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I am wondering if it possible to configure my DD-WRT router so that the wifi network I am using is undetectable - impossible to tell if there is any network broadcasting even if there are clients connected, with all the transmissions indistinguishable from the noise. If not, why is that technically impossible? Is it a matter of no support from the protocol side, or do some radio-related constraints come into play?

Edit:

To answer @Francois Valiquette's question, here's how I saw it: as far as I know, perfect encryption is indistinguishable from the noise unless you know the key. The clients would send encrypted messages to the frequency and if the key is correct, they would notice the reply in the "not-noise".

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Even by using noise and not-noise signals a device that can listen to a range of frequencies will notice the unusual transmission signal. I am not an expert in that field, but the men made signals are probably easy to distinguish from the natural ones. Therefore, this would give suspicion to any attacker. –  Francois Valiquette Apr 23 '13 at 19:51
    
Imagine that your Wifi network emitted sound instead of radio waves. To someone without the decrypting receiver, the encrypted stream would sound exactly like noise -- i.e. a hissing sound. If several people in a crowded room were using that noise to communicate, you could easily find them by following the sound, even if you can't decipher what they are sending to each other. –  Johnny Apr 23 '13 at 21:38
    
To comment on your edit - I believe that's the difference between encryption and steganography. Encryption doesn't try to hide the existence of a signal, just the data being transmitted. You are right in the sense that encrypted data should be indistinguishable from "noise", however (in order to be resiliant to various analyses), but here you're comparing apples and oranges - data being sent as part of the signal, and things that are not part of the signal at all. –  Daniel B Apr 24 '13 at 8:38

4 Answers 4

up vote 3 down vote accepted

You need a new air-protocol to make it work (ie: hardware/firmware), but the protocol you are thinking of would use asynchronous CDMA:

Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal."

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No, the spectrum that is used is well-known and not concealed. You can't conceal that activity is occurring, you can only prevent the SSID from being directly broadcast and encrypt the traffic such that the network can not be spied on without breaking the encryption. The fact that there is radio activity in the spectrum will be apparent to anyone listening in and the encrypted messages will be able to be captured without trouble.

In order for a radio to be able to send a signal, it has to appear above the noise. The only exception to this is when using specialized jamming equipment that allows signals to be sent through it (which is also not WiFi) (generally by not blocking frequencies in a pattern derived from a key), but even then, it is obvious that there is a transmitter there.

You are correct that a good encryption algorithm and key should make the cipher-text look random, but there has to be some kind of way for signal to stand out from random noise. Say that you have a signal that looks random 10011011 and then have actual random noise 01011010. When you combine them, you 11022021, but now since the noise was truly random, how do you know what was noise and what was signal? You can't because you don't have a pattern to the noise.

If you generated the noise, then you could have a pattern and drown out the natural noise, but then we are back to the situation I described for signal jammers, you are going to know they are there because it needs to be enough louder than passive noise to be distinguished from background noise. Such a system would also have to drown out any other signals in the spectrum in the area as they would also be effectively random interference which would make such a device illegal to operate for most purposes in the US due to FCC requirements that a device not cause interference.

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When you are using your Wi-Fi your hosts sends wireless signal to communicate. Therefore anyone can intercept those signals. If your signal is undetectable, how come your legitimate hosts would detect the communication?

Wi-Fi signal is like the one you use when you communicate when you speak. The only difference between both is the frequency of the signal. So how could you speak to someone without others hearing it? It is impossible unless they are outside your signal range.

What you could do, change the frequency of your transmission. Most detection tools are made to listen on a certain frequency, so if you change the frequency most tools won't hear your signal. But I don't think you can achieve that with DD-WRT.

Edit for @d33tah
Even by using noise and not-noise signals a device that can listen to a range of frequencies will notice the unusual transmission signal. I am not an expert in that field, but the men made signals are probably easy to distinguish from the natural ones. Therefore, this would give suspicion to any attacker.

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I answered your question from the first paragraph in an edit of my original post. –  d33tah Apr 23 '13 at 19:38
    
I updated my answer... –  Francois Valiquette Apr 23 '13 at 19:54

Spread-spectrum emissions are very often used in military and clandestine communications. This is not new, they can be detected and they can be jammed, yet doing so requires some not-so-cheap equipment (for detection, not for jamming) and much more time to integrate the signal. You cannot implement this stuff on top of conventional 802.11 wireless transceivers, though - the underlying transmission modes are incompatible.

One note of caution: if you ever try to use existing frequencies for spread-spectrum operation, you may have to obtain necessary licenses first! You may be in for a whole lot of trouble (including criminal persecution in some jurisdictions) for unlicensed wireless transmissions.

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