In Bitcoin and Ethereum, the longest chain with the most proof-of-work (POW) invested in it is accepted as the true chain and used to determine balances and history on the network.

Because of this, if an adversary wants to change a block in a blockchain of height at h, he has to solve the POW for all blocks from h up to the current block. This is very difficult.

However, with IOTA, the entire Tangle is not broadcast to the network similar to the way that blocks are mined and broadcast with Bitcoin.

What takes the place of nodes honoring the longest chain?

How does IOTA ensure that the consensus is permanent with an immutable Tangle?

  • 2
    What do you mean by "the entire Tangle is not broadcast to the network similar to Bitcoin"? – Austin Powers Aug 9 at 13:58
  • When you say "unique Blockchain" and "unique Tangle", do you actually mean "unanimous Blockchain" and "unanimous Tangle"? – Zauz Aug 9 at 15:05
  • @Austin Powers I mean for example in Bitcoin to achieve a consensus a miner who creates a new block, he broadcasts his new blockchain (as the longest one) to the network and every node after receiving this longest chain will replace it. Thanks – sas Aug 9 at 15:17
  • @Zauz I mean every node in the network keep the same blockchain at every time interval. In other word, the blockchain that every node maintains is the same. (All local blockchain are equal.) Thanks – sas Aug 9 at 15:20
  • What do you mean by "removable" i.e. what counts as a "removed" transaction for you? – Zauz Aug 9 at 18:59

At the moment, we rely on Milestones issued by the Coordinator. In the future we might be able to rely on weight, which is the DAG equivalent of chain length.

I don't know if Iota will need a Coordinator forever. If the honest transaction rate reaches a certain point, I think we could get rid of it entirely, but with current PoW (Proof of Work) and difficulty that point is just too high. Maybe Network bound PoW will change something about this but we know hardly anything about it.

As for immutability, Iota uses the same mechanism as Blockchains:

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The block with hash value cof points at the previous block eef. If someone changes the content of eef, the hash value of the block changes as well. It will no longer be eef, in fact eef will no longer exist.

tangle tangle 404

The transactions with hash value PMF and VQT point at the transaction JOE. If someone tries to change the content of JOE, the hash value of the transaction changes as well. It will no longer be JOE, in fact JOE will no longer exist.

  • Thank you, Yes, but as I mentioned in my question, since IOTA PoW takes only milliseconds, so adversary can calculates the PoW for all vertices (tx) again, such that there will be a new Tangle that nobody is capable of distinguishing correct Tangle from malicious Tangle. – sas Aug 13 at 15:09
  • Once the honest transaction rate gets high enough, Iota could theoretically (!!!) rely on weight only. With the current PoW system, the number of transaction needed for security without COO would be higher than the amount of transactions the network can handle at all. Migrating to network bound PoW (whatever that is) might help. Also, increasing the MWM would work but then it would be more "expensive" to make transactions which we also don't want. – Zauz Aug 13 at 15:22
  • "If the honest transaction rate reaches a certain point, I think we could get rid of it entirely". You mean, if it ever reaches the 1.2 billion TPS required by using protection against 1% of the current BTC hashrate? iota.stackexchange.com/questions/48/… – Austin Powers Aug 13 at 17:53
  • As many people already pointed out, your number is arbitrary. But yes, the number would probably also be incredibly high. But not 1.2 billion! Maybe higher, maybe lower, probably higher if ternary hardware works as predicted, but not 1.2 billion. – Zauz Aug 13 at 18:05
  • @Zauz I admire that at least you are attempting to be objective in your answer. However, even 'theoretically' is theoretical. There is no proof that TPS secures the tangle. None. Only conjecture. The fact remains that IOTA has not provided a proof that the tangle works. Now, as to 1.2B TPS being too high, you and I both know that every year the '1% of the current BTC hashrate' is growing a lot. So it is reasonable to expect that to be much greater in the future. Which is impossible of course because there is no way nodes could ever process even a smallest fraction of that TPS. – Austin Powers Aug 15 at 16:18

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