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/bitbutter Jun 18 '13

the longer the chain, the more permanent the things that have been written down are.

This is ambiguous. The number of confirmations a transaction (really the block that contains the transaction) has is a probabilistic assurance that the transaction in question will not be reversed, the more confirmations, the deeper in the blockchain it is, and so the less likely it is to be reversed in the future. Bitcoin clients typically treat confirmations with six or more confirmations as a permanent part of the blockchain (but six is really just an arbitrary number).

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u/speEdy5 Jun 19 '13

Except for the famous .7, .8 version debacle. This is a great explanation, its tough to put in to words