aliashraf,
Your original question and discussion in your thread is a total mess. You are using very unclear terms, and you are bending Bitcoin terms in a way that shows that you do not understand Bitcoin's security model.
You don't understand what a "full node" is (as you confuse it with SPV), and you don't understand its role in the network.
It's a headache to read what you write as you appear to be ranting and make half-points all along the way.
Confusion..Here is only one example of your confusions:
There is a very efficient, secure way of running a bitcoin full node on commodity mobile devices very efficiently in economic mode, called UTXO commitment.
Here is more confusion:
According to this model miners would have to commit to the UTXO (state of the bitcoin machine) by somehow including its hash in their blocks and a light full-node could besides synchronizing headers like a usual SPV wallet continue to downloading the full state of the machine as of latest couples of hundreds of blocks and act like an ordinary pruned full node thereafter. Right?
For this to happen, you need a simple soft fork which is not getting support, why? Because people are paranoid about miners being susceptible to commit to bad UTXOs for say 500 blocks!
Even more confusion:
Are we clear? This full-node, full-node propaganda is totally fake and misleading. It is not a campaign in favor of empowering bitcoin users by giving them an opportunity to run a full node, it is just a trick to relying less and less on miners in a baseless and paranoid way:
Even more confusion:
Who in the hell can imagine a scenario in which miners are colliding for 3-4 days to inject a few false UTXOS in bitcoin blockchain? A paranoid or a politician.
I started out reading your thread coming from a place of being willing to help and to clarify how Bitcoin works, but after reading your responses, I'm not convinced you are interested at all.
Bitcoin safe to double spend, based on fully validating nodesJust to start somewhere, Bitcoin does solve the double spend problem. This is achieved by that participants run fully verifying nodes, and each such node enforce Bitcoin's consensus on every transaction of every block, and this will mean that in one node's view and perception, no double-spend ever occurs.
Miners are then free to mess up with blocks as much as they like, but they do so at their expense, so as long as the value of your transactions are insignificantly small compared to the Bitcoin miners' electricity expenses, you're safe - it's highly unlikely that someone will want to play appear-disappear reorg games with your 100USD transaction when it costs 10,000USD to mine a block.
UTXO commitments discussionRegarding UTXO commitments, again you discuss these without understanding Bitcoin.
I'll share with you here that UTXO commitments are somehow similar to Ethereum 1's blocks' merkleized state snapshots.
What happened in the Ethereum community then was that, they fell for the temptation of validating blocks at all - when the Ethereum network is too busy, it will trig a feature that they call "fast forward", where a fully validating node will skipover one or more blocks, and thus simply blindly trusts the other nodes and the miners that they didn't forge anything. This is extremely risky and a viable attack vector for attacking their blockchain.
Further discussion of Bitcoin's security model, the definition of decentralization, and of that, here:
https://hackernoon.com/the-ethereum-blockchain-size-has-exceeded-1tb-and-yes-its-an-issue-2b650b5f4f62UTXO commitments infallible as you suggest is not correctYour assertion that "UTXO commitments would always be valid because the miners are always attentive, correct and would never want to trick anyone" does not hold.
My take (which is a surprise somehow):
Unlike what is said ever and ever, one could put
trust in miners as long as there is proof that:
- Miners are not inflating the supply illegally,
- The costs involved in defrauding him/her (personally) by re-org attacking the bllockchain are orders of magnitude higher than the assets he/she has put in stake.
Again, this reads confused, you need to be more specific and clear.
51% attack does not allow double-spendLast to respond to your question in the thread's title, "Is 51% attack a double-spending threat to Bitcoin?", I guess you have the answer already: It is not.
Asterisk