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The Truth on the Blockchain

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Re: The Truth on the Blockchain

#21

Post by riptheworld »

p4bl0 wrote: post #11
aeternum wrote: post #7

Earlier quoted context omitted.

This isn't the best analogy since PoW was declared beforehand that more work = more trust. In the manufacturing example though, there are many examples of collectables that trade at a huge premium due to a very subtle manufacturing defect.

> more work = more trust This is not true. The trust you are talking about is the "solidity" of the blockchain, i.e., it's immutability feature, what (supposedly) makes it a trustable source. But it is not PoW that makes the blockchain immutable. A lot of people are saying that, but they're wrong, I'm sorry. What actually makes the blockchain immutable is that it is distributed and that it cannot be done to rewrite i…

> But it is not PoW that makes the blockchain immutable. A lot of people are saying that, but they're wrong, I'm sorry.

You are incorrect. The proof of work mechanism is central to ensuring the immutability of the blockchain.

As a concrete example, suppose there are N blocks in the blockchain, and I want to change a transaction in the (N - 2) block. Obviously, this will change the hash of the (N - 1) and N-th blocks, so the decentralized network will not accept this arbitrary change.

However, if I can produce four PoWs corresponding to the new (N - 2), (N - 1), N, and (N + 1)th blocks, then I can convince the network to accept. Of course, I have to produce these four PoWs before the honest users on the network mine the next block at index (N + 1). This is extremely difficult by design. In theory, you would need the majority of the network’s computing power to carry out this attack.

In other words, the security level of the proof of work is directly related to the difficulty in modifying the blockchain.

Re: The Truth on the Blockchain

#22

Post by errantmind »

p4bl0 wrote: post #9
abss wrote: post #6

Well.. this articole has some points but is not 100% fair or correct. The value of distributed ledger (aka blockchain) is given by the fact that you can find the latest version of a record in a decentralised way. This is usefull outside cryptocurencies (even for university diplomas you need a good mecanism for revocation). You can do revocation with a centralised system but a decentralised version could actualy impri…

There are other ways of having a distributed immutable ledger than a blockchain, if you don't have the constraints of a decentralized adversarial context. You don't have this context for diplomas. Also, neither revocation nor security and privacy does seem like good examples of what a blockchain would allow that a non-blockchain based approach wouldn't.

That is a big 'if'. The adversarial context is everything.

As for diploma's, I'm not sure what your point is. There is nothing decentralized about them.

Re: The Truth on the Blockchain

#23

Post by cuteboy19 »

galaxyyy wrote: post #15

Imo the point of the blockchain is that it can do many things (such as payments, storage, immutable records) all in one place which will lead to an overall increase in societal productivity. In the case of your diploma, it's true that there are other ways to have a distributed immutable ledger that contains a signature leading to the diploma. However, by having one agreed upon global ledger for any university to uplo…

Why not just use a docusigned diploma pdf?

And if you care about revocation (why?) then you can just store the pdf on git

Re: The Truth on the Blockchain

#24

Post by p4bl0 »

riptheworld wrote: post #21
p4bl0 wrote: post #11

Earlier quoted context omitted.

> more work = more trust This is not true. The trust you are talking about is the "solidity" of the blockchain, i.e., it's immutability feature, what (supposedly) makes it a trustable source. But it is not PoW that makes the blockchain immutable. A lot of people are saying that, but they're wrong, I'm sorry. What actually makes the blockchain immutable is that it is distributed and that it cannot be done to rewrite i…

> But it is not PoW that makes the blockchain immutable. A lot of people are saying that, but they're wrong, I'm sorry. You are incorrect. The proof of work mechanism is central to ensuring the immutability of the blockchain. As a concrete example, suppose there are N blocks in the blockchain, and I want to change a transaction in the (N - 2) block. Obviously, this will change the hash of the (N - 1) and N-th blocks,…

Okay so now suppose that some one finds a way to break the hash function used in a way that allows to quickly compute partial collisions.

This person is able to mine valid blocks very quickly because they can have a number of zeros at the beginning of their hashes thanks to the vulnerability they found.

So they do what you're saying and rewrite history. In a few minutes, they start with block (N - 10), and rebuild an alternative chain up to block N and then add two new blocks so their version of the chain in longer.

Yes, what is supposed to happen, in theory and according to you, is that their version of the chain will prevail.

What would happen in practice? People would notice. A fork will be decided. Such things have happened with blockchains already, either the community, or worse, the developers, decided against the rogue version to hard switch to the original version. And everyone except the attacker will agree…

So I repeat: the only way to actually modify the recorded history is to do so discretely. And that requires computing full hashes collision. Even for a single one it is much much expensive than mining hundreds if not thousands of blocks.

Another point: if PoW is what guarantees immutability, what about PoS blockchains?

The truth is: PoW and PoS are not immutability mechanisms, they're adversarial distributed consensus mechanisms: i.e., complex ways of selecting someone at random such that no one else can dispute the choice.

Re: The Truth on the Blockchain

#25

Post by p4bl0 »

errantmind wrote: post #22
p4bl0 wrote: post #9

Earlier quoted context omitted.

There are other ways of having a distributed immutable ledger than a blockchain, if you don't have the constraints of a decentralized adversarial context. You don't have this context for diplomas. Also, neither revocation nor security and privacy does seem like good examples of what a blockchain would allow that a non-blockchain based approach wouldn't.

That is a big 'if'. The adversarial context is everything. As for diploma's, I'm not sure what your point is. There is nothing decentralized about them.

The adversarial context is everything, but it doesn't exist in most case, because the blockchain is not self sufficient, it needs an external authority to make what's written on it relevant.

