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One of the questions that comes on quite often is about WiFi encryption.

Now imagine that you have multiple users connecting to one AP. With a simple WPA2 protection setup they can still sniff each others traffic.

What technology would allow a different encryption key per user? So that when multiple users are on the same AP they would not be able to see each others plain traffic?

Does RADIUS implement this? How does the initial setup work?

Can this also be done with a simple home AP?

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2 Answers

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I'm going to assume your goals are the following:

  1. Prevent unauthorized access to the WiFi AP.
  2. Maintain the confidentiality of traffic between WiFi clients and the AP.
  3. Maintain the integrity of traffic between WiFi clients and the AP.
  4. Allow multiple users to authenticate to the AP with different credentials.

RADIUS implements goals #1 and #4, but not in the way that you might expect. In traditional 802.11 authentication, the client sends an authentication key to the AP, which verifies it and, if correct, allows the client to connect. The actual key exchange is more complicated, and varies depending on what security protocol you're using (e.g. WEP, WPA, WPA2), but that's somewhat irrelevant to our scenario.

RADIUS sits on top of these protocols, acting as a credential provider. With RADIUS, the AP sends all authentication requests over to the RADIUS server (usually via wired ethernet) responsible for managing the AP. One of the major benefits of RADIUS is that different users can log in with different credentials, and all attempts can be logged and audited. Some configurations allow for client certificates as an authentication mechanism, which is much stronger than traditional passwords. However, the use of RADIUS does not solve #2 and #3, because the underlying security protocols (WPA / WPA2) are flawed, allowing traffic to be sniffed, decrypted and (in certain scenarios) modified by authenticated clients.

In order to fulfill goals #2 and #3, you need to implement IPsec on your network. IPsec acts as an end-to-end cryptography mechanism at the Internet layer (i.e. IP layer), providing confidentiality, integrity and authenticity. Authentication is part of IPsec, so it partially supercedes RADIUS in this purpose.

In any case, IPsec should act as a strong authentication mechanism, using client certificates. Further security via RADIUS is a good thing, because it allows the following benefits:

  • Protection from standard WiFi cracking attempts.
  • A credential store for VPNs, remote NAS, etc.
  • Full audit / logging for device authentication on the AP.

In terms of your last question, "can I do this on a home AP?", it depends. Almost all will allow for RADIUS, but I'm not sure how many will support IPsec. RADIUS on its own will not prevent sniffing between legitimate clients, but it will provide a barrier to unauthenticated clients. You need IPsec to enforce confidentiality.

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"the underlying security protocols (WPA / WPA2) are flawed" Can you give a reference? –  curiousguy Aug 28 '12 at 12:01
    
@curiousguy They're flawed in the sense that they allow authenticated clients to read each other's data. Furthermore, the WPS extension makes it trivial to break into WPA/WPA2 networks. –  Polynomial Aug 28 '12 at 12:56
    
"Furthermore, the WPS extension makes it trivial to break into WPA/WPA2 networks." The WPS designers may believe that because some secure systems are hard to use, an easy to use system must be very insecure. Quite a lot of people say that security is inversely proportional to ease of use, ignoring all counter-example of this relation. –  curiousguy Aug 28 '12 at 13:29
    
@curiousguy Not sure what your point is. I agree that security is often counter to productivity and ease of use, but that's somewhat out of the scope of this question. WPS is a veritable security problem, due to its small keyspace. –  Polynomial Aug 28 '12 at 13:36
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WiFi access point usually cannot use several encryption keys simultaneously. So no, they cannot do encryption which locks out connected clients from each other. WiFi protection aims at emulating the physical protection of an ethernet LAN: people who can connect get full access to the LAN (at the ethernet layer). Encryption is meant to prevent unauthorized access (yeah, it is a bit weird to use encryption for an authentication job, but such is WiFi).

If you want encryption for what it is meant (confidentiality of data transfer), then concentrating on the WiFi layer misses the point: the data does not cease to be confidential once it reaches the access point. Confidentiality is best addressed if ensured end-to-end, from sender to receiver. The access point is neither, so it should not be able to see the data. But with WPA2 and similar techniques, the access point performs the decryption and encryption, and has access to the clear data. Also, we call it an "access point" because it "gives access" to a larger network (up to an including the whole Internet), and the encryption services of the AP do not go beyond the AP itself.

Therefore, for confidentiality, you should use data encryption systems which operate at upper levels. That's what happens when you access a HTTPS Web site: encryption from your browser to the distant site, regardless of the access points and networks which are used in between. More comprehensive solutions include various Virtual Private Network technologies, and, ultimately, opportunistic encryption with IPsec (IPsec is often used as a basis for a VPN, but, theoretically, it could be active transparently for every connection over the Internet -- the standards are done, the implementations are widespread, it "just" needs everybody to take a bold step into the future, simultaneously).

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"WiFi access point usually cannot use several encryption keys simultaneously." Actually each Wifi station has its own keys, and there is a broadcast key shared by all stations. –  curiousguy Aug 28 '12 at 12:00
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