In my military scifi novel the military is, obviously, very concerned with security aboard its spaceships. They don't want other people to be able to receive any information about what happens on their ships. They also don't want other spaceships to be able to send any signals to the computers and other devices used by the crew on the ship, at least no signals that didn't go through the "comm tower" that handles normal ship-to-ship or ship-to-planet communication and data transfer.

For ship sizes think modern destroyers/cruisers up to aircraft carrier size in space, just with a big engine at the back and all around armor plating.

Combat happens through missiles at distances between one and sixty lightseconds.

Technology is closer to The Expanse than Star Trek, i.e. no shields, no FTL communication and no teleporters.

Groundside on todays earth that might mean that wireless communication is forbidden, because the signals might be detected outside and someone with a powerful enough computer might eventually decrypt whatever was transferred through the wireless connection.

There are a few emissions that can't be prevented, e.g. heat and engine, but what other kinds of emissions could give information about what happens on the inside of the ships away? How would EmSec work in space?

My current idea:


All ship internal communication works exclusively via wall terminals using wires.

All handheld devices, Star Trek Padd analogues, are offline, unless connected via a cable to the ship computer.

Personal entertainment and communication devices, i.e. game consoles and mobile phones, are strictly forbidden unless the wireless capabilities are physically removed.

You're in a steel box, it shields internal wireless communications from the outside by virtue of being a big steel box. See Faraday Cage for details. I work in a steel frame building, we basically have no mobile signal inside. Your only external comms is through the comms tower because all other signals are blocked by the nature of the hull.

What's going to be far more interesting is tracking the tiny vibrations in the outer hull caused by people and equipment moving around inside. They're going to know when your shifts change, they might even be able to track individuals moving around in the ship depending on how sensitive their equipment is.

Do not make a problem where there is none, or you will be ridiculed

I am sorry but this...

They also don't want other spaceships to be able to send any signals to the computers and other devices used by the crew on the ship, at least no signals that didn't go through the "comm tower" that handles normal ship-to-ship or ship-to-planet communication and data transfer.

...is an almost nonsensical concern.

Getting communications to work is not an easy task, especially not in the digital age. For reasons too long to explain here, I will state the following as a given fact:

You cannot "send a signal" to a device unless it is explicitly listening for one

If we were to rip out the radio units of your phone (4G, WiFi, Bluetooth), there is no way in hell you can "send a signal" to that phone without being physically connected to it. Magic Hollywood Hacking has led us to believe that we can access any device anywhere just because we want to but reality / realistic fiction begs to differ. Your readers will laugh at you if you create an environment where intruders can access computers at a distance and at will if those computers do not have a wireless communication interface.

That said...

...even if Eve & Mallory cannot intrude on systems that are not actively listening for outside communications, you still have the comms tower, and you still have devices that are listening for internal communications.

This means you must have communications protocols that are specifically made to avoid unauthorised communication and to reject intrusions. Especially handshaking is required to set up a communications channel before you can start moving any data. But — again — the device must have been set to listen for the signals to begin with.

The comms tower is set to listen. There you need to have protocols that are made so that no handshaking is being performed until the incoming transmission has authenticated themselves. Otherwise Eve & Mallory could fool your ship to reveal its location by broadcasting transmissions in the blind and triggering a handshake attempt. This however is not a big issue and can easily be automated.

Plot hook

Eve & Mallory have managed to steal the authentication keys.

Same thing for internal devices: they must be set to not trust any old transmission, but only those that can be authenticated. Again: this is not much of an issue, even present day communication protocols are designed for this (although not always successfully).

Plot hook

Eve & Mallory have gotten their hands on such a device that was not set to lock itself or self-destruct.

EMCON

Finally there is EMCON... emissions control. This is good old fashioned radio silence and behaviour procedures for that are in effect even today. And — again — these are not always successful.

You had the notion that internal communications should be made only over wired devices.

That is a good idea, and is used in real life

Personal anecdote: I did my military service in the Swedish Airforce. My task was to ready aircraft. When military aircraft are on the ground and being serviced in a war-like situation, it is procedure to hook them up to a land-line. In our case the connector was clipped to an eye in the concrete pad the aircraft was being serviced on. So when the plane started rolling, the connector with be yanked out by itself. That way the plane could be radio silent the entire time it was on the ground, no confidential orders would leak, and it would not even announce its presence through its emissions.

Apart from that there are several schemes to make artificial emissions disappear among the natural noise, for instance LPI radars. But out in space, any emission from a point source is likely to arouse suspicion.

Levels of EMCON

Just like submarines have different "rigging" when it comes to sound management — which can be anything from We Do Not Care If We Sound Like A Rock Festival, in various levels all the way down to Do Not Even Talk Loudly — your space ships most likely also have that. The EMCON can take many levels from Noisy (all comms operative, space radar/lidar, jamming) to Quiet (no active comms, low intercept mode) to Stealth (everything that can emit is shut down).

In order to enforce this, I would recon that all emitting devices on-board can be turned off down by a "shut up" command from the ship itself.

Plot hook

— Captain? I am picking up an unauthorised signal... from within the ship!

— Who the hell brought a friggin' PlayBox on board?! Kill it! Turn it off, turn it off now before the enemy... oooh crap...

Van Eck phreaking of some kind will always be an issue with space craft, even one with a Faraday Cage, which you want your ship to be for the protection of the crew from incoming radiation anyway. The Faraday Cage lights up, a little, at certain frequencies when EM radiation hits it, whether from the inside or the outside. In theory you can read the response of the Faraday Cage, AKA the hull of the ship, to the radiation hitting it and differentiate the internal signals. Then if you know the layout of the vessel and how the ship's display systems work you can work out what is being displayed. This is an extremely advanced technique that would require very discerning sensory systems but it is possible. To avoid this kind of surveillance direct retinal displays are your best bet as they are extremely, personally, directional rather than being in fixed positions within the hull and they put out very little signal to start with.