Which of the following statements about blockchain are correct?

A) A transaction is accepted once it is included in some block.

B) A transaction is accepted once it is included in a block that is at least 6 blocks deep.

C) The number of leading zeros required for the hash value of a block is dynamically changing over time.

D) A miner gets a reward whenever it finds a block that hashes to a value with required number of leading zeros.

E) Consensus is achieved by proof of work and the longest chain rule.

My Attempt:


B - I came to know from Wikipedia

I know the D,E are correct but I'm not sure about the B. Can you please tell me if my above options are correct or not.

  • Is this some HW question :) ? – sanket1729 Dec 10 '20 at 7:01

Actually they are all wrong:

A. Some sellers may accept one block (one confirmation) for small-value transactions that are time sensitive like buying a coffee.

B. Some sellers will want six confirmations for high value transactions where a one hour delay is acceptable. Like buying a house.

C. D. It isn't the number of leading zeroes that is tested. Only that the hash is numerically less than the current target number.

D. The reward cannot be spent until 100 blocks have been mined on top of the block. So the reward is not really realized at the time of finding the block with the required hash. Also, because of naturally occurring blockchain forks, some miners will occasionally have a block made extinct and any associated reward will be void. Because of the reward halving process, eventually in some future decade, miners will receive no reward for mining (only transaction fees).

E. It isn't the longest chain but the chain with the highest cumulative amount of work that gets chosen.

So, all wrong but also all pretty close to the truth.

  • Not all answers are wrong – R0bert Dec 10 '20 at 13:14
  • @R0bert. Your homework evaluator may have some answers they are looking for but the truth is all the statements are wrong in the way I described. Any expert will confirm this to you. – RedGrittyBrick Dec 10 '20 at 13:17

For A and B it depends on the user/system that receives bitcoin. You can read this article to understand finality of a transaction:

“It’s the settlement assurances, stupid” by Nic Carter https://link.medium.com/2oOWBym25bb

E looks true because nodes accept longest chain as valid. And full nodes enforce consensus rules.

Is the longest chain the one with the most blocks? The phrase “longest chain” refers to the blockchain that has taken the most energy to build. For the most part this is usually the chain with the most blocks in it, but to be more precise it’s the chain with the most amount of work in it.

How do you calculate the longest chain? Chainwork is the total number of hashes that are expected to have been necessary to produce the current chain. https://bitcoin.stackexchange.com/a/26894/

  • What about option D – R0bert Dec 10 '20 at 12:52
  • Not sure. You can refer to answer by RedGrittyBrick – Prayank Dec 10 '20 at 13:10

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