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Comment by capableweb | Hacker News Reader

I don't quite understand how something you run yourself on your own hardware can be tamper-proof (digitally, not physically). If you're running the software you can modify it, so no matter how many processes there are in place for resisting mutability, you'll always be able to find some way to mutate it.

Compared to blockchain which is running on X number of nodes that you'd have to have access to in order to modify something, immudb doesn't actually seem to replace the use case when you need something actually tamper-proof.

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You can build a Merkle tree from any data that is append-only; Git does this, as do ZFS and Dat/Hypercore. That lets you make strong assertions about data integrity, even without blocking local writes.

Now add a mirror: git upstream, FS snapshot, immudb replica, etc…or even just an outside log of the merkle proofs themselves. Then, if your database ever fails a check against that proof, you know the data has been modified, not just appended to.

To use a familiar Git workflow example: you can do whatever local writes you want, but if you disallow force pushes no one can erase history on the upstream repo.

Put another way: if you have immutable backups you don’t need a blockchain to ensure data integrity. OTOH, if you can’t trust your own infrastructure even as far as a secure remote backup you have other problems that a blockchain won’t solve either.

Again though, the entire point of blockchain tech is that it's decentralized. In the git analogy, someone is "disallowing force pushes", but the person that is disallowing can modify the database.

You can use git in a secure decentralized way - you have people sign commits.

by the way, you can play with immudb Merkle tree visually on https://play.codenotary.com

Blockchains like Bitcoin are actually not tamper-proof. They can be attacked by 51% attacks where you can even rewrite history if you have enough hashpower. The protocol is explicitly designed to always follow the longest chain thus the only defense is to hash faster than the attackers. This might vary for other blockchains but the biggest and most mentioned is particularly unsafe in that regard.

You can't rewrite the history of blocks that have already been distributed. You may fool SPV nodes but any node with a copy of the blockchain (even if pruned) will reject your version.

This is false. Please read the white paper. It clearly states in section 4:

> The majority decision is represented by the longest chain, which has the greatest proof-of-work effort invested in it. If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains.

If the majority of the hashpower (not nodes!) is dishonest, you can rewrite history. It's the reason why the current difficulty is part of a block.

This has been done numerous times in the past for small but highly traded altcoins.

There are wo things you skip over:

1. You can't rewrite everything. Given enough hash power to create a longer chain, you can create a block that a) removes any transactions from a block in the original chain and b) contains new valid transactions (must be signed by you so you must be the owner of the Bitcoins used in the tx), allowing you to double spend your coins, but you can't change other people's transactions.

2. With each new block changing a past one becomes harder, while you make it sound as if you could arbitrarily rewrite history. Merchants usually wait several blocks before accepting your on-chain payment. Exchanges wait 6 blocks as that's seen as infeasible to change a block that's buried under 5 other blocks for non-nation state actors.

TL;DR: 1) Other people's transactions can at most be removed but not be changed and 2) data on the Bitcoin blockchain is tamper proof after x blocks.

This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on.

Sure, someone, somewhere will know the truth but who cares - it's all about information control, and you control 51% of the network. Now your opponents have to scramble with social countermeasures to try and discredit your chain. A politically savvy attacker will simply buy off the main social nodes favorably.

EDIT: The problem with "oh but people will know" argument is that it assumes credibility outside the network, the thing which cryptocurrencies like to claim is unnecessary. In reality, if the network is 51% attacked, your entire recovery strategy is assuming enough people believe you about what's real or should be real. This isn't even theoretical: this is literally what happened to Ethereum.

> This is ignoring that if you own 51% of the network...

Are you talking about some kind of node sybil attack? Because the only way I can make sense of what you've said is if a system had client implementations maintaining zero state - relying completely on unsecured network consensus. But in that scenario hashing power wouldn't enter the equation. I don't know of any cryptocurrency that operates that way, and doubt that one ever has outside of a home lab.

