I wonder whether it would be possible to use isotopic ratios of gases in negative pressure containers or something like that to ensure that any puncture will disrupt the mixture before it can be measured. Hardly foolproof, but anything that an adversary can measure you have to assume that they can reproduce unless you have some way to prove that the process required to reproduce that physical state _must_ take longer than the transit duration.
The other thing that comes to mind would be quantum systems that can only be measured once. Unfortunately I think that practically you would need a system that is "only twice" so that it can be compared, but I have this sense that anything that can be measured twice can be measured 3 times.
Lots of great links here to people working on practical solutions, but in the limit I wonder whether for many of the "black box in enemy territory" models you just have to go with self destruction as the only safe solution because anything less than a fully trusted human being is at risk for being tampered and pwnd (and even then you might still worry).
> The other thing that comes to mind would be quantum systems that can only be measured once. Unfortunately I think that practically you would need a system that is "only twice" so that it can be compared, but I have this sense that anything that can be measured twice can be measured 3 times.
Just spitballing, but you could do it with a "once only" system if you could generate it reliabilly/deterministically enough that you don't need to measure it post-generation
This thought crossed my mind, but I couldn't figure out quite how to get it to work. I think the fundamental flaw with using a deterministic process to create the state is that an adversary can immediately reproduce a matching system if they make the measurement, and then we are back to needing a deterministic process that takes longer to occur than transit time.
After a bit of tangentially related thinking (see below), here is one possible way, and why I don't think it works. One could deterministically create a metastable state in a quantum system, e.g. by pumping a certain specific amount of energy into it. Then to figure out how much energy there was present, any additional amount of energy from by the measurement would cause the state to collapse. Unfortunately having the measured value in hand an adversary could now reproduce that state because the original process is deterministic.
For example, a classic "easy to produce hard(er) to measure" is creating aqueous solutions, where a bunch of different solutes are mixed together (I always think of ACSF, artificial cerebrospinal fluid, because I used to have to make it all the time). The creation of these can be entirely deterministic.
Unfortunately all you need is a good analytical chemist to get an approximation. Even if you used specific ratios of different isotopically pure salts they could probably reproduce it, and you would want something that would cause an irreversible change on physical tampering, such as an oxidation, cleaving, or isomerization so that you couldn't just dump the contents and put them back. All of these are tactics that delay an adversary by presenting them with a measurement and combinatorial problem, but doesn't provide the "measure once" property we need.
The other thing that comes to mind would be quantum systems that can only be measured once. Unfortunately I think that practically you would need a system that is "only twice" so that it can be compared, but I have this sense that anything that can be measured twice can be measured 3 times.
Lots of great links here to people working on practical solutions, but in the limit I wonder whether for many of the "black box in enemy territory" models you just have to go with self destruction as the only safe solution because anything less than a fully trusted human being is at risk for being tampered and pwnd (and even then you might still worry).