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Nowdays we have biometric doors that can scan your eye, palm, fingerprint, ear, etc, etc...

But all of them rely on electronic readers and computers to match the sample against the stored pattern.

How would you design a biometric door that does not need to rely on a electronic scanner and computer, and could only be opened by the designated person (or someone very alike).

Of course these biometric locks would be easier to pick and less accurate. That is not the concern. They could use electricity or any modern device, just not an electronic reader and/or a microprocessor device.


Please do mind the hard-science tagging in your answers.

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  • $\begingroup$ I don't believe it's possible to not involve a computer in the analysis of biometric information. Biometric information requires a scanner to input information, software to convert that information into useable data, and more software to compare the data to valid data. Without a computer, you're probably going to need magic, which you don't want for this answer. $\endgroup$ – Frostfyre Apr 8 '15 at 1:39
  • $\begingroup$ The problem with your question is that the boundary between "can use electricity" and "can use a microprocessor" is rather fuzzy. Is a hardwired pattern-recognition circuit allowed? $\endgroup$ – Mark Apr 8 '15 at 1:42
  • $\begingroup$ @Mark such circuit would fall under electronic reader. $\endgroup$ – Mindwin Apr 8 '15 at 1:49
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    $\begingroup$ @Frostfyre How do you recognize your family members? Personally, I mean. Do you bring out the tricorder every time, or do you have some streamlined, non-electronics-dependent approach? $\endgroup$ – a CVn Apr 8 '15 at 8:27
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    $\begingroup$ Out of curiosity, what is the purpose of the restrictions on this door? It might help with thinking up relevant ideas. Computers don't exist? There's a high-tech big-brother which could hack into any computer that was used? The user simply doesn't like them? It's more of a nature-ish society, so most things are made out of wood and/or work mechanically as opposed to electrically? Is the door supposed to be hidden? I would also try to make the point that all a computer is, is a complex electric circuit. So anything which uses electricity is in fact a computer, if just a basic one. $\endgroup$ – HammerN'Songs Apr 8 '15 at 21:50

19 Answers 19

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What about a lock based on the Pin Art toy:

"Pin art, Flickr" by Eduardo Habkost (ehabkost) - Eduardo Habkost's flickr account. Licensed under CC BY-SA 2.0 via Wikimedia Commons.

It could be loosely modeled after a pin and tumbler lock.

As an added bonus, not only would some one have to have a replica of the lock owners body part, they would also need to know which body part, and in what orientation it needed to be placed into the lock.

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    $\begingroup$ The "resolution" is not nearly good enough for a single body part, but a "fakir's bed" of these taking the body shape would work. $\endgroup$ – SF. Apr 10 '15 at 7:47
  • $\begingroup$ Hah! Awesome, I was going to suggest the exact same thing - with these as the "pins" in a lock, your face could work as the key. You'd need a way to keep you head perfectly oriented, some kind of hat/mask approach. Beard growth and expression would affect it so you'd need to be cleanshaven and maintain a neutral expression. But it would work. $\endgroup$ – Dewi Morgan Apr 11 '15 at 4:27
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Bloodhound

The dog is trained to recognize specific scents/people. It barks if it knows you, at which point the guard opens the door. New people can be introduced, at which point it knows your scent and you're authorized.

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    $\begingroup$ And now I shall show you how to pick this door, using bacon! $\endgroup$ – AndyD273 Apr 8 '15 at 4:38
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    $\begingroup$ @IsmaelMiguel Get a new dog periodically (like resetting a password periodically). $\endgroup$ – Styphon Apr 8 '15 at 9:25
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    $\begingroup$ @IsmaelMiguel Whether it's a waste of time or a reasonable security reason would be determined on a case by case basis. If you've got a top security vault holding the most secret information that has an unlimited budget then no, it's not a waste of time. $\endgroup$ – Styphon Apr 8 '15 at 9:51
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    $\begingroup$ On Mythbusters, bacon did not work. The only thing that distracted the trained (male) dog was pheromones. $\endgroup$ – JDługosz Apr 8 '15 at 11:12
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    $\begingroup$ This also helps keep out Terminators. $\endgroup$ – KSmarts Apr 8 '15 at 14:46
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As far as practical goes, the best way is to have a guy sitting next to the door. If he recognizes you, he opens the door.

