Follow-up from this question. I just learned that all one needs to decrypt WPA2 traffic is the network key. So for example if I go to a coffee shop and they have the key on the wall I can listen to everything, and if people are visiting sites over HTTP I can MITM them. My question is: why??? Why can't the router and client exchange a secret using public key crypto and encrypt their connection so that other people on the network can't listen? And then each packet or whatever could have a MAC so MITM would be impossible.

  • That means you can give me points on the other question? ;) pulling your leg. The protocol was designed from the begging to be an ethernet-over-air, and as such suffer from the same limitations. I actually have a always VPN (on demand) policy installed in my iphone partly because of this, partly because I do consider DNS services giving over in wireless hotspots trustworthy. Jan 17 '16 at 18:20
  • sorry man :) I'll accept it in a few days, I want to wait for some more answers tho. I upvoted tho. Jan 17 '16 at 19:20
  • No worries. I was just commenting actually. You have plenty of literature if you want to read further on the subject. Jan 17 '16 at 19:24

From my knowledge WPA2 does this (encrypting the connection between client and router with a key different for each device)

WPA2 uses the authentification handshake to generate multiple keys. 2 of these are:

  • Temporal Key (TK) – Used to encrypt/decrypt Unicast data packets
  • Group Temporal Encryption Key

The first one is used to encrypt unicast connections like communication between client and router. The second one is used to encrypt multicast/broadcast messages.

In the case of the mitm attack the attacker persuades the victim to think he needs to send specific/all packets to his address. So the client encrypts the packets with a key the attacker is able to decrypt because the client thinks the attacker is the one he wants to talk to.

Think of it this way: all letterboxes in a city (wifi-network) can only be opened (decrypted) by the correct owner. You want to send secret data to your friend Joe who lives at Goodstreet 1. But you don't know that. So you ask the people of the city where he lives. Alice (also living in the city) convinces you that your friend lives at Badstreet 1. So you put your letter in the letterbox of Badstreet 1, Alice reads it/alters it and forwards it to Goodstreet 1. So even when the letterboxes are secure Alice managed a mitm-attack.

Edit: Professional routers/network equipment have countermessaures against things like ARP-poisoning to try prevent MITM-attacks.

Saying "So what's the point of encryption then? MITM renders it useless!" is not different than saying that a burglar alarm is useless in a public building or theft by a friend you invited to your house.

  • So what's the point of encryption then? MITM renders it useless! Jan 17 '16 at 19:18
  • To stop passive attackers and people not having the key. ARP-poisining is a protocol flaw on another layer which doesn't have anything to do with WPA2. You have the same/better security than on an ethernet network except you need physical access to the ethernet network. Professional routers/network equipment countermessaures against things like ARP-poisoning. Expect prices like 1000$/€ and more for a professional router.
    – H. Idden
    Jan 17 '16 at 19:25
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    This goes more in less in line what I have been saying to René in a related thread. Please do keep in mind 802.1X can be configured in any wired networks too. I configured Cisco catalyst switches at work to authenticate users. I upvoted you. Jan 17 '16 at 19:28
  • MitM does not render WPA2 truly useless you see. I feel quite confident my neighbours are not able to eavesdrop my activity and (ab)use my connection; what I have at home, and at work is a visitor´s network that is separated from our network, and which password is freely given to guests. Jan 17 '16 at 19:36
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    @RuiFRibeiro You are correct. I was just going with the commonly used default configuration as he mentioned a coffee shop which is mostly SOHO-area and doesn't have professional network admins. To be honest I have never seen 802.1X deployed on ethernet including even multiple enterprise organisations with more than 1000 machines. From what I heared the problem was with it's bad support in network devices like printers and giving more problems than it was worth the risk They used authentification by physical port/client MAC/VLANs/... and some vendor specific authenification for non-guest rights
    – H. Idden
    Jan 17 '16 at 19:47

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