What is the "certain criteria" to be a valid Bitcoin block?

Question

I'm looking for information about Bitcoin. And I read this, it's really helpful.

I already asked the question and many of you guys said it's duplicated, and link to the question "What exactly is the mining?" or something.

The thing is, I already read it, and I just need some more specific information about criteria.

One of the answer for "What exactly is the mining?" said

"The way Bitcoin makes sure there is only one block chain is by making blocks really hard to produce. So instead of just being able to make blocks at will, miners have to compute a cryptographic hash of the block that meets certain criteria."

In here, the cryptographic hash should meet the certain criteria. And I wonder what it is, and who decided the criteria. I mean, if computers keep mining hash block to get the bitcoin, there should be a certain criteria like it says above.

Hope anyone can answer this question.

Have a nice day.

Answer 1

A valid block is found when any miner authors a block candidate whose block hash interpreted as a number is lower than the target. This is equivalent to meeting or surpassing the required difficulty.

To this end, miners create templates of a block that they would like to confirm by selecting a set of transactions and creating their unique coinbase transaction to collect the block reward. Each block template can be tried a number of times by cycling through all possible values of the arbitrarily picked nonce in the block header. After that, the template can be slightly adapted e.g. by changing the block time in the header, by incrementing the extranonce in the coinbase transaction or introducing any other change into the block template.

Answer 2

Ok, let’s try to give a helping hand: this certain criteria is the nonce, that must be lower than a “certain” value. The nonce is calculated as a hash value with a certain number of 0 at the beginning. It is not the block hash! This nonce value is automatically adopted by the system, there are formulas behind, that try to achieve a block creation every ten minutes. If there is too many miners with too much hash power, the difficulty increases, otherwise it get’s lowered (or stays the same). The explanation in detail can be found in Andreas’ book “Mastering Bitcoin”, or on Bitcoin.org.

--> correction: from Andreas' book: "With all the other fields filled," (which is Version, Previous Block Hash, Merkle Root, Timestamp and Difficulty Target) "... the block header is now complete and the process of mining can begin. The goal is now to find a value for the nonce that results in a block header hash that is less than the difficulty target. The mining node will need to test billions or trillions of nonce values before a nonce is found that satisfies the requirement."