If the node of the 51% of the total cpu power decides to cheat, it can change the value of the bitcoin reward (from 6.25 to 100 bitcoins) and add them to his account?
he cant change the rules of all network node users. but he can change what he sees in his local copy. but thats just him having his own stack of blocks he and he alone is following. whilst everyone else rejects that bad block that doesnt follow the rules. and they instead receive good blocks from the other 49% that they decide to keep
In case of the 51% attack, since the block generated by the scammers is invalid (in this case), even though its blockchain is longer, the other legit miners in the bitcoin network will not shift to it. This will make the attacker solo in its own branch. He or she can accumulate more bitcoins, but no one outside it's branch will accept those.
correct.. unless he can convince other people to download new edited/updated software that he tweaked to meet his new rules then they will join him on his altcoin. he would then have to try to convince merchants/exchanges to accept that altcoin too to then be able to spend/exchange his altcoin coins
But, there are some other things an attacker can do if it owns 51% of the total CPU power. Double spending is an example . For example I buy something for 10 bitcoin — and post the transaction to the bitcoin network. The scammer can mine a block with that transaction, and updates the blockchain. Now after the merchant confirms that transaction, the scammer can mine that block again with a new transaction with the same inputs, but outputs the amount to the buyer’s bitcoin account. Now that block is a valid block, and the attacker can repeat this on top of that to make it the longest blockchain, with its 51% in computational power.
he cant change who YOU have signed YOUR transaction destination of funds to go to..
instead when he goes back to edit the confirmed block. he can just remove the transaction and make it unspent.. (unconfirmed) once he catches up and makes his chain the re-organised default chain that includes the block without your 10btc transaction.. then YOU can spend your coin again..
there is no advantage to him.. so he wont do that. he would probably want to re-exclude one of his transactions he already spent. thus he could double spend that and gain/profit from his effort.
however. the cost to mine so much hash just to edit out a transaction.. that transaction better be of large volume to be worthy of the effort
Also, the scammer can block some transactions being added to the blockchain. attacker can have it's own preferences and keeps-on mining the blocks with the set of transactions it wants. This will defer certain transactions — even though they happened quite earlier in time. Satoshi Nakamoto has suggested a way here to prevent such situation , but he himself concludes that there will not be a need to do such, where a miners want to explicitly drop some transactions.
empty blocks is and can be annoying and cause chaos. but what is gained from that apart from some drama.
if people cant move their funds then it negatively affects the market price thus ends up costing the scammer in the long run
By this moment , nearly all the miners are mining through pools, very few miners solo. A mining pool lets miners contribute their cpu power — and paid to each miner based on the hash rate they contribute. The following link shows the percentage hash rate generated by popular mining pools at the time of this writing.
its no longer CPU power. nor GPU, its now ASIC power
a CPU is about 50,000,000 hashes
a asic is about 140,000,000,000,000 hashes (140 terra hashes)
the network on average has about ~1.5m asics which if it was CPU standard would be 4-5 trilion PC's
