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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?


For the bounty I'd like to see some expansion in regard to EAP protocols.

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In Re: EAP, is 802.1x what you are asking for? I mean, it sounds like what you want because it allows for individual user auth and each session will have its own encryption pair. But, as an answer it well explored even on this SE site: security.stackexchange.com/questions/88695/… –  Jeff Meden May 11 at 15:25
@JeffMeden one canonical answer would be better, if you combine the different answers, you can have the bounty. –  Lucas Kauffman May 11 at 18:45
@JeffMeden, that question does not really answer the question whether or not all stations use the same key (and thus are able to see each others plain traffic). In the comments to that answer there seems to be some confusion. –  Michael May 12 at 6:17
@Michael, Agreed there is a lack of user-friendly material regarding how the keys are generated, mostly just proof-in-the-pudding examples of how easy/hard it is to break knowing certain pieces of the puzzle. Here is another answer, this time from SuperUser: (specfically "But with 802.1X, the authentication process securely generates keying material that is used to create a unique Pairwise Master Key (PMK) for the connection, so there's no way for one user to decrypt another user's traffic."): superuser.com/questions/373453/… –  Jeff Meden May 12 at 14:23
@JeffMeden if you compile all of that in an answer and could post that would be great –  Lucas Kauffman May 12 at 14:29

3 Answers 3

up vote 2 down vote accepted

Different Encryption Key Per User

It is important to start by clarifying your question since terminology is important when discussing encryption: I think a more pointed question in pursuit of your goal is "What technology would allow a sufficiently different encryption key per user, so that when multiple users are on the same AP it would be very difficult to see each others' plain traffic?"

The WPA2 system (currently the standard-and relatively secure-solution to over the air confidentiality and integrity) uses individual keys for each client (called Pairwise Keys) which are based on either a Preshared Key (where the same simple key is given to each user) or a random initialization (for 802.1x backed authentication).

When using a PSK, there is a four-way handshake (good discussion of 4-way process here: How exactly does 4-way handshake cracking work?) that starts with the PSK on the client and AP, and a zero-knowledge proof is used to verify that the other really knows the right PSK without divulging what it is before proceeding (to prevent giving the key away to a rogue). The end result is a seeded encryption session based on the PSK, but using newly generated information. (Great answer about keying here: WPA2 Enterprise AES encryption key size?) This means that, if starting with just the PSK and observing another user's session already in progress, you will not be able to easily decrypt the data since you don't know the random nonce. You need to have recorded the 4-way handshake to know the nonce, AND posses the PSK, to easily decrypt a WPA2 session in progress.

Does Radius Implement This

The 802.1x protocol is port-based authentication backed by some sort of AAA server (commonly Radius). 802.1x does not form the encryption session, it only sets the stage for it. Upon successful authentication of the username and password (done in an encrypted tunnel) the AP starts a WPA2-encrypted session with a seed sufficiently secret and random that guessing by other users authenticated to the same AP is sufficiently hard (see discussion: http://superuser.com/questions/373453/802-1x-what-exactly-is-it-regarding-wpa-and-eap and real world product: http://freeradius.org/enterprise-wifi.html).

Can this also be done with a simple home AP

As of 2015, off the shelf APs in the very low end (under $50 MSRP) can be found with support for WPA2-Enterprise which implements 802.1x based on a Radius server. Most, however, do not also include the Radius server so it is not a self-contained solution. You will need to have or implement a Radius server on your network (Free and commercial packages are available), or subscribe to a service like AuthenticateMyWifi (http://www.nowiressecurity.com/#!hosted-cloud-radius-8021x-service/c1739). See http://www.ciscopress.com/articles/printerfriendly/1576225 for additional information about the pieces involved in 802.1x implementation.

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I would add that it is very easy to de-authenticate a client and capture the 4-way handshake then. Plus, I don't understand what you mean exactly by 'the AP starts a WPA2-encrypted session with a seed sufficiently secret and random that guessing by other users authenticated to the same AP is sufficiently hard'. How is this seed communicated? Can I capture this seed to guess the key? It's just that I always read that with Radius, it's impossible to see traffic of other users, but I haven't seen a complete explanation of this fact. –  Michael May 13 at 7:19
The reason that capturing the 4-way handshake and guessing the pairwise key is easy, is that you know the preshared key too. With an 802.1x session, the only preshared information is the user/pass entered into the supplicant, which is hopefully unknown to eavesdroppers. I admit that I don't know the exact steps taken to ensure key strength and uniqueness, but if it were as easy as reversing a PSK session I suspect we would know by now (WPA2-Enterprise has been mainstream for almost a decade) –  Jeff Meden May 13 at 11:28
In the official standard, they seem to say the following: "When not using a PSK, the PMK is derived from the AAA key." And a bit before that: "When the IEEE 802.1X authentication completes successfully, this amendment assumes that the STA’s IEEE 802.1X Supplicant and the IEEE 802.1X AS will share a secret, called a PMK. The AS transfers the PMK, within the AAA key, to the AP, using a technique that is outside the scope of this amendment; the derivation of the PMK from the MSK is EAP-method-specific." So, EAP-specific. –  Michael May 13 at 12:37

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

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
@curiousguy Not to resurrect this too much further but the OP term "encryption key" in the first sentence is inspecific, what he described was the preshared key (PSK) which most consumer routers will only accept one of. You are right that each connection maintains a unique pairwise key plus a group key, but since it is based on the PSK it is trivial to reverse if you start with the PSK and a transcript of the handshake. So for "intents and purposes" the device is operating on one encryption key; the multitude of unique variants only serves to make bruteforce a little bit harder. –  Jeff Meden May 11 at 16:32

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