryptic. So, chances are better that if we try 45 million intelligently generated passphrases, we might get lucky and find a winner.

It takes my system about five days to crank through 45 million passphrases. This isn't exactly lightning fast. But given the fact that I could have passively captured your key's hash, by the time you found out, it would be too late.