52 lines
3.4 KiB
Plaintext
52 lines
3.4 KiB
Plaintext
Electronic Hotel Card Locks
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These are wonderful little microcomputer projects masquerading as door locks.
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Inside there's a processor running a program, with I/O leads going to things
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like the magnetic strip reader, or the infrared LEDs, and the solenoid, and the
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lights on the outside. They are powered entirely by a battery pack, and the
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circuitry is designed such that it draws almost nil power while idle. The cards
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are usually magnetic-strip or infrared. The former uses an oxide strip like a
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bank card, while the infrared card has a lot of holes punched in it. Since IR
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light passes through most kinds of paper, there is usually a thin layer of
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aluminum inside these cards. The nice thing about these systems is that the
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cards are generally expendable; the guest doesn't have to return them or worry
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about lost-key charges, the hotel can make them in quantity on the fly, and the
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combination changes for each new guest in a given room. The hotel therefore
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doesn't need a fulltime key shop, just a large supply of blank cards.
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Duplication isn't a problem either since the keys are invalidated so quickly.
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The controlling program basically reads your card, validates the number it
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contains against some memory, and optionally pulls a solenoid inside the lock
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mechanism allowing you to enter. The neat thing about them is that card changes
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are done automatically and unknowingly by the new incoming guest. The processor
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generates new card numbers using a pseudorandom sequence, so it is able to know
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the current valid combination, and the *next* one. A newly registered guest is
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given the *new* card, and when the lock sees that card instead of the current
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[i.e. old guest's] card, it chucks the current combination, moves the next one
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into the current one, and generates the new next. In addition there is a
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housekeeping combination that is common to all the locks on what's usually a
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floor, or other management-defined unit.
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There is no wire or radio connection to the hotel desk. The desk and the lock
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are kept in sync by the assumption that the lock won't ever see the "next" card
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until a new guest shows up. However if you go to the desk and claim to have lost
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your card, the new one they give you is often the "next" card instead. If you
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never use it and continue using your old card, the guest after you will have the
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wrong "next". In cases like this when the hotel's computer and the lock get out
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of sync, the management has to go up and reset the lock. This is probably done
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with a magic card that the lock always knows about [like in ROM], and tells it
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something akin to "use this next card I'm going to insert as the current
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combination". The pseudorandom sequence simply resumes from there and
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everything's fixed. If the lock loses power for some reason, its current memory
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will be lost but the magic "reset" card will work.
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Rumor has it that these locks always have a back-door means of defeating them,
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in case the logic fails. Needless to say, a given manufacturer's method is
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highly proprietary information. In theory the security of these things is very
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high against a "random guess" card since there are usually many bits involved in
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the combination, and of course there is no mechanical lock to be manipulated or
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picked. The robustness of the locking hardware itself sometimes leaves something
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to be desired, but of course a lock designed for a hotel door probably isn't the
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kind of thing you'd mount on your house.
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