[Cryptography] Crypto and rustling
leichter at lrw.com
Sat Nov 19 05:48:50 EST 2016
> I've just started reading "Robbery Under Arms" (a story about
> cattle/horse/sheep rustling in Australia), and given the recent thread
> about how Enigma could be improved using only technology available at the
> time), a thought occurred to me:
> How would an owner design a brand that could not be altered, and how could
> a rustler get around it? For example, the owner would not use a single
> letter (a circle for example could be branded around it), and would not
> use letters that could be altered (such as F -> E, L -> E, O -> Q, T -> I,
> etc), whereas a rustler would have to find a compliant blacksmith.
> In crypto terms, it should be infeasible to turn one brand into another.
If the set of possible brands is open-ended, it's clear that no solution is possible: An altered brand is just a new symbol that's as valid as the original.
If there's agreement up front about what the set of legitimate brands is, you just need to ensure that no brand in the set, when viewed as a collection of curves, is a subset of any other. A set of brands all consisting of the same number of constituent curves is an obvious example.
A real-world example from way back when: DEC wanted to ship a "limited" VAX that would be cheaper but would only run some software, not other software. (I don't recall for sure, but I think they wanted it to run a real-time OS called ELN, but not VMS.) The limitation would be implemented through an ID delivered from the boot ROM. It was assumed that copyright law could prevent anyone from copying the existing ROM from an unlimited machine and burning a new one; but that couldn't stop someone from modifying a ROM they had received with their machine. Given the realities of the ROM used, it was possible to change bits in only one direction, not the other - say, you could change a 0 to a 1 but not a 1 to a 0. So the problem was stated as: Devise an ID code (or set of ID codes) with the property that changing 0's to 1's in a valid ID code for a limited VAX never gave you a valid ID code for an unlimited VAX.
Not a particularly difficult puzzle, but amusing.
(DEC - and others - were casting about for ways to maintain margins without ceding the low end to cheap PC's. Some here may remember the MicroVAX II RC. RC stood for "Restricted Configuration" - restricted to keep you from adding more cards to the backplane. The "restriction" was implemented by pouring epoxy into the otherwise-open slots in the Q-bus. Some sophisticated users simply bought Q-bus backplane modules - DEC sold them separately - and replaced the epoxied versions.... In general, all these attempts went nowhere.)
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