WPA2 Process + 4 way handshake

Answered Question
Mar 28th, 2014
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Good Morning all

I am trying to understand the process used that allows a client to connect to a WPA2 wireless network. I have searched and found many resources but I can't seem to gel it together. So a little help would be appreciated, in particular I'm not clear on what happens after a probe-response or beacon has been received by a client.

My understanding so far:

Beacons sent by an AP or probes sent by a cleint allow the discovery of available WLANs, within the beacon / probe-responses are the names of the WLANs and the security they use; WPA, WPA2, Enterprise etc.

I understand how the WPA 4 way handshake works ( to a degree ) but what's happening in the interim of 'clicking connect' on the client and the start of the 4 way handshake?

The reason I ask is that whilst reading info on how the 4 way handshake works it occoured to me that when the nonce and snonce are being sent between client and AP to create the keys for encryption they are 'exposed', unless there is already some form of encryption taking place?

Many thanks.

 

 

Correct Answer by Rasika Nayanajith about 3 years 4 months ago

 

Here is a good reference for the above topic.

CWSP Certified Wireless Security Professional Official Study Guide

HTH

Rasika

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George Stefanick about 3 years 4 months ago

Hello,

 

Great question .. This is complicated stuff and normally people don’t dive this deep. 

 

Once the probe, authentication (not security authentication btw this is open / shared) and association are complete the ap and station move to the 4 way should WPA/WPA2 / PSK or WPA/WPA2 Enterprise is configured. 

 

The handshake is the same if its a PSK or 802.1X.However the orignal keying material used to start the authentication process is derived differently. The PSK uses the actual phase phrase for seeding material in the  4 way key handshake.While the radius server uses a random AAA key generated during the EAP process. This keying material is used for the seeding martial in the 4 way handshake. 

 

5 Things are needed for the handshake - 

PSK or Master Key

Snonce

S mac address

Anonce

A mac address

 

The Ap sends the first handshake. Inside this frame is its Anonce and mac address both derived from the PSK or Master Key. The client at this point has the UNICAST keying material, because it knows its own Snonce and S mac address. The client then sends its Snonce and S mac address  derived from the PSK / Master key to the AP. The Ap now has enough to derive the key for  UNICAST. 

 

Both transactions are encrypted with a KCK,KEK, TL, keys. Going off memory here… 

 

The 3rd and 4th handshakes are for the broadcast key. 

 

Hope this helps .. 

 

Yes CWSP is a great book. Here is a blog post I wrote on the subject 

http://www.my80211.com/cwsp-george-stefanick/2010/10/3/george-stefanick-...

Please rate if you find this helpful !

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Sean Haynes Mon, 03/31/2014 - 01:52
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Thanks for the link I'll have a look to see if it has the answers I'm looking for.

George Stefanick Wed, 04/02/2014 - 14:19
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Hello,

 

Great question .. This is complicated stuff and normally people don’t dive this deep. 

 

Once the probe, authentication (not security authentication btw this is open / shared) and association are complete the ap and station move to the 4 way should WPA/WPA2 / PSK or WPA/WPA2 Enterprise is configured. 

 

The handshake is the same if its a PSK or 802.1X.However the orignal keying material used to start the authentication process is derived differently. The PSK uses the actual phase phrase for seeding material in the  4 way key handshake.While the radius server uses a random AAA key generated during the EAP process. This keying material is used for the seeding martial in the 4 way handshake. 

 

5 Things are needed for the handshake - 

PSK or Master Key

Snonce

S mac address

Anonce

A mac address

 

The Ap sends the first handshake. Inside this frame is its Anonce and mac address both derived from the PSK or Master Key. The client at this point has the UNICAST keying material, because it knows its own Snonce and S mac address. The client then sends its Snonce and S mac address  derived from the PSK / Master key to the AP. The Ap now has enough to derive the key for  UNICAST. 

 

Both transactions are encrypted with a KCK,KEK, TL, keys. Going off memory here… 

 

The 3rd and 4th handshakes are for the broadcast key. 

 

Hope this helps .. 

 

Yes CWSP is a great book. Here is a blog post I wrote on the subject 

http://www.my80211.com/cwsp-george-stefanick/2010/10/3/george-stefanick-...

Please rate if you find this helpful !

Rasika Nayanajith Wed, 04/02/2014 - 13:58
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George,

You are a master of this topic... I am still in learning stage & your blog posts on this topic is really great...

Rasika

George Stefanick Wed, 04/02/2014 - 14:20
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LOL .. The CCIE W calls me the master ... Im a dumb red neck that likes WiFi ..

Sean Haynes Wed, 04/02/2014 - 23:19
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Morning George, many thanks for the input - I'll take a look at your blog later on.

It is a complicated subject, but it seems the more I begin to understand the more questions it rasies in my mind.

The CWSP book recomended by Manannalage is a fairly easy read  - it's been a big help. I think it may take a while to get this commited to memory, but at least now I can confidently write a report.

So thanks one and all.

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