Is calendar time used in the Bitcoin network?

[gmaxwell]

Does that mean that if the Bitcoin protocol is changed so that all nodes check for double spending, then that would prevent a 51% attack?

Double spending is absolutely and completely precluded in a blockchain. Everyone always checks for double spending already, it's one of the rules.  I suspect you've missed this.

(I'd really recommend some quiet time trying to understand the system instead of so many posts.)

[Anders]

Does that mean that if the Bitcoin protocol is changed so that all nodes check for double spending, then that would prevent a 51% attack?

If two different nodes receive two different competing transactions, how would they decide which transaction is the "real" one, and which is the "double spend"?

Right now the solution that bitcoin created was a consensus system where proof-of-work is used to establish the order of the transactions.

If a single entity can accumulate more proof-of-work than the entire rest of the world combined, (more than 50%), then they are able to choose the order of the transactions.

If two or more transactions are competing, then they can all be rejected I would guess.

Have you discovered a better distributed consensus system than proof-of-work?

Yes! My proposal is a distributed hash table (DHT) where each coin has a unique id and is locked into the DHT with a digital signature. I don't know yet if it would work though.

[Anders]

Does that mean that if the Bitcoin protocol is changed so that all nodes check for double spending, then that would prevent a 51% attack?

Double spending is absolutely and completely precluded in a blockchain. Everyone always checks for double spending already, it's one of the rules.  I suspect you've missed this.

(I'd really recommend some quiet time trying to understand the system instead of so many posts.)

Yes, I need to learn more about the Bitcoin protocol. The problem is that there are so many details. The Satoshi white paper is only a very high level description and the real Bitcoin implementation is much more complicated with all the details. Plus I think asking amateur questions can be helpful also for other people who are learning about Bitcoin. But ok, I need to check the details better beforehand I agree.

[DannyHamilton]

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I think asking amateur questions can be helpful also for other people who are learning about Bitcoin.
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Then please ask your questions in the "Beginner & Help" section instead of the "Development & Technical Discussion" section.

Have you actually read the whitepaper?

Because, while it might be a "high level description", it answers the questions that you are asking here.

[Anders]

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I think asking amateur questions can be helpful also for other people who are learning about Bitcoin.
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Then please ask your questions in the "Beginner & Help" section instead of the "Development & Technical Discussion" section.

Have you actually read the whitepaper?

Because, while it might be a "high level description", it answers the questions that you are asking here.

Yes, I have read this document: https://bitcoin.org/bitcoin.pdf

I took a look at it again and it doesn't mention how calendar time is handled. At least I couldn't find any information about it.

[DannyHamilton]

I took a look at it again and it doesn't mention how calendar time is handled. At least I couldn't find any information about it.

True.  That particular question is not answered in the paper. And was answered very well in this thread.

The questions that are less well though out are:

Can a 55% attack on timestamps be done to mess up the blockchain?

Clearly not, since as can be seen in that paper, the time has no effect on the ordering of blocks in the blockchain.  According to the paper, the blocks themselves in the blockchain are "the timestamp" that is used to order transactions (a block that occurs higher in the chain is "later" than a block that occurs lower in the chain regardless of any calendar time stored in the block).

But wouldn't the majority of nodes accept the false timestamps in a 55% attack? Hmm... And that would mean that they can even set timestamps backwards in time.

Since it had already been made clear that nodes will only accept a timestamp "if it is greater than the median timestamp of previous 11 blocks, and less than the network-adjusted time + 2 hours.", then it should be clear that a >50% attack can't change the timestamps significantly. The paper specifically states that nodes "reject invalid blocks by refusing to work on them." and that "Any needed rules and incentives can be enforced with this consensus mechanism."  Furthermore, it states that "An attacker can only try to change one of his own transactions to take back money he recently spent".

Then how is double spending possible in a 51% attack?

This is well covered in section 11 of the paper.

[Anders]

Can a 55% attack on timestamps be done to mess up the blockchain?

Clearly not, since as can be seen in that paper, the time has no effect on the ordering of blocks in the blockchain.  According to the paper, the blocks themselves in the blockchain are "the timestamp" that is used to order transactions (a block that occurs higher in the chain is "later" than a block that occurs lower in the chain regardless of any calendar time stored in the block).

I think I understand it now. It's easier for the nodes to check calendar time, since that's a global value. The transactions on the other hand can be spread out on different nodes. Calendar time is only needed for determining the next difficulty.