As for diplomas, I didn't choose this example at random. A lot of startups are trying to sell exactly that. I've been approached myself multiple times about that. And some higher education institutions already do it…

Re: The Truth on the Blockchain

#26

Post by imtringued »

aeternum wrote: post #2

The author gets it wrong with this quote: >The only truth that is guaranteed by the recording of a given information on a given blockchain, is that this particular information is written on this particular blockchain. The second part is that $X worth of real-world resources have been consumed in building this blockchain, so this one blockchain and the particular information inside is worth paying attention to. Whethe…

Consuming $X amount of resources makes you poorer in the real world. At the end of the day it is just an overengineered accounting system.

Re: The Truth on the Blockchain

#27

Post by riptheworld »

p4bl0 wrote: post #24
riptheworld wrote: post #21

Earlier quoted context omitted.

> But it is not PoW that makes the blockchain immutable. A lot of people are saying that, but they're wrong, I'm sorry. You are incorrect. The proof of work mechanism is central to ensuring the immutability of the blockchain. As a concrete example, suppose there are N blocks in the blockchain, and I want to change a transaction in the (N - 2) block. Obviously, this will change the hash of the (N - 1) and N-th blocks,…

Okay so now suppose that some one finds a way to break the hash function used in a way that allows to quickly compute partial collisions. This person is able to mine valid blocks very quickly because they can have a number of zeros at the beginning of their hashes thanks to the vulnerability they found. So they do what you're saying and rewrite history. In a few minutes, they start with block (N - 10), and rebuild an…

> What would happen in practice? People would notice. A fork will be decided.

There's nothing in the Bitcoin protocol that explicitly prevents a longer and valid chain from being accepted. Forks happen every day where miners are competing to construct the longest accepted chain on the network (it's part of the protocol).

However, the difficulty in maliciously changing previous blocks in the Bitcoin blockchain is explicitly parameterized by the security level of the PoW mechanism. There are undergrad CS courses where this analysis is probably a homework problem.

In PoS, immutability is baked into the protocol since the block selection algorithm doesn't explicitly cause forking as it does in PoW based blockchains.

Decentralization creates trust that newly added blocks contain valid transactions, not to ensure that previous blocks don’t change. This is why the finality times of Bitcoin are strictly worse than the finality times of Algorand, or other PoS blockchains. Not because one network is more decentralized than the other, but because Bitcoin’s immutability property is hamstringed by the PoW security level.

EDIT: Anticipating your response requesting citations - here are the lecture notes from the "Foundations of Blockchain Systems" course at the University of Washington, Seattle [1]. From section 3.2, I quote:

"One property of the blockchain is immutability. This means that the chain cannot be changed given the last block in the chain. This is because it is difficult to find a hash of x that is equal to hash of y in computationally feasible time (collision-resistance). We add the qualifier “given the last block” because if we disagree on which is the last block, we may have problems. With distributed mining, there is a possibility of simultaneous mining, however these ties should be resolved quickly with proper tuning of parameters. Therefore, although we may not agree on the last block all of the time, all blocks will lead back to the genesis, starting block, and the large majority of the chain will be in agreement."

[1] https://ece595uwseattle.github.io/Scribe_notes_Lecture_3.pdf

Re: The Truth on the Blockchain

#28

Post by boublepop »

abss wrote: post #6

Well.. this articole has some points but is not 100% fair or correct. The value of distributed ledger (aka blockchain) is given by the fact that you can find the latest version of a record in a decentralised way. This is usefull outside cryptocurencies (even for university diplomas you need a good mecanism for revocation). You can do revocation with a centralised system but a decentralised version could actualy impri…

Amazing, we might finally be able to solve the great university diploma verification problem.

It’s really held back society that we haven’t been able to verify diplomas issued by centralized trusted institutions in a decentralized trustless fashion. /s

Re: The Truth on the Blockchain

#29

Post by p4bl0 »

riptheworld wrote: post #27
p4bl0 wrote: post #24

Earlier quoted context omitted.

Okay so now suppose that some one finds a way to break the hash function used in a way that allows to quickly compute partial collisions. This person is able to mine valid blocks very quickly because they can have a number of zeros at the beginning of their hashes thanks to the vulnerability they found. So they do what you're saying and rewrite history. In a few minutes, they start with block (N - 10), and rebuild an…

> What would happen in practice? People would notice. A fork will be decided. There's nothing in the Bitcoin protocol that explicitly prevents a longer and valid chain from being accepted. Forks happen every day where miners are competing to construct the longest accepted chain on the network (it's part of the protocol). However, the difficulty in maliciously changing previous blocks in the Bitcoin blockchain is expl…

Your citations proves my point, not yours.

> This is because it is difficult to find a hash of x that is equal to hash of y in computationally feasible time (collision-resistance).

This is exactly what I was explaining.

Re: The Truth on the Blockchain

#30

Post by riptheworld »

p4bl0 wrote: post #29
riptheworld wrote: post #27

Earlier quoted context omitted.

> What would happen in practice? People would notice. A fork will be decided. There's nothing in the Bitcoin protocol that explicitly prevents a longer and valid chain from being accepted. Forks happen every day where miners are competing to construct the longest accepted chain on the network (it's part of the protocol). However, the difficulty in maliciously changing previous blocks in the Bitcoin blockchain is expl…

Your citations proves my point, not yours. > This is because it is difficult to find a hash of x that is equal to hash of y in computationally feasible time (collision-resistance). This is exactly what I was explaining.

This is literally proof of work. I don’t know what else I can do to explain that Bitcoin’s immutability property is directly parametrized by the security level in the PoW mechanism.

You are arguing that decentralization creates immutability. Decentralization has nothing to do with hashing.

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