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Randaly comments on Open Thread, September 1-15, 2012 - Less Wrong
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I'm thinking about a fantasy setting that I expect to set stories in in the future, and I have a cryptography problem.
Specifically, there are no computers in this setting (ruling out things like supercomplicated RSA). And all the adults share bodies (generally, one body has two people in it). One's asleep (insensate, not forming memories about what's going on, and not in any sort of control over the body) and one's awake (in control, forming memories, experiencing what's going on) at any given time. There is not necessarily any visible sign when one party falls asleep and the other wakes, although there are fakeable correlates (basically, acting like you just appeared wherever you are). It does not follow a rigid schedule, although there is an approximate maximum period of time someone can stay awake for, and there are (also fakeable) symptoms of tiredness. Persons who share bodies still have distinct legal and social existences, so if one commits a crime, the other is entitled to walk free while awake as long as they come back before sleeping - but how do they prove it?
There are likely to be three levels of security, with one being "asking", the second being a sort of "oh yeah? prove it" ("tell me something only my wife would know / exhibit a skill your cohabitor hasn't mastered / etc."), and the third being... something. Because you don't want to turn loose someone who could be a dangerous criminal just because they were collaborating with a third party to learn information, or broke into the National Database of Secret Person-Distinguishing Passphrases, or didn't disclose all their skills to some central skill registry - but you don't want to lock up innocent people who made bad choices about who to move in with when they were eight, either.
Is there something that doesn't require computers, or human-atypical levels of memorization/computation, or rely critically on a potentially-break-into-able National Database of Secret Person-Distinguishing Passphrases, which will let someone have a permanently private bit of information they can use to verify to arbitrary others who they are? (There is magic, but it is not math-doing magic.)
Why use cryptography? If I understand the problem statement correctly, there's a simpler solution. When a prisoner wants to go to sleep, they signal and a guard walks over and renders them unconscious, presumably using drugs. Since we know that nobody would go to sleep outside of jail, you can figure out who is who by counting the number of times they've been sedated.
(This is vulnerable to troubles telling who is who at the start, but so is any knowledge-based method. This is also vulnerable to people falling asleep outside, but so is any knowledge based method. It's also fairly dangerous, given that most drugs capable of rendering somebody unconscious are dangerous; however, giving guards some training and then handwaving away or saying the society isn't concerned by the (minimal) danger sounds reasonable. It assumes certain things about going to sleep and drugs that may not be true in this universe, but it at least sounds reasonable- and this is fiction.)
Sedatives would cause physical sleep, and the reason people share bodies in this world is because having your body be asleep will cause your soul to be eaten by insubstantial demons. Sleeping-while-someone-else-pilots-your-body is safe in large part because it cuts off interventions regarding your soul from outside sources - demons, drugs, magic, etc.
Also, this method relies on cooperative criminals, not just cooperative cohabitors-with-criminals. The criminal has an incentive to make being in jail really inconvenient for their cohabitor - by, for instance, not notifying anyone before going to sleep. They're already in jail, so making their cohabitor mad at them has limited power to make their situation worse, but if the guards wind up having to imprison the cohabitor too to be safe, the cohabitor might work on ways to get out.
I suppose reallocating cohabitors (say, criminals with criminals) is out of the question?
Moving one person in with another person is already very magically challenging; this might not be strictly impossible but your average community would not have access to even one person who could do it. Perhaps this would be a good last resort on a national level for anyone with a demonstrated propensity to actually escape, or whose escape would be particularly dreadful.