not the nonce, but the blockhash has to be lower than the target. to achieve this, a miner needs to add a random number (nonce) into the block, hash it, check if the the blockhash is low enough. this gets repeated until you find a random number (nonce) that prodcuces a blockhash lower than target.
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oops, indeed, I knew it, I was just being too fast in answering. 
Thanks Fabrice.
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- Why is it important to make sure that miners spend money (on electricity) while mining?
Because they spend a lot of money so that they will appreciate the opportunity to get a reward from mining in the correct way.
2. Why would it be very difficult for someone to go back to change a previously added block?
Because it refers to two things, the content of block and the special link between each block.
- How does the network regulate mining difficulty?
When more people went mining, the target number will be low and difficulty to guess.
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- To be incentive. When mining to follow protocol.
- He need to have 51% of the hash power in the network which is impossible.
- Whit a target - as per the rule your hash of the block need to be bellow the target.
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spending money/electricity makes it verry unatractive to try and change a block in order to make BTC out of nothing. theoretically it is possible, pratically it is not. but if miners would have no risk at all trying to do this some would. if they do that messes with the blockchain and increases the hashrate.
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because if they do so, the other blocks will reject it. in order to succefully carry out this “dick move” they would have to catch up with all the blocks that came after the one they changed. because all node’s combine their computer power one person/computer could never catch up.
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more miners = higher difficulty… the block changes difficulty to keep hash at 10 minutes
more miners = better
less miners = less secure
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- Why is it important to make sure that miners spend money (on electricity) while mining?
it is important to for miners to spend money on electricity in order to add value to every mined block. It costs money for people to mine so it will always have a value to those that mine it.
- Why would it be very difficult for someone to go back change a previously added block?
If someone went back to change an old transaction that is already on the blockchain, that same person would have to spend the electricity to mine every block afterwards seeing as the hash value will completely change and negate all transactions moving forward.
- How does the network regulate mining difficulty?
Mining difficulty is regulated by the amount of miners participating.
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to add more value to securing the network
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because it would change all the blocks after and invalidate them
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buy adjusting that target based on how many miners there are.
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- Miners must spend money on electricity so that they are incentivized to mine honestly or they will loose the money they have invested.
- Because they would have to solve a new nonce for the block they have altered a transaction in, then solve the nonce in every block that comes after that. Plus miners would already be mining new blocks in that chain making it nearly impossible for the dishonest miner to catch up.
- The network regulates mining difficulty by lowering the target for the miners to guess, the hash rate is then increased in difficulty. If more miners are solving the blocks then the target is lowered further. When less miners are in the network the target is adjusted up and mining becomes more easy.
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1] Mining expends electricity to solve the cryptographic puzzle (proof of Work). In so doing they are rewarded by incentives (Block Reward and mining fees)
2] To do so would require re-mining that and all subsequent blocks
3] The more miners the lower the value of the target, and the lower the target the higher the difficulty. This may serve as a disincentive for least efficient miners forcing them to stop mining and thus reducing the hashrate.
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- To make sure they are playing by the rules; because they are the consensus of the blockchain.
- It is very difficult because they have to remine all the blocks after the changed one in order to re link all blocks on the blockchain.
- By lowering the target (Nonce)
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- To keep them honest and secure the network. This is the only way that they will recover their costs by mining. They receive bitcoin in transaction fees and in block reward.
- They would have to change every block afterwards while the current miners are also mining new blocks. It would take too much computational power and is almost impossible.
- The more the hashrate increases the more difficult it is to mine. The network lowers the target number which makes it more difficult to mine each block.
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Why is it important to make sure that miners spend money (on electricity) while mining?
To encourage miners to play by the rules of bitcoin+blockchain. If they break the rules they lose their investment ($/Electricity). If they follow the rules they get rewarded for their efforts = Proof of Work. -
Why would it be very difficult for someone to go back change a previously added block?
They would have to change the target block and all blocks following it, else their efforts are wasted and they only fork off the chain. The cost and infrastructure needed would be staggeringly enormous. -
How does the network regulate mining difficulty?
Greater mining resources dedicated to the network will increase the Hash rate. Greater hash rate causes the Puzzle Difficulty to increase proportionately. This is to help regulate block creation rates, keeping it close to 10 min/block.
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- So they have skin in the game, incentive to follow the rules. BTW, this is a problem because FREE energy is a real possibility in the future. Perhaps we’ll have to go to proof of stake instead.
- Because they would have to remine that block and all blocks created since.
- You didn’t say, but the protocol must look at the amount of time taken for the last block and adjust the difficulty to maintain average block times of 10 minutes.
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- It is important for miners to spend money to be incentivized to follow the rules and protocol as they are invested.
- If someone tried to change a prior block that would break the link of that block and all the one following it.
- If there are more miners than the Target is lowered making it more difficult to guess the nonce.
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It incentivizes them to play by the rules. Also, it makes cheating expensive.
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The hash of that block would change and the link to the blockchain would break. The offending miner would then have to attempt to remine all future blocks with significantly less hashpower than the main fork. Basically impossible to do.
