Been thinking on this for hours to try and fully grasp mining, and this scenario is still confusing to me -- makes me realize I don't fundamentally grasp some concepts about mining.
First, I understand that there is a shared
criteria for the newest block (based on difficulty target?) that all miners have to go by in order to complete the SHA256 algorithm puzzle and win the latest block. However, I understand that all miners are separately targeting different specific numbers? And the number is informed by the specific transactions they choose to incorporate in their block, which informs the block hash(?) that they're trying to solve via bruteforcing the SHA256 algorithm? And finally when they do win the block they beam it to the network and other honest nodes accept it. (feel free to clarify if I'm not understanding that right)
That said, I'm trying to understand if dishonest nodes can purposefully ignore other nodes with longer chains, and meanwhile work on an alternative timeline. Based on an article (see the "Why You Can't Cheat Bitcoin" diagram) , that to create an alternative history w/ a double spend in an altered block, the attacker would be required to recalculate all subsequent blocks... but they would have to complete them before
everybody else in the network completed computing the newest block on the honest active longest chain, or else restart again (making it even harder for them). As I understand, this means they have to compute all their subsequent blocks in roughly 10 minutes, or otherwise restart...
If this is true, why? Why can't the attacker ignore the latest chain? Why does an attacker restart every time another miner broadcasts that they found a new block / created the longest active chain?
In other words, what prevents an attacker from altering a block 6+ blocks back and re-mining subsequent blocks at a rate slightly
faster than the rest of the network -- meanwhile ignoring the network's newest blocks. In this theory, they should eventually overtake the network given enough time (and assuming they have the hash power to move this fast?)? Meaning, even as the network is averaging new blocks every 10 minutes, the attacker would still eventually catch up based on its faster mining rate and overcome the network, making it officially the longest chain and legitimizing its double spending history... Is something preventing them from doing this?
On a related note regarding the factor of a miner altering the difficulty of mining in this scenario -- why is it that: "blocks released by malicious miners that do not meet the required difficulty target will simply be rejected by the other participants in the network." ? In other words, what specifically prevents an attacker (using only a small amount of total hash power) from changing past blocks and then mining subsequent blocks at an easier difficulty level than the rest of the network in order to quickly become the longest chain?
Thanks in advance!!!
edit: clarification / formatting
I understand - at a meta-level - what Merkle trees are and how they are used by SPV clients.
I don't understand a specific example from "Mastering Bitcoin".
In the image, there is a path of blue blocks for block "HK" (green). The blue blocks are an "authentication path" (according to the book). The path of blocks with dashes around them apparently prove to SPV clients that "HK" is in the block.
The dashed blocks have a direct path from "HK" to the root hash. The blue blocks do not.
I don't understand the point of the blue blocks. What do they do? How do they relate to the blocks with dashes?
In this thread by Roger Ver
I posted this comment
. User CRAB_WHORE_SLAYER challenged me to provide a diagram in response. The problem is reality is messy and full of nuance, not lending itself to simple, clear and pretty diagrams that are also completely accurate. I'm also no graphic artist, so this is the resulting mash-up that I cobbled together.
I'd welcome anyone skilled in graphics to clean this up.
NOTE: For greater accuracy and completeness, I should mention there's actually a fourth block chain coming out of the August 2017 forks that is not displayed here: the original non-SegWit 1 MB BTC. That block chain is also now abandoned.
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