Is modular security possible?

In the context of an embedded device, is it possible to create an add-in module that will be responsible for both communications and security, with his role being creating a single secure link to access to a cloud server while limiting all other access.

The idea is that the main system won't have another another communication path, and using this module, whatever you do in the main module, your system will be secure (assuming the add-in module is secure). So that basically, the main system programmers couldn't introduce security vulnerabilities however hard they try.

I assume that system update is also handled by the add-in module.

To make it more concrete, the system I'm thinking about is a photography UAV. The threat model is either remote attacks or physical attack to a single UAV which can give control over a fleet of similar UAV's.

Can a modular security architecture with a security module separate from the main module work in this scenario?


When you have the main module only communicate with the radio module through the security module, the security module could do all communication with the base server signed and encrypted without the main module even noticing it.

But the communication between base server and UAV is not the only attack vector one could think about.

UAVs are often equipped with different kinds of sensors. When you have a vulnerability in interpreting that sensor data in the main module, that would be exploitable (example: trigger an integer overflow when interpreting the brightness level of a photo sensor by pointing a laser at it). And you can forget about also filtering the sensor data through the security module, because the security module can't know what kind of sensor input will trigger a bug in the main module.

The same attack vector applies when there is any malicious data which originates from the base server itself. When the base server isn't trustworthy, the security module won't be able to do anything about it because it doesn't know what malicious activity to look for. This attack vector could also manifest when the base server receives information from some outside source and relays it to the UAV unfiltered.

Also, it would not protect against physical access. An attacker with physical access would be able to place a third chip between main module and encryption module to MITM the unencrypted communication between these two chips (assuming we are talking about two modules which are physically separated and not just logically separated features of the same chip)

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either remote attacks or physical attack to a single UAV which can give control over a fleet of similar UAV's

If that's your only goal, then the security architecture of the UAV itself is irrelevant. All you need is to avoid class secrets, i.e. don't have anything that you both want to keep secret and include in every device. Any good security architecture avoids class secrets. This means don't rely on security by obscurity and, for any cryptography, use different keys in every device. Each UAV generates its own secret keys for internal use, and has its own private key(s) for communication with the outside.

However, if you're worried about attacks, then presumably the UAVs have assets (and they're probably assets in themselves). For that, the security architecture of the device obviously matters.

Against software attacks carried from the outside, a dedicated security module makes sense. Have it mediate all communication, and be the repository of cryptographic keys that are used for communication. Also make it responsible for ensuring the integrity and performing software updates on other components — perhaps it stores the firmware for all other components.

However, you'll still need to ensure that individual components are bug-free. The dedicated module acts as a firewall against malicious requests from the outside, but if the other modules misbehave on valid commands, the game is lost. If a bug in a servo controller causes the UAV to go down instead of up and it crashes, the security module can't help with that.

You need to think about your device's trusted base. What parts of the device are critical for its correct operation? As Philipp explains, the answer is lots. A lot of sensors, motors, etc. are necessary for the UAV to operate correctly (starting with: not crashing). If the controller module gets incorrect data, or if the motors don't turn at the speed requested by the controller, then the device will misbehave; thus all the sensors and motors are part of the trusted base.

The best you can hope for is an architecture where most components can only affect themselves, e.g. one misbehaving sensor can't modify another sensor's readings or prevent the other sensor from reporting. This example requires proper isolation on the data buses and proper authentication of each sensor in their controller.

Against hardware attacks, this model breaks down even more. If someone has physical access to the device, then they'll have fairly easy access to a lot of data buses. What the sensor reports is not necessarily what the controller receives: it could be data injected by the attacker. To avoid this, you need to bring cryptography into all the sensors. This raises the bar for an attacker, but it isn't a panacea. An attacker with physical access can always perturb the sensor's readings, causing the sensor to make accurate reports of useless information, e.g. by obscuring a camera, holding a heat source near a thermometer, etc.

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  • Thank you. you wrote:" but if the other modules misbehave on valid commands, the game is lost." - sure it's bad. But it isn't a security issue. more of a reliability/safety-critical issue, right ? which is something non-security people do work on. – hulkingtickets Dec 16 '15 at 11:58
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    @hulkingtickets It's both. The role of the security module would typically be to protect the system from unauthorized commands, but not from authorized-but-invalid command. However, which commands are authorized can depend on context, e.g. maintenance vs on the field. Furthermore valid commands can have a security impact, for example if the UAV is commanded to fly into hostile territory. – Gilles 'SO- stop being evil' Dec 16 '15 at 12:32

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