Or are you talking about someone trying to hijack a cryptocurrency by creating brand confusion and convincing people to run modified code, like the failed bcash campaign against bitcoin?

A 51% attack on a blockchain has no rewrite ability past the point that where an attacker establishes a longer chain. That is why it is a "chain"... where do you think the hashing function inputs come from that are being fed into the adversarial mining hardware? An earlier block - the one where the attack started, you can't go back further than that without having to throw out and rehash your entire attack fork.

You are both mostly right, as I understand it.

If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". To re-write actual history takes a lot more work, and wouldn't be possible with 51% for 1 transaction.. If I was able to maintain 51% control for a long time, then I can do anything I want for as long as I have 51% control. Though I imagine after that very 1st transaction, the world of BTC would blow up and everyone would stop hashing BTC as there would be zero point as the current chain is now effectively useless.

This is the real issue that I see, 1 transaction of 51% power is enough to permanently wipe out all of BTC's worth. As far as I'm aware every crypto currency basically has this same problem.

What I'm saying is that, if enough big players like Coinbase et al. started saying "oh there's a BTC glitch, you need to pull a new state file..." then a huge number of people would do it. Not all, but you don't need all - just enough.

Again, if they don't control 51%, their new statefile would be pointless, unless they did a hard-fork, but if I have the resources for a 51% attack on the current fork, chances are I have the same capability on the new fork.

The only way what you propose would work would be if they could cobble together enough resources to break my 51% control.

So BTC would go 100% bust if someone managed a 51% attack. The question is, can someone with 1 transaction of 51% get enough converted to USD/etc before enough people noticed. Otherwise the financial incentive isn't there to try. I'd guess no matter what, it would be a huge fricking mess and if you did it in a country that didn't like you, it probably wouldn't end well for you years later when whatever govt you live in gets around to ruining your life, even if you managed to extract a few billion.

Because you know the exchanges like Coinbase as soon as they noticed would do their best to stop you(as it's in their best interest).

> You are both mostly right, as I understand it.

> If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent".

You clearly don't understand it, and it is kind of amazing that you think such a design would survive for as long as bitcoin has (going on 12 years) - by relying on altruistic miners not taking advantage of such a silly flaw. The Satoshi white paper is 9 pages long and written in very plain language... do like everyone use to do back in the day: read the paper and take a peek at the source, it goes a long way in inoculating you from misinformation that you subsequently repeat, accidentally (?) misinforming others.

BTC has changed a bit since the original white paper, and I have read the white paper(though it's been a while). Back it up with sources to convince me I'm wrong, don't just say I am with nothing but YOU ARE WRONG, that doesn't accomplish anything.

To help, here is my understanding.

In a < 51%(i.e. normal transaction), if I win the BTC mining lottery and get to write the next transaction, I can make it say whatever I want, but then 51% of the mining network has to agree that what I said was sane(as defined by the current mining software).

If I control 51%, then I can make the transaction say whatever I want AND I can make everyone else accept it as sane, because a majority of the network agrees with me.

This is how BTC changes over time, > 51% of the network agree that they will accept X as the new reality, and it then becomes the new reality.

The old question about how one eats an elephant comes to mind... you've expressed such a fundamental misunderstanding that the only sensible correction is: YOU ARE WRONG, START OVER. For example, this:

> ..make the current chain say anything I want, like give me all the BTC..

You know that transactions are cryptographically secured by public and private keys, right? That would be like saying "I can h4x0r all the hotmails and rewrite every PGP armored message to say whatever I want!" Do you think that miners, upon building the next block, have the opportunity to ignore all the rules with regard to PKI?

> ..and it would become valid..

There is a very straightforward block validation sequence, your attack would impotently collapse against it for any number of reasons: https://en.bitcoin.it/wiki/Protocol_rules

> I have read the white paper.

