MovieChat Forums > Robot & Frank (2012) Discussion > Brute force would not work

Brute force would not work

posted 12 years ago by sth128
52 replies | jump to latest

So the robot says that it takes between 4 seconds and 1 hour 43 minutes to crack a 3-number combination safe. That is complete rubbish. The 4 seconds is likely the estimate for opening the safe on the first try, this means it takes the robot 4 seconds to rotate the dial 3 times. 1 hour 43 minutes equals 6,180 seconds, divided by 4 gives 1,545 trials.

A combination lock dial usually scales between 1 and 100 (search "combination lock dial" on Google and you'll see). 3 number combination means at least 100 x 99 x 99 permutations (I'm assuming you can't have 3 identical numbers). That is 980,100 possible combinations, 634 times more combinations than the trials the robot can perform in the given time.

Ignoring that "little" problem and say the robot does brute force its way through the lock, another problem with spinning the dial so many times in such a short duration is heat generation. If every 3 trials span one single revolution (that is, the equivalent of rotating the dial exactly 360 degrees), that means 326,700 revolutions over the 103 minutes, which is over 3000 RPM. Imagine spinning the lock dial at that speed for over an hour. That thing will be so frigging hot the gears would probably expand and lock the entire assembly.

Even if the safe was using a high school combination lock (ie. 30 numbers), there will still be 25,230 possible permutations, 16 times more than the 1545 trials the robot can perform in the given time.

That robot might be able to cook and garden, but it sure as *beep* can't do basic math.

[–] camaleonyco 12 years ago

I think you are forgeting some details... Let's say we have a 4 digits safe. The dial has 1 at the top, 2 at the right, 3 at the bottom and 4 at the left. I need to find a 2 digits combination, and my first guess will take 0.720 seconds. According to you it will take me 11.52 seconds to get every possible combination. However the safe doesn't reset after every pair of numbers, I will not have to input: 11-12-13-14-21-22-23-24-31-32-33-34-41-42-43-44

Instead I could use: 1121314223243441 and I have every possible combination.

So, if my first guess took me 0.720 seconds, you might think that every turn takes 0.360 secs, therefore the entire sequence will take 6.120 seconds. But lets say that, for the first pair of numbers (11), I have to turn the dial 360°, first clockwise, second counter-clockwise, that's why it takes me 0.720 seconds, 1 millisecond per degree. For the next number I will need to turn it just 90° CW, then 90° CCW, 180°,180°, 270°, etc. Total number of degrees or milliseconds: 3960.

Not 11.52s as you would guessed, but 3.96s...

If we take into account the previously mentioned document (http://www.crypto.com/papers/safelocks.pdf), the safe might have only 22,330 "good" combinations... If 4 seconds is not an average, but the time needed for the first permutation, and if that is the one that takes more time, we can assume that each turn takes an average of 0.667s (a Gaussian assumption, lol). According to my empirical theory, each turn will throw a new permutation, so it will take 22,332 turns to get all the possible combinations. That's still more that 4 hours, that's why I'm suspecting Robot knew something that we don't!

Accurate or not, I loved the movie!

Sorry for my english.

[–] hisloverisolde 12 years ago

Your thread title is wrong.

Brute force WOULD work - it would just take longer than was stated and depicted in the film.

[–] cableaddict 11 years ago

Maybe it takes the robot 1/2 second to initially get his arm & hand in position to start, 1/2 second to actually dial three numbers, and then, once the correct numbers have been dialed, 3 seconds to rotate the arm & then pull the door open.

It would thus still take 4 seconds to open the safe if the first guess were correct, but 1/8th the total time calculated here, by whomever is correctly calculating the number of dials needed, to go through every possible combination.

(And I really, REALLY don't care who's correct!)

A little knowledge is a dangerous thing.

Oh, and I bet the OP never gets laid!

Ever.

[–] cableaddict 11 years ago

So, has anyone actually figured out the correct time?

[–] sgreen4 11 years ago

Unless you know the type of lock, you can really never know the real answer.
The OP has a point in saying you couldn't crack the lock using the brute force method. It would obviously take too long. Therefore, the Robot obviously used another method. The Robot must have exaggerated when he was telling Frank he was using a brute force method. He must have used some short cuts. short cuts that are probably too complex or too boring to explain to Frank or the audience so he just says he is using a brute force method. The Robot is able to lie and exagerate as seen in the film. He is even able to steal just to make his master feel better. Cracking locks for him is child's play.

[–] 11 years ago

[deleted]

[–] shoobe01-1 11 years ago

The door was on a second floor balcony, or something.

The various posts about combination dials having fewer available combinations than they appear to is spot on. For your typical Master lock on your school locker, around 1/3rd as many numbers as it seems. Try it. Enter a combination off by exactly one on every single value and it'll work.

They skipped a bit, but yes they clearly had additional knowledge of the type of safe. And: this goes great with the absolutely true statements Frank made several times about no security being perfect, everyone in the industry knowing that, and you just delay entry and make best guesses. Plus, the victim wasn't a security expert, so just had some consumer safe. Easy.

OTOH, would have been cool if, because it's a vaguely medical robot, it had enough sensors to listen to the lock mechanism and open it basically immediately instead.

[–] daddie0 11 years ago

All I know is it took him less time that it took R2D2 to deactivate the trash compactor on the Death Star, but it was probably because there was less yelling and another robot wasn't banging on his head.

[–] QTKeet 11 years ago

Interesting thread, and you're not completely wrong to question the validity of Robot's method, but I think you're concentrating too much on details that might just be complete moot points.

Yes there's many combinations, give or take depending on limitations on the particular safe. Doesn't matter.

You've already heard from another poster about your gross assumption that dialing one combination takes 4 seconds, and I strongly agree that assumption kills your whole math.
Four seconds could just be a reasonable bottom estimate for opening the safe altogether and not a mere measure of time for a single combo attempt. I'm sure Robot knows that opening the safe on the first try is HIGHLY unprobable any way.

But I laughed when you said spinning the dial fast (3000RPM) would generate significant heat; it's not a car engine.
For example, CNC machines often run spindles at very high RPM but only need coolant when doing any actual work, drilling, etc. You can free-spin the machine at max RPM all day and you wouldn't even burn your hand on it.

If you want to go all Nuclear Physicist on us, then you should be more concerned with Robot's drive motors overheating before the safe dial ever does.

Another poster already grazed on my final point, but just to clarify: we don't know the exact model of the safe and how it works, mainly what happens after 3 numbers are dialed in to verify the combo. Let's say you want to try 35-21-7 followed by 21-7-16 then maybe you could just treat both as a set 35-21-7-16, so as long as you can verify after each number following the first 3. So a sequence of twelve numbers doesn't give you just four combos, it would give you ten. That is, assuming you don't have to reset after each combo.

It's just so ironic to see a post basically criticizing the ONE THING shown in the movie that a machine can actually do in a superior way to any human, even today. Nevermind all the total displays of understanding that Robot had for the social/intellectual/psychological nuances of human thinking and language. Pfft "near future" my donkey!

-"But you know what's on my mind right now? It AIN'T the coffee in my kitchen..."

[–] djsnafu23 10 years ago

I think the reasoning is as follows. The set up for the first operation and combo to try would take 3.99999999 seconds and the rest would be the time needed for all permutations.

...

http://soundcloud.com/dj-snafu-bankrupt-euros

Coz lifes too short to listen to Madlib