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    $\begingroup$ Such guys however could be more or less easily "persuaded" to open the door to unauthorized people by various means (bribes, brute force, blackmail, ...). $\endgroup$ – Ghanima Apr 8 '15 at 8:20
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    $\begingroup$ Well sure. And other types of door are immune to sledgehammers or shaped charges? $\endgroup$ – WhatRoughBeast Apr 8 '15 at 15:03
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    $\begingroup$ I am not trying to invalidate your answer. Just naming ways to lock-pick this particular system. $\endgroup$ – Ghanima Apr 8 '15 at 15:16
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Possible ideas, assuming the "only the designated person" part implies each device only has to work for one person:

Possibly you could have some series of slots into which a hand could be placed and then moved around inside the door, which can only fit someone with that person's hand dimensions or smaller. You could combine that with spring-loaded tumblers within each of those slots which can only be moved by a person with long enough fingers. Or maybe put them all around the hand, so it would have to have the correct palm and wrist size as well. Increased security if the person is also willing to remove a portion of a finger - requiring anyone trying to open the lock to have removed the exact same amount. This wouldn't work if the person's hand changes size due to muscle/fat gain/loss though.

Perhaps a number of sheets of specific materials inside a hidden chamber, each of whose resonant frequencies corresponded to a different component of the person's voice when they sing a specific note. Each sheet will have a long thin tail at the bottom, to amplify the movement of vibration. Each tail is attached to a tiny tiny oiled switch, which when flipped becomes hidden inside the wall in which its housed for 2 seconds (using a spring timer). Each switch turns a different tumbler in the lock - only when all switches are "on" at the same time ( meaning all tails are vibrating, and therefore the person singing is someone with a nearly identical voice to the intended user) will the lock open. This could be gotten around if someone recorded the user opening the door, and played it back at high enough fidelity - this requires computers though, and so is only a concern if they actually exist in your world. It also would probably not be able to be opened if the intended user has a cold.

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    $\begingroup$ You don't need a computer for music playback, it can be done in an analogue way. $\endgroup$ – akaltar Apr 8 '15 at 5:53
  • $\begingroup$ Finger length doesn't really change once you're fully grown, as far as I know. So it doesn't have to be a perfect width match, if it's just based on (stable) length. $\endgroup$ – Bobson Apr 8 '15 at 21:03
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Assuming whoever makes the locks has biological engineering capabilities, a fully biological solution (a 'wet' solution) is possible.

Say the owners of the lock engineer a virus to replicate only in host cells matching certain, very specific tidbits of DNA unique to the people authorized to enter the door. Once an authorized person has swabbed a few cheek cells into a culture in the locking mechanism, the virus would replicate and, at high concentration, could trigger the door-opening mechanism.

I see a couple ways of accomplishing this: the virus could be engineered to release a special chemical only at high concentration, which could be detected by a special receptor molecule that releases a few electrons (towards a detection circuit connected to the electric motor etc. that physically opens the door)

Or, if you want to go fully biological, the virus could somehow trigger a bunch of muscle tissue controlling the door, through a similar chemical mechanism of some sort.

A few things you'd have to watch out for: the engineered virus could mutate while replicating and have unintended behavior (e.g. recognizing unauthorized people), so the culture would have to be replaced frequently.

An attacker could directly introduce the chemical the virus produces, if the chemical was known and producable through other means in high enough concentration. Or an attacker could swab some cells from an authorized person in secret (by grabbing a piece of hair, etc) and introduce those.

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    $\begingroup$ If a virus was too much, you could also have some form of enzyme or protein that bound to particular features of the DNA, and similarly produced some noticeable chemical product from to work that machinery. Could be as simple as producing an acid, and reading the pH. $\endgroup$ – psycotica0 Apr 8 '15 at 11:32
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    $\begingroup$ Actually, if we assume that wrong DNA produces no result, rather than a negative result, it'd be easy to allow multiple people entry by just filling the bath with enzymes for each person. If any of them match, it will affect the change, and if none of them do, nothing will happen. $\endgroup$ – psycotica0 Apr 8 '15 at 11:34
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The pin tumbler lock is thousands of years old and still in common use today; the key is a rigid object with several protrusions of varying lengths, which depresses pins in the lock to the correct depths. All we need is a part of the human body that is rigid and unchanging, irregular in a way that is unique to each individual, and which can be pressed against a lock.

