Is it possible to detect encrypted communications [duplicate]

Question

I'm writing a detective novel, in which a series of terror attacks are carried out in the United States by a group of mercenaries using military grade, channel hopping radio to communicate.

I would like to create tension, in the lead up to a subsequent attack, by having my protagonist alerted to the activity and location of the mercenaries by picking up on these radio signals.

I don't need my detective to decrypt the contents of the comms (I assume this to be impossible without knowing their encryption keys), but the mere presence of channel hopping radio signals and the location of their source would be enough.

I read in another post that it's possible to determine the source of such radio signals, using a wide band receiver and directional antenna but I hope to determine if it's possible to have a system such as this set up to constantly monitor for these signals and to set off an alarm when it detects activity?

I don't intend to explain the intricate details of this system but I would like the idea to pass the credibility test and not have my readers pooh poohing it as impossible.

To give a little more background, the first attack in my story is the bombing of a dockside wharf which my detective is staking out on a case of drug smuggling and he's got support from police surveillance, who are monitoring the radio frequencies. This is how he first realizes he's up against people with military equipment.

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In answer to your query.. A/ the story is contemporary (excluding COVID19) and B/ my protagonist is a cop, therefore his assistance includes whatever surveillance gear a modern stateside urban precinct might have access to.

I appreciate your suggestion to modernise my tech scenario, which, I guess, might include cell phone tower monitoring.

On the plus side.. The first attack, at the wharf, happens in the dead of night in a deserted industrial zone, relatively free from local mobile traffic noise.

What I am looking for.. A self evident way to link a later terror attack back to the first one, so my protagonist realises that the perpetrators are one and the same group. I hoped that the military radio signals could achieve this but perhaps there are other unique attributes in the more modern tech scheme which could achieve the same thing?

Also, it's important that the unique identifier occur BEFORE the second attack, so that my hero gets a brief opportunity to avoid being blown to bits.

I've currently got it written such that my protagonist gets a phone call from the cop who's been running the monitoring system. Said cop announces excitedly that he's picked up a signal from the same mercenary crew as the wharf bombing. My protagonist asks where the signal is located and the reply makes him realise he's in immanent danger.

I hope this gives you enough insight to ruminate on a solution.

I really appreciate your expert insights here. I hate it when I see movies hinging important plot points on unbelievable technology...

"Hey, just stick this USB into the alien ship's computer and upload the virus to save the world" or "Zoom in to this wide shot photo of a massive crowd, to the engraving on the earring of that girl.. Enhance... Enhance..."

Answer 1

by a group of mercenaries using military grade, channel hopping radio to communicate.

Few things:

• military-grade doesn't actually mean anything
• channel-hopping was the hot stuff in the late 80's, early 90's, most mobile communications have moved on
• Using military bands and modulations is a 100% surefire way of being noticed

by having my protagonist alerted to the activity and location of the mercenaries by picking up on these radio signals.

Is this an apocalyptic setting? What about the state?

The spectrum is monitored. Especially the spectrum useful for medium- to long-range mobile communication is under constant surveillance. There's military/intelligence/commercial products you can buy (from more than one potential supplier) that will do that for you, and the companies behind that will help you set up a network to do that throughout your nation.

If any nation, then the US will have a very reliable network of spectrum surveillance.

Your best chance of not being detected hence is doing something everyone does – use the internet on their phones in a populated area. Encrypted internet traffic is the norm, and a phone with an LTE/5G subscription is something everyone has.

I don't need my detective to decrypt the contents of the comms (I assume this to be impossible without knowing their encryption keys), but the mere presence of channel hopping radio signals and the location of their source would be enough.

You make it sound as if channel hopping being very special is a plot device. It's not very special – but depending on which band they use, you can just say "John McClane, trying to use {usual police usage of that band} was irritated by the regular interference he got, so he took his SDR dongle and observed the spectrum, noticing that where he was expecting to see only transmissions from his {own radio device}, he could make out a {low/high bandwidth, burst-y or constant transmission, strong or weak signal: depends on your story}".

Maybe our hero actually does administrate some radio communications network or such, and he gets automatically alerted to such things, alternatively.

I read in another post that it's possible to determine the source of such radio signals, using a wide band receiver and directional antenna but I hope to determine if it's possible to have a system such as this set up to constantly monitor for these signals and to set off an alarm when it detects activity?

Yep, exactly what nation states do. But it's not hard per se, if you know what you're looking for. Hardware-wise, I'd calculate 1800 $ per station, add a mast, need at least three stations for reliable localization of an emitter. The more wideband the emitter, the higher the localization resolution, and the higher its transmit power, the lower the expected error of locale estimation. (The way companies like the linked above make their money: their products reliably work, and they detect a couple hundred different radio standards, log that, present it in a user-usable manner, they sell very good receivers...)

To give a little more background, the first attack in my story is the bombing of a dockside wharf which my detective is staking out on a case of drug smuggling and he's got support from police surveillance, who are monitoring the radio frequencies. This is how he first realizes he's up against people with military equipment.

Police usually don't do that, in the US that would be the FCC for ensuring everyone uses their spectrum; the spectrum surveillance for intelligence reasons is probably within the job description of the NSA, but you're right, in case of drug smuggling, the FBI and CIA might be involved, as far as I understand the secret services of the US.

But really, at a harbor/dock, it's pretty realistic that the maritime authorities do have ship radars – and that the bad guys' radios simply interfere with that, which is unusual, so our hero overhears that and asks the policeman John McClane, see above.