Ndtyjky

I understand how a distributed ledger ensures integrity using a chained linked-list data model whereby each block is chained to all its previous ones.


I also understand how in a PoW/PoS/PoET/(insert any trustless consensus mechanism here) context, the verification process makes it difficult for a malicious (or a group of) individual to tamper with a block because they would not have the necessary resources to broadcast an instance of the ledger to all network members so that they update their version to a wrong one.


My question is, if let's say some one does manage to change a block, does an integrity checking process ever happen? Is it an actual part of the verification mechanism and if so, how far in history does it go?
Is it ever necessary to verify the integrity of i.e. block number 500 out of a total of 10000 and if so, how do I do that? Do I need to start from block 10000 and verify all blocks from there until block 500?

My question is, if let's say some one does manage to change a block, does an integrity checking process ever happen?

Change the block where? If you mean change my copy of the block in my computer, how would they do that? By breaking into my computer? And how would that affect anyone else?

Is it an actual part of the verification mechanism and if so, how far in history does it go? Is it ever necessary to verify the integrity of i.e. block number 500 out of a total of 10000 and if so, how do I do that? Do I need to start from block 10000 and verify all blocks from there until block 500?

The usual rule for most blockchains is that every full node checks every single block it ever receives to ensure that it follows every single system rule for validity.

While you could re-check every block you already checked to ensure that your particular copy hasn't been tampered with, this generally serves no purpose. Someone who could tamper with your local block storage could also tamper with your local checking code. So this kind of check doesn't usually provide any additional security.

To begin with, tampering with a block is not made almost-impossible because of resource shortage for broadcasting the wrong ledger to all nodes. Broadcasting is not necessarily resource intensive. It is a chain-reaction which you only have to trigger. The challenge with tampering a block-chained block arises from the difficulty of recomputing the valid hashes (meeting the block difficulty level) of all the successive blocks (the blocks that come after the one being tampered). Because altering a block alters its hash, which in turn changes the previous hash attribute of the next block, hence invalidating its previously correct hash, and so on till the latest block. If say the latest block index is 1000. And if you tamper the 990th block. This implies that you would have to re-mine (recalculate a valid hash by randomly changing the nonce value) blocks from 990 - 1000. That in itself is very difficult to do. But say somehow you manged to do that, but by the time you broadcast your updated blockchain, there would have been other blocks (of index say 1001, 1002) mined by other miners. So yours won't be the longest valid blockchain,and therefore would be rejected.

According to this article, when a new block is broadcasted to the network, the integrity of its transaction is validated and verified against the history of transactions. The trouble with integrity check arises only if a malicious longer block-chain is broadcasted to the network. In this case, the protocol forces the nodes to accept this longest chain. Note that, this longest chain maintains its own integrity and is verified of its own. But it is verified against its own version of truth. Mind you though, this can only be possible if the attacker has a hashing power that is at least 51% of the network's hashing power. And this kind of power is assumed practically impossible. https://medium.com/coinmonks/what-is-a-51-attack-or-double-spend-attack-aa108db63474