Per person, but most people in ordinary day-to-day life will have plenty of opportunity to observe and practice mimicking their cohabitor’s handwriting if they feel like working on that—they can’t talk to each other directly, so they leave notes (“watch out for our left foot, it’s still tender, I dropped something on it”, “so how are you doing, what are you up to”, “we’re pregnant”).
So handwriting is secure between a pair; then all you need is some sort of authentication. Why not use a very simple random number generator? Each member of a pair knows it, of course, and they occasionally set up fresh seeds. Each day is one iteration. To ‘sign’ a message, one simply writes down today’s random number afterwards. (You said handwriting is secure, so you don’t worry about someone tampering with the message and making an authentic number testify to a faked message.)
What RNG? Dunno. Blum Blum Shub has a hilarious name, but the multiplying is a bit painful. Depending on how much accuracy you want, you could make up your own simple recurrence (imagine a list of 5 integers, which shift each day, and the first is defined by the sum of the last two modulo 5). But it turns out geeks have already discussed PRNGs you can do with mental arithmetic:
Set up another pair of RNGs; both write down on a piece of paper and show the paper simultaneously, something like that. With third parties, you lose the time-delay aspect which makes things hard in the case of temporally separate pair members trying to authenticate to each other.
Well, first, handwriting is extremely hard to mimic perfectly, but maybe it’s easier if you are using the same hand (and brain). Think of other individual traits that are harder to observe in your other half. Maybe speech patterns, or mannerisms, or some other subconscious manifestations. Maybe have a separate hypnotic induction for each person when they become of age. Judging by your writings, you don’t suffer from the lack of imagination. The goal is to have a cheap version of the same feature, and “There are likely to be three levels of security” sounds pretty complicated already.
Oh, come on, it’s an obvious consequence of the premise.
Hypnosis has some promise. Speech patterns/mannerisms seem like they’d rely on the testimony of people who know both of the cohabitors really well and who probably aren’t cops, which has the problem of those people being corruptible in various ways.
I don’t suffer from lack of imagination, but I’m just one person. An entire civilization which has had this problem for a long time should be able to come up with a solution that’s more robust than what I’ve been coming up with, so I solicit help—I’d feel especially silly if there were some trivially implementable noncomputerized version of RSA that someone could tell me about. Also, the entire setting does this thing where people share bodies, and there are multiple cultures in the setting—ideally they’d have different approaches, so if I can come up with more than one workable idea, so much the better.
Without introducing more magic and without there being at least some kind of database, this is an unsolvable problem. I would say use a one-time pad, but the key would have to be stored in a database.
If the technology of the time is at least that of, say, the 1940′s, you could use quantum key distribution to at least be alerted if the crypto is broken (more useful than any other solutions), but would still require a database.
Oh, come on, it’s an obvious consequence of the premise.
Maybe it would be obvious, were I female.
I’d feel especially silly if there were some trivially implementable noncomputerized version of RSA that someone could tell me about.
Good point. RSA in a nutshell is “I’m the only one who knows a certain secret, and I’m the only one who can unconditionally and repeatedly verify this fact without divulging the secret itself”. Well, this is one half of it, the authentication part, not the encryption part.
So you need a way for a person to produce some output from a given input that can be unique both to the person and to the input. but easily verifiable. What kind of non-technical output is available? Visual? Aural? Motor functions?
For example, maybe a way one’s eyes follow a complicated pattern is while unpredictable, but unique enough and easy to check. Or a rhythm one drums in response to something. Or the interpretation of the Rorschach test.
By the way, if you find something that works in real life, you will be famous and set for life, as this is an open problem with multiple applications.
These people are humans, although there is much more potential for magical alteration of the base plan than real humans have. They have human capacities to memorize and transmit information.
I’m reminded of this. Although the technique in the article was taught using a computer game, one could plausibly develop an analog equivalent. Give someone a musical instrument and teach them to play specific sequences in response to the sequences somebody else plays, or something.
But the teaching would be really time-consuming, and of course you’d have to make sure that the right person was in charge of the body while they were being taught.
If it’s something you can teach children, then wealthy societies (which can afford to wait longer before having people move into each other’s bodies) can be sure to teach only the correct people, but indeed time consumption remains an issue.
Per person, but most people in ordinary day-to-day life will have plenty of opportunity to observe and practice mimicking their cohabitor’s handwriting if they feel like working on that—they can’t talk to each other directly, so they leave notes (“watch out for our left foot, it’s still tender, I dropped something on it”, “so how are you doing, what are you up to”, “we’re pregnant”).