No you didn't. Sorry to have to put it so bluntly, but you couldn't have read it and still be so comically wrong - like I said, it is written in very plain language.

I don't think you understand software like you think you do. Those protocol rules do exist, and yes cryptography is involved, obviously, but those rules exist because the miners all agree on it, see: BIP2: https://github.com/bitcoin/bips/blob/master/bip-0002.mediawi...

I.e. These rules CHANGE, and since they change(and have changed in the past) if you can convince a majority, you can change the rules to be whatever you want.

Also see: https://en.bitcoin.it/wiki/Economic_majority

Which is exactly what I said. If I have 51% of mining I can make BTC do whatever I want, but that doesn't mean the majority(or any) of the exchanges will accept it, which is basically what the above is saying.

Also see: https://en.bitcoin.it/wiki/Bitcoin_is_not_ruled_by_miners

Again, what they are saying is what I've said, they just put flowery language around it saying, see, miners can't do EVEYRTHING, which isn't technically true, but practically true. Miners can technically do whatever the hell they want, assuming they have the majority, but that doesn't mean exchanges like Coinbase will accept it and exchange the BTC for USD.

so what I said above is generally and technically true, see my other comment in this thread as well, where I said it would be a giant mess and likely ruin BTC forever if someone ever did execute a 51% attack. So there is little incentive(financial or otherwise) to do so.

The closest real-life example we have(that I'm aware of) is the BTC cash stuff, where it hard-forked and became it's own crypto currency because they couldn't get a majority to agree, but enough agreed to fork themselves.

> I don't think you understand software like you think you do.

Sorry, can't hear you above the noise of your furious backpedaling. If somebody rewrites the protocol rules in their software to allow anything approaching what you've described (lol, "yes cryptography is involved"), then why are they worried about btc hashing power? I mean, you specifically said:

> If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent".

I'll tell you what it would look like to the rest of the network if anybody enacted your diabolical plot: suddenly somebody starts submitting invalid blocks to the network at regular intervals and only those who are monitoring for weird traffic like that even notice, then the block solve rate slightly sags until the difficulty automatically adjusts. Congratulations, you've turned your massive hashing advantage into an incredibly expensive joke.

> I.e. These rules CHANGE, and since they change(and have changed in the past) if you can convince a majority, you can change the rules to be whatever you want.

False, everyone running a node is enforcing whatever rules their node is written to enforce.

You could have 99% of the hashpower, if you generated a block saying that Coinbase is handing over all their Bitcoin to you (with an invalid signature because you don't have their private key), their node and the nodes of most/all exchanges and businesses would just go "lol, wtf is this shit, not a valid block because it has a transaction with an invalid signature, ignored".

A majority of hashpower is not the same as a majority of economic participants.

> If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent".

No, this is a big misunderstanding of how it works. Having more than 50% doesn't give you superpowers, it just means that you're able to create VALID blocks faster than the rest of the network combined (creating invalid blocks is useless), which allows you to censor transactions (by not including them in your blocks, which will be the chain with more work because you have 50%+) and you can TRY to do double-spends.

A double-spend gets exponentially harder and more expensive to pull off the longer your target waits before considering the original transaction final, because you'll have to rebuild all the blocks generated since that transaction was included in the chain, so that your parallel chain that doesn't include that transaction becomes the chain with the most work that everyone follows.

> This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on.

Not exactly, for every block you rewind there is extra work you have to do, to the point where it may take you close to 2 years to rewind 1 year's worth of blocks (because while you're doing that the rest of the network is still creating new blocks).

It gets exponentially harder/more expensive the further back you want to go.

You can still have the extra verifier nodes, but those don't have to be on the critical read/write path.

Presumably you can create a config where have your "main" beefy server where all the activity is -- which is backed up, redundant, etc... And a bunch of "client" servers, which just pull and verify the data all the time. And the client servers notify if there are any errors using some out-of-band channel, probably using the same system you use for general server health monitoring.

