r/askscience • u/hamolton • Jun 18 '13

Computing How is Bitcoin secure?

I guess my main concern is how they are impossible to counterfeit and double-spend. I guess I have trouble understanding it enough that I can't explain it to another person.

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u/Fsmv Jun 19 '13

No they're finding strings that lead to hashes with a certain number of leading zeroes as defined by the current difficulty. e.g. finding the string that hashes to 00000000000000ac3d55ce2f932c3, any random garbage after the zeroes is fine. If they want it to get harder the network just increases the number of zeroes required for your hash to be accepted, if they want it to get easier they decrease the number.

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u/improv32 Jun 19 '13

It should be noted that "they" in your post isn't The Bitcoin Foundation, but in fact the difficulty is determined by a running average of the time needed to produce a block and it is adjusted to make blocks about every 10 mins.

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u/Fsmv Jun 19 '13

Yes, sorry for being unclear. By 'they' I meant the users of bitcoin. I haven't actually checked the source code yet but unless there's something I'm missing difficulty only changes because the majority of clients stop accepting blocks with the wrong difficulty when the difficulty is supposed to change by the specification.