So handwriting is secure between a pair; then all you need is some sort of authentication. Why not use a very simple random number generator? Each member of a pair knows it, of course, and they occasionally set up fresh seeds. Each day is one iteration. To ‘sign’ a message, one simply writes down today’s random number afterwards. (You said handwriting is secure, so you don’t worry about someone tampering with the message and making an authentic number testify to a faked message.)
What RNG? Dunno. Blum Blum Shub has a hilarious name, but the multiplying is a bit painful. Depending on how much accuracy you want, you could make up your own simple recurrence (imagine a list of 5 integers, which shift each day, and the first is defined by the sum of the last two modulo 5). But it turns out geeks have already discussed PRNGs you can do with mental arithmetic:
http://ask.metafilter.com/191135/Help-me-get-random-numbers-by-mental-arithmetic
http://blog.yunwilliamyu.net/2011/08/14/mindhack-mental-math-pseudo-random-number-generators/
http://stackoverflow.com/questions/3919597/is-there-a-pseudo-random-number-generator-simple-enough-to-do-in-your-head
http://ask.metafilter.com/20334/Random-sequences-in-your-head
For the looks of them, at least one suggestion should work for you.
This allows pair members to authenticate themselves to each other, but not third parties to tell members apart.
Set up another pair of RNGs; both write down on a piece of paper and show the paper simultaneously, something like that. With third parties, you lose the time-delay aspect which makes things hard in the case of temporally separate pair members trying to authenticate to each other.
OMG!
Well, first, handwriting is extremely hard to mimic perfectly, but maybe it’s easier if you are using the same hand (and brain). Think of other individual traits that are harder to observe in your other half. Maybe speech patterns, or mannerisms, or some other subconscious manifestations. Maybe have a separate hypnotic induction for each person when they become of age. Judging by your writings, you don’t suffer from the lack of imagination. The goal is to have a cheap version of the same feature, and “There are likely to be three levels of security” sounds pretty complicated already.
Oh, come on, it’s an obvious consequence of the premise.
Hypnosis has some promise. Speech patterns/mannerisms seem like they’d rely on the testimony of people who know both of the cohabitors really well and who probably aren’t cops, which has the problem of those people being corruptible in various ways.
I don’t suffer from lack of imagination, but I’m just one person. An entire civilization which has had this problem for a long time should be able to come up with a solution that’s more robust than what I’ve been coming up with, so I solicit help—I’d feel especially silly if there were some trivially implementable noncomputerized version of RSA that someone could tell me about. Also, the entire setting does this thing where people share bodies, and there are multiple cultures in the setting—ideally they’d have different approaches, so if I can come up with more than one workable idea, so much the better.
Without introducing more magic and without there being at least some kind of database, this is an unsolvable problem. I would say use a one-time pad, but the key would have to be stored in a database.
If the technology of the time is at least that of, say, the 1940′s, you could use quantum key distribution to at least be alerted if the crypto is broken (more useful than any other solutions), but would still require a database.
Maybe it would be obvious, were I female.
Good point. RSA in a nutshell is “I’m the only one who knows a certain secret, and I’m the only one who can unconditionally and repeatedly verify this fact without divulging the secret itself”. Well, this is one half of it, the authentication part, not the encryption part.
So you need a way for a person to produce some output from a given input that can be unique both to the person and to the input. but easily verifiable. What kind of non-technical output is available? Visual? Aural? Motor functions?
For example, maybe a way one’s eyes follow a complicated pattern is while unpredictable, but unique enough and easy to check. Or a rhythm one drums in response to something. Or the interpretation of the Rorschach test.
By the way, if you find something that works in real life, you will be famous and set for life, as this is an open problem with multiple applications.
These people are humans, although there is much more potential for magical alteration of the base plan than real humans have. They have human capacities to memorize and transmit information.
I’m reminded of this. Although the technique in the article was taught using a computer game, one could plausibly develop an analog equivalent. Give someone a musical instrument and teach them to play specific sequences in response to the sequences somebody else plays, or something.
But the teaching would be really time-consuming, and of course you’d have to make sure that the right person was in charge of the body while they were being taught.
If it’s something you can teach children, then wealthy societies (which can afford to wait longer before having people move into each other’s bodies) can be sure to teach only the correct people, but indeed time consumption remains an issue.