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Comment by agumonkey | Hacker News Reader

Last month I had the disappointing surprise of finding a lock on my bike. I forgot I put it (it's been years since I last used it). And it's not a key lock.. so I'm screwed. Not willing to cut the cable.. I decided to brute force it. 999 space is fun. Luckily you quickly find similar tips than the alignment one to speed up the process. And since it's not a full rotary lock, I could also "DDR" the tests by testing when going from 0 to 9 then from 9 to 0. 5 minutes later and 800 attempts I was lifting my bike like a king. Until I realize the rear tire was dead. TL;DR; do not succomb to the seductive power of cryptanalysis.. check rubber first !

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It probably would have been faster to apply tension to the lock, and then rotate the rows of digits until you find the "binder" (the one that the lock is resting on under tension).

Once you have found the binder, rotate it until you feel the lock open slightly and the digit ring falls into the gate. The digit ring will move freely for about 1.5 digits until it starts to seize again.

Repeat until you've found all the digits. I've done this one a number of occasions to help friends unlock their bikes, or to troll them by changing the combination on their bike.

tl;dr: combination locks suck. Don't use them, ever.

Case in point: https://youtu.be/uMvx0GtfEOk

That's exactly what I did.

Then it should not have taken you 800 attempts?

Without hyperbole stories are not as fun.

In reality there are other issues:

- mechanism is rusty

- has backlash/play

you're never sure you isolated the empty slot, so you assume so, go deeper, then backtrack when you reached the leaf of that exploration tree. Also, the double scan thing made it quick enough to avoid trying to probe too long for the right slot, hence the hybrid solution.

800/999 is worse than brute force at that point…

How so ?

By bruteforcing randomly, you'll find the combination in 500 attempts on average.

To have to make 800 guesses, you need to have pretty bad luck, or a strategy worse than bruteforce.

Average says nothing about how often you'd expect to take more than 800 attempts or more unlock, which I guess is 199/999 or one in five times

bruh... you think I can remember all the random attempts in my head ?

On average it would take around 500 guesses. One would have expected that with your "method" you could have reduced this by an order of magnitude (i.e. not 800) at least. Otherwise you're really just wasting time with additional steps for each trial.

When I was 10, I found a rotary bicycle lock + connected chain, locked alone to a post in a park. Having read up on locks even then (god knows how), I tried the method of "pull on it and spin the dials".

About 30 seconds later, it was unlocked.

I used that lock for about 10 years, until I went to University, where the thieves were a bit smarter.

Just a note that the lock you found may have not been abandoned.

Many bike posts in my university's undercover bike shed often had several locks without a bike in sight. It turned out many people leaving bike locks on posts they often use (it doesn't take up much space or prevent someone from using the post).

It saves those cyclists the hassle of carrying the the lock with them (only the key), which is particularly an issue with those very heavy bike U-locks and D-locks.

Agreed. I have a lock on the work bike lockers I use. Everyone seems to do this. Means you can have a very hefty lock but not carry it. Speaking of we moved buildings I need to snag it.

hehe nice story.

My cherished (and uber low grade) bike was stolen in Paris. The robber didn't bother to find the code, they ripped that tiny cable apart. They were nice enough to leave the bits in a McDonald's bag where the bike was attached (and of course, the McDonald's was just in front of me). Anyway I know have a 2kg keylock chain on my new bike.

At university, I had someone accidentally lock their bike to mine with one of those. So I just unlocked and relocked it, using a similar process. (There was a lot of play in the wheels - it was a fairly cheap one.)

In a bike workshop you frequently remove locks. It always takes seconds.

It is a bit like taking on a project and not knowing the admin password, it takes seconds if you know the codebase to fix it.

After taking assembler my freshman year (1970's) i wrote the combination lock code in the octal representation of 1-9. I wrote it on the bottom if the storage unit rented to?store furniture between terms..

When i returned to unlock it, the numbers were faded and unreadbale.

Got lock pliers from the office

lol. Funny someone suggested to engrave similar encoding into the frame.

And since I was eyeing on CNC milling machines .. that might be a nice project. I am thinking about stealing the correction scheme used on CD to be scratch resistant.

For 1000 possible combinations, wouldn't you expect to have to try ~500 combinations on average to find solutions? Since you found it in 800 attempts, this suggests that whatever method you used may have actually made your odds of success worst.

Well if you enumerate in order and your code is 800 then you have to try 800 cases.

I'm not sure how you make that inference. Could you explain your logic?

> on average

You can "expect" but if you only have one lock to unlock, it can be anything from 1 to 1000 attempts.

If you had 1000 locks, you can expect that the average would be 500 (if the numbers were truly random), but it still means that you might need one try for some locks, and 999 tries for some other lock.

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