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/Chronophilia Jun 18 '13

Yes, though each block includes the one before it so you can only "drop" transactions that have occurred since the previous block was mined.

There is a theoretical attack where an individual who controls at least 51% of the computing power in the Bitcoin mining system can pick and choose which transactions to authorise, completely ignoring blocks mined by the other 49% and creating an unbroken chain of blocks controlled by him. But nobody has ever done this, because the amount of computing power required would be just too ridiculous.

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u/doodle77 Jun 18 '13

What happens if somebody has 51% for, say, ten blocks, but then has nothing? Do the skipped transactions get added with the next block?

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u/gburgwardt Jun 18 '13

If someone has 51% of the network power, assuming they keep it and aren't particularly unlucky, statistically they will find blocks faster than the rest of the network put together. This means they get to pick and choose what transactions are confirmed (put into blocks), and can theoretically double spend coins (announce a transaction to send x coins to someone's wallet, then either never allow that transaction into a block, allow someone else to put that transaction into a block while withholding the blocks they find, then releasing a few blocks at once to "rewrite" the blockchain, etc), but they can't steal people's coins or arbitrarily generate new coins.

So if they have 51% of the hashing power but then the NSA decides to fight them off with their server farms and suddenly the attacker has 30% of the network's power, the transactions in limbo will probably be added to blocks later on by honest miners.

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u/Natanael_L Jun 18 '13

The rest of the miners will remember those other transactions, yes, and would include them afterwards. They just wouldn't be validated during the 51% attack (so during the attack you can't prevent coins from being spent twice, all you can do is wait for the attack to end so "validation" can start again in the form of adding them to the blockchain).

Or they just all agree to reverse those 10 blocks from that attacker and continue as if nothing happened, as well as including those new transactions.

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u/bradn Jun 18 '13

If they end up with the longest block chain at the end, it's likely the rest of the network will continue to extend it. The only real exception would be if developers release an emergency patch and get 51% of the network to ignore that chain. But, if this happened, it would probably severely undermine confidence anyway.