In a word, teeth.

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    $\begingroup$ I admit, I was wondering where you were going there for a moment :) $\endgroup$ – Dewi Morgan Apr 11 '15 at 4:30
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    $\begingroup$ Unchanging? What about a filling? $\endgroup$ – Loren Pechtel Apr 12 '15 at 0:36
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A finger or a palm will fit exactly into a mold made from it. By making the mold of a transparent material, light will reflect differently when the fit is exact, i.e. when there is direct contact between skin and the mold.

I imagine a row of such plates on a wall, one per each authorized person. To enter you press your hand to your own mold and a simple light detector activates a motor to open the door.

This is somewhat easily hackable by pressing some soft material against the mold. Extra holes can be designed into the mold as traps, which trigger an alarm when covered.

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    $\begingroup$ Equally adding a height / weight / handsize / footsize etc to each lock would work. One "key" would be easy to break, but having multiple required keys to open the lock would make it more tricky. $\endgroup$ – franglais Apr 8 '15 at 11:37
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How good are you at micro-engineering? If you're sufficiently good at building on small scales, you could make a purely mechanical fingerprint reader that uses an array of miniscule probes (fractions of a millimeter across) to detect the pattern of ridges. The main drawbacks are that it would be highly sensitive to the orientation and positioning of the finger, and that it would be extremely fragile.

If you're willing to use electronics, an optical system using an array of micro-lenses (think: the method a CD drive uses to read the pits of a CD) would be more robust, but with the same positioning limits.

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There's a good low-tech solution if the authorised person is significantly smaller than the unauthorised person:

enter image description here

(image credit: Wikimedia)

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Implant subcutaneous magnets in a special 2D pattern which must be held over a magnetic amplifier which in turn moves the tumblers to the lock. The device is made to look like an ordinary fingerprint reader to fool those trying to crack the system. The subcutaneous magnets can be scrambled using a degausser between uses so that they won't be discovered, then through induction they can be recharged.

Alternately, you can use the techniques found in this article:

http://www.extremetech.com/extreme/133067-unbreakable-crypto-store-a-30-character-password-in-your-brains-subconscious-memory

The lock can be mechanical, but the person does not know the code, only their muscle-memory knows the code. They can't be tortured to reveal it. This is based on "implicit learning". The research was conducted at Stanford and Northwestern. Read the article - it is fascinating.

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When I was a kid, my father and I used to do some weekend electronic projects (he is a Electronic Engineer). One of our projects was a way to change the doorbell pitch according to who was pressing it, so we could easily known who was there.

We did that by simple hiding some circuitry under the doormat. The sensor could detect two things: (1) how apart were the feet and how heavy was the person. Of course that could be easily cheated and prone to errors (like when carrying extra weight or not standing correctly over the doormat.

You could use this very same principle, but changing the mechanism for a spring-based one. You would also have to change the lockpins to the bottom of the door. Also, this design is adjusted for a single person. A multi-user version is out of my league.

So, here how it works:

Planks

Six planks under the doormat. Each plank with a set of 8 springs. As I said, the springs must be calibrated according to a person weight and how apart the feet are. this is important, as the way the person steps over, trigger the lockpin mechanism:

Details

Of course the height of the pin must match the deepness of its housing in the door, the same way a the cuts on a key matches the notches on a tumbler. This way, people with different weights and feet aperture, would displace the pins too a specific position.

Well, that is a simple idea and a simple sketch. I'm looking forward to see people improving this concept!

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It might be possible to do a facial scan. Shine a light at the face through a slit and scan from the top to the bottom.

Back in the old days, a light shined through movie film with something that looked like a waveform, a vacuum tube would pick up the amount of light that went though and play the sound through speakers.

You would have a strip of film in the device that would get scanned at the same time as the face was scanned (that was made from the reflection of the user's face). If the "sounds" matched, then it would open. Think of 2 VU meters bouncing at the exact same time. Certain reflections would be enough for the needle to pass a certain point. If there were those two VU meters, there could be a light sensor that gets blocked when the needle goes past that certain point. Lets say the during the scan, it must pass a point 8 times for the lock to open. There would be a wheel that would increment every time it passes that point and on the 8th time the bolt would drop. If it didn't get to 8, the wheel would reset back to 0.

