Quote from: johnyj on December 14, 2015, 05:13:52 PM

- snip -
hackers can spend your money without your consent

Under the current protocol rules, this is a lie.

Changing the protocol to allow others to spend your bitcoins without the private keys would require consensus of all full nodes. Since I would refuse to support such a change (as would many other operators of full nodes), consensus on such a change would be impossible. Therefore, the protocol cannot change to allow "hackers to spend your money without your consent".

Quote from: johnyj on December 14, 2015, 05:13:52 PM

How is that possible? Because bitcoin is essentially an agreement (protocol) among nodes, if majority of the nodes around you agree that your bitcoin is gone, then it is gone!

This is not true. You don't seem to understand what the word "consensus" means, or how bitcoin actually works.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

It does not matter how strong ECDSA is, all it takes is a group of nodes around you changed their rules (or so called sybil attack)

It takes a LOT more than just a "group of nodes around you". Also, a group of nodes changing their rules is not a "Sybil Attack".

Quote from: johnyj on December 14, 2015, 05:13:52 PM

Somebody might wonder: Aren't miners suppose to be the honest nodes and stop all this?

Nope. Bitcoin does not require miners to be "honest". It requires solo miners and mining pool operators to be self-interested, and it requires that no single self-interested entity control a majority of the hash power.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

Unfortunately, in this case, miners or so called most powerful computer network in the world can not do anything about it

Why? Because everything in bitcoin is decided by its agreement among nodes.

Solo miners and mining pool operators that don't want to be scammed or attacked should be running at least 1 full node. Therefore, they participate in that "agreement among the nodes" that you are talking about.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

If the nodes changed their way of calculate blocks, then all the miners will be dropped from the new network, and all those ASICs in large mining farms will just become paperweight

If some nodes change their way of accepting blocks, then those nodes will be dropped from the network. The Bitcoin miners and Bitcoin nodes will continue running without the modified nodes and ASIC will continue to work as they always have.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

This becomes a real threat when mining has become too centralized, e.g. only a few large pools are doing mining. So, even they are running the original version of bitcoin, if large group of nodes have upgraded to a different version, these miners will just be ignored as minority

As I already said, you don't seem to understand what the word "consensus" means, or how bitcoin actually works. If a large group of nodes ignore existing consensus rules, then they will fork off onto their own non-bitcoin blockchain. Meanwhile the remaining nodes and all the existing mining will continue to operate as it always has.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

(new version can easily change the highest diff rule to highest length).

I don't even understand what you are trying to say there. Anyone can change their node to do anything they like, but if it doesn't follow the existing consensus rules it will be ignored by the rest of the network.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

Of course without hash power the new version will worth nothing later on, but I guess the thieves only need to sell their stolen coins before others realize the loss

They will find it very difficult to sell their useless coins that won't be recognized as valid by anyone or any service that is still running the current consensus rules.

Quote from: johnyj on December 14, 2015, 05:13:52 PM

The critical point that have real financial impact are exchanges and web wallet services. If one of these nodes together with a group of malicious nodes changed their protocol, then they could easily take others' coin, sell on exchange and profit.

If you are really paranoid and assume that every exchange might be a potential malicious actor like MTGOX, then they have many ways to profit unethically through a protocol change

Exchanges and Web wallet services don't need to bother changing any protocol. The users have already sent the bitcoins to them. They can simply refuse to give the bitcoins back.