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By controlling the size of the target block hash (i.e. how many leading zeroes in the block hash) and thereby mining difficult. The aim is to keep a block time of about 10 minutes.
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Homework on Mining and Proof of Work - Questions
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Why is it important to make sure that miners spend money (on electricity) while mining?
Ans: This provides the incentive for them to find the correct hash so the new block can be appended to the blockchain and they will this get paid. -
Why would it be very difficult for someone to go back change a previously added block?
Ans: Because changing a previous block is virtually impossible. It will be mean that everything is changing with every other block and that is next to impossible. Once the block is appended to the blockchain, it is permanent. -
How does the network regulate mining difficulty?
Ans: The network makes it easier to find the hash for a new block when there are less people mining and more difficult when there are more people mining by changing the location of the target. This keeps everything balanced.
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It incentivizes miners to keep mining in an honest manner to re-coup their costs.
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It would change the hash and then all the hashes, to get back to the present so much extra work would be needed to re-mine old blocks on your own, and then catch up it is practically infeasible.
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The target hash rate. The greater the number of miners, the lower the target rate = the greater the difficulty.
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- Why is it important to make sure that miners spend money (on electricity) while mining?
The fact that money must be spent on electricity in the proof of work consensus model creates an incentive structure that provides a basis for securing the network. It’s the “work” in the proof of work. The electricity is spent before up front in the mining process before a block is awarded to a particular miner. Each miner is competing with their own block at the same time and only the miner that solves the puzzle gets the block reward. If the block contains errors it will not be accepted and the electricity would have been spent for nothing and no reward would be given.
- Why would it be very difficult for someone to go back change a previously added block?
In order to change any transaction in any block on the chain, the whole chain must be altered. This is due to the fact that each block contains a pointer from a previous block that is a hash of the block itself and a hash of the pointer from that block. Due to hashing algorithm property number 4, once any change is made to any block it creates an automatic chain reaction that would alter all of the blocks that come after it in the blockchain. Once the chain is altered no other nodes would agree that the altered chain is the correct one and it would be rejected by the network because the nodes always only agree on the longest version of the chain. This is how the history of the chain is made permanent. It’s a monumental innovation that has forever altered our digital landscape.
So if a person were to make an alteration to any block, they would then have to go back and re-mine every succeeding block and solve every puzzle in order to create new links between the blocks. Since the difficulty of solving the puzzle is directly proportional to the hash rate on the network at any given moment and person would have to gain access to computational power at least equal to 51% of the hash rate. A.K.A. the 51% attack.
Even further, after each block is re-mined and each puzzle re-solved they would then have to catch up to the blocks that were being found by other miners as they were re-doing the chain.
A further incentive structure exists in that if a person were to successfully perform a 51% attack they would invalidate BTC in principal and it would lose its trust and therefore its value. So there’s not point in doing it. So, if you have enough hashing power to do the attack the much more profitable option would be to just legitimately mine BTC yourself. And, even though it’s seems counterintuitive at times, human action seems to always follow profit incentives.
At this point, the BTC network has enough hash power that performing a 51% attack is virtually out of the question. Unless maybe you had a quantum computer capable of operating at 1500 Q-bits or higher.
- How does the network regulate mining difficulty?
The difficulty level is adjusted via the adjustment of the target number required to guess the nonce. The is what was referred to before as “solving the puzzle”. The size of the target number is proportional to the hash rate on the network. The lower the number is, the more difficult it is to guess the nonce. This builds in an automatic loop that increases the difficulty as more miners come onto the network or decreases it as they leave. Guessing the nonce is virtually random. A miner basically rehashes over and over until they’ve found it. If the target number is lower it decreases the possible space in which the nonce may appear and therefore is harder to hit. And thereby increases the difficulty.
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Why is it important to make sure that miners spend money (on electricity) while mining?
Miners then have an incentive to play fair so that they can be the first to solve the cryptographic puzzle to get the block rewards and all transaction fees. -
Why would it be very difficult for someone to go back change a previously added block?
Because this makes all other previous block after that the block which has been altered invalid. They will become rejected. It takes enormous amounts of computing power to restore the blocks. Meanwhile, other miners who play by the rules are already adding new blocks to the blockchain. Which means the miner who cheats, will have to re-mine all other rejected block + all current new added blocks. In theory this is possible but in practice this is nearly impossible to pull off. -
How does the network regulate mining difficulty?
More active miners on the chain = higher difficulty for miners to guess the NONCE in order to get blocks validated, meaning the target is being lowered.
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It is Important to make sure that miners spend money (on electricity) while mining since there is where their incentive coming from; by spending money, they would like to get a reward, and to get this reward they have to follow the rules, otherwise it doesn’t matter if they solved the puzzle if they haven’t followed the rules.
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It would be very difficult for someone to go back change a previously added block since the link would be broken and would be Invalidated and then he will have to re-mine the new blocks since the network will always choose the longest chain.
3.The network regulate mining difficulty by increasing the difficulty when the hash power is high ( high volume of miners ) and vice versa.
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