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  • $\begingroup$ Even better, make the reference copy a negative. The sum of the original face and its negative will be a constant throughout the entire scan, at a level defined by the way the negative is processed. If, at any point during the scan, the level drops outside of a tolerance range (high or low) centered on that constant value, it trips a latch. If it hits the end of the scan with the latch still untripped, the door opens. $\endgroup$ – Matthew Najmon Apr 11 '15 at 6:37
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You can create a stylus type device rigged to a fixed angle against the flat surface with wires/rods on the x and y axes. The target has to 'write' the appropriate passphrase. As they write, the wires are pulling mechanical pins or some such underneath corresponding to the motions being made with the stylus. Any person would need to know not only the unique passphrase, but also mimic the person's idiosyncratic style for writing that particular phrase. For the permitted person, it comes natural, but it's EXTREMELY hard to replicate something like that.

It's not just imitating a signature or other handwriting - the physical motion of the body of the stylus, positioning of the hand, possibly timing, etc. must match, not just the tip.

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  • $\begingroup$ Would your solution work even when the person writes her signature a bit smaller/bigger, a bigger more on the left/right, a bit rotated, etc? If yes, could you please detail how the mechanism handles scale differences? Or does it have to be the exact same signature, triggering the exact same wires? Thanks! $\endgroup$ – Nicolas Raoul Apr 8 '15 at 6:40
  • $\begingroup$ @nicolausraoul unfortunately, I am not skilled or trained in mechanical engineering. I'm not entirely sure what's feasible on that front. But you could design a margin of error based upon the system you use. I imagined a system in which sudden shifts in stylus direction (or pressing down at the end of a stroke) trigger pin pushing (or crossing a letter). The two guy wires determine the x and y location of the pin pushed, with some racheting springs powered by the lateral movement adding to the pin location. If the passphrase is correct and the signature is roughly the same size it should work. $\endgroup$ – Isaac Kotlicky Apr 8 '15 at 13:16
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An iris scanner may work with low level electronics: An image of the iris could be projected onto a matrix of photo receptors and if each of these is within a certain threshold of the correct brightness for the rightful person's iris, the door unlocks.

Might be fooled by a photo of the iris (as in real life)

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Ok, this could be quite complex, so bear with me.

The person who is allowed entry is placed in front of a panel on which are marked glyphs. Through a complex neurological process, this causes a chemical change in their brain, which will be the identifying mark that the lock uses.

The lock itself consists of a collection of rotating discs with notches arranged such that when properly aligned, they allow the passage of a bar or slider. A spring provides a little positive pressure to hold them in place, and a ratchet-type system allows them to settle into predefined locations. The bar or slider interfaces with a set of levers or hasps that hold the door closed, so that the door cannot be opened without moving the levers, which means that the bar has to be slid back, which can only be done when the discs are properly aligned.

In order to detect the presence of the identifying protein structure in the person's brain, glyphs are marked onto the discs that correspond to the priming pattern used earlier. On seeing these glyphs, the subject's brain will respond chemically, triggering a response in which their fingers can be positioned to rotate the discs into the position that aligns the notches so that the locking bar can pass. Subjects without this protein structure will fail to correctly position the discs, unless given sufficient time that random motion will cycle through the statistical combinations - this can be mitigated by having a guard posted who tells people to move on after an allotted time. After a successful entry, this guard can also be employed to reset the discs to their initial positions, although it is also possible to use a spring catch to rotate them once the locking bar passes back through.

I took the liberty of preparing this concept image of how such a system might look:

enter image description here

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    $\begingroup$ Very droll, but since this goes back to the wrong fork of the essential trichotomy of security ("something you have, something you are, something you know"), I don't think this counts as biometric in a meaningful sense. $\endgroup$ – Nathan Tuggy Apr 11 '15 at 19:35
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If electricity is allowed, you can make a set of devises that measure some parameters of the body: weight, height, microwave and X-ray transparency, magnetic field, CO2 and water content in breath and breath volume, voice frequency, conductivity and spectre of the skin etc. All this can be accomplished with simple electric devices, but each method given is imprcise. If the door is unlocked only with all tests passed, this would work.

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Since you mention by name fingerprint, palmprint, and iris recognition, I'll exclude biometrics that cannot be read externally (i.e. no blood). This is preferable for reasons of convenience and hygiene.

I'll also exclude biometrics based on gross properties (height, weight, etc.), since the variation from measurement to measurement is significant compared to the variation from person to person, requiring the tolerances to be set too high. (Essentially, a single or small number of measurements don't contain enough information to identify a single person out of billions.)

The difficult part of your question is the limitation on electronics. The only components not allowed are "an electronic reader and/or a microprocessor device." This is pretty vague. After all, isn't a doorbell an electronic reader that reads the presence of a finger? I'll assume that you don't want to disallow all transducers, so I'll interpret the restriction as "no complex digital logic." (We can't exclude all digital logic, since the output of the lock, locked/unlocked, is a digital state!)

This pretty much eliminates any sort of optical sensor. The only option we're left with is fingerprint recognition.


My phone uses a linear capacitance sensor to read the fingerprint as it slides across. This could be accomplished without anything more complex than a flip-flip. Basically the way it works is that a tiny metal pad (insulated from your finger) is wired to a bistable multivibrator. The capacitance of your finger loads the oscillator, changing its frequency. The frequency will depend on the capacitive coupling between your finger and the sensor pad, which depends on whether it's below a ridge or groove on your fingerprint.

The hard part will be matching to a stored fingerprint without complex logic. We are limited to a very simple direct comparison algorithm; we can't say, count and locate features. If the finger is scanned at a constant rate and with a fixed orientation and position, the resulting signal should be almost exactly repeatable. This could be accomplished by placing the finger in a linear stage that gently grips and aligns the finger, then slides it past the sensor at a fixed rate.

The stage can be electrically coupled to a magnetic tape or wire containing the stored pattern to ensure that they move at the same rate. You'll probably want to have a PLL in the loop so that you can tolerate up to a ridge-spacing of misalignment. You can use analog circuitry to compare the two signals (maybe by integrating the difference between the two) and trigger the locking mechanism to open if the match is close enough.

The system would be a little finicky, but should work. All of the electronics you'd need could be found in a pre-digital era tape player or radio. And, it can easily authorize multiple people by swapping the tapes (via either a guard or some sort of mechanical CD-changer-like-mechanism).

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I think you could devise a fingerprint scanner.

The device would require inserting your finger into a mechanism rather than simply against a plate as it can only accept one degree of freedom in the finger placement. You would push against a stop and in pushing you move the scanning element back accordingly. (It's pushing forward based on a counterweight, not a spring, thus ensuring a constant pressure no matter how deep you push it.)

Your finger is illuminated, optics focus the image onto a plate that is a photographic negative of your finger. The plate is slid sideways to address possible alignment issues (one sideways scan can be done quickly. Scanning two dimensions would be a very slow process, thus you must restrict the finger to one degree of freedom.)

The light bounces off your finger and goes through the plate. Behind the plate is a pair of analog TV cameras of as high a resolution as practical. They are doing an interlaced scan of the underside of the plate, the retrace intervals are 50% of the time so as each camera completes a scan line the other is ready to do the next scan line. The output signal is from whichever camera is currently scanning.

The resulting signal goes through a capacitor (to remove the average signal intensity, retaining only the variations from the average), then a bridge rectifier. The voltage now present is a measure of the mismatch between the finger and the plate.

The plate is slid sideways, making many scans as it goes. If the plate matches the finger at some point the image will become of nearly perfectly constant intensity, the output voltage drops to near zero and the door opens.

If you're limited to tube electronics you'll need an amplifier stage. I'm not sure but you might also need some bias voltage after the capacitor to drive the rectifiers--you would know how much comes through from this and consider that acceptable for unlocking the door.

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Keeping it simple. I was gonna suggest the guy behind the door but we can go one step further.

You could do the same with fingerprints (index cards with the fingerprints, and make the user do a fingerprint each time he wants to come in - compare the size and fingerprint patterns for entry manually). User identifies himself, makes a fingerprint impression, and that's filed away with the time (by someone else?). Also means you can't 'just' chop off someone's hand and use it. Designed correctly, you could seperate tasks so bribery is harder (one guy collects the fingerprint, a second one verifies it and tells a third one to open the door...)

Alternative identification schemes may involve tatoos (which can be copied) or physical tokens (which can be lost or stolen) or some combination of these.

User sticks his hand into a hole, and never has to see the person doing verification (so you don't know if its the same guy). It also could have a little guillotine in case someone tries to be smart and sticks a weapon in.

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