So you are getting same security guarantees as "private blockchain", but with drastically higher performance, and only needing one beefy server. And the downside is that you won't auto-stop all operations on tampering, you'll only get an alert for it.

> immudb doesn't actually seem to replace the use case when you need something actually tamper-proof.

I think that's an unrealistic requirement. There's tamper-evident and tamper-resistant but AFAIK nothing is tamper proof. Best you can do is an HSM with a tamper resistant HMAC with keys and a running checksum in unrecoverable ROM coupled to the packaging.

> nothing is tamper proof

I beg the differ.

If I place a signed message in the Bitcoin chain, can you then modify that message?

If you can prove that you can somehow modify the message, I'll give you $1,000,000 USD tomorrow.

Not after you post it, but by infecting your device before you make that message, and tampering when you place it in the Bitcoin chain

So you agree, once it's on the blockchain, it's tamper-proof?

That's "tamper proof" in the colloquial sense. As a term of art, it means something very specific. For example, see FIPS 140-2/3 [1]

It makes no sense to say that the blockchain is tamper proof because the blockchain is just information. Tamper "proofness"/resistance is first a property of the devices storing the information - once you get into custody chains, provenance documents, etc. that's when a system becomes tamper resistant. At best the blockchain as a system is "tamper evident" in the colloquial sense because the network of all the other nodes decides which bits of information form the "real" blockchain. However, without verifying the (physical) identity and data integrity of the devices that run (at least?) 50%+1 of the nodes, you have no idea whether the system has been tampered with.

[1] https://en.wikipedia.org/wiki/FIPS_140

If you want to raise the bet to a few billion dollars I'll happily take you on.

But that's just a question of scale - if you have a rando-blockchain you use for immutability internally then how trivial would it be for me to spin up five servers to outhash you and rewrite history?

the entire state of the database gets captured by a hash value. By having light-weight clients (or auditors) keeping track of it is how tampering is detected in despite of where the database server is running

I see. It's a blockchain without calling it a blockchain, so people who hate blockchain can use it without having to realize they use a blockchain.

And with git being the most superior blockchain of them all.

Except it has no way to achieve consensus automatically. That's left as an exercise to the reader.

Blockchain is just a special case of Merkle trees, there isn't anything original about them other than that Bitcoin served as a marketing engine for the term blockchain because some people made a ton of money with it.

https://en.wikipedia.org/wiki/Merkle_tree

Rekor is just that. It's a merkle tree implementation (with extras such as timestamping)

https://github.com/sigstore/rekor

No block chains are different than Merkle trees entirely. Block chains include previous hashes in each block, whereas Merkle trees, as the name implies are trees of hashes. In Merkle trees blocks do not include the previous block's hash.

In git each "block" includes the previous "block"(s)' hash. Is it a blockchain or a hash tree?

I would say that in practice what differentiates a blockchain from other applications of hash trees is a mechanism for consensus, not whether the blocks being formed into a trees conceptually represent time or not.

Git is a block chain. Block chains require a previous block's hash to denote sequence. The hash for any block of data can be appended to Merkle tree, even duplicate blocks.

In a block chain it is easy to find history because the link to the history is included. Merkle trees require n-1 additional hashes.

Disagree. There are not formal definitions for these terms, but I don't think your definition here is what most people would think of as a blockchain.

Looking at the wikipedia article I can see where one might be confused.

    A blockchain is a growing list of records,
    called blocks, that are linked together using 
    cryptography. Each block contains a cryptographic
    hash of the previous block, a timestamp, and
    transaction data (generally represented as a
    Merkle tree).

The Merkle tree referenced here is with respect to the organization of the transaction data contained within a block, not the blockchain itself.

Merkle trees are used in various ways within cryptography in general and cryptocurrencies specifically, but blockchains and Merkle trees are distinct data structures with different uses. The colloquial use of "blockchain" has perhaps made the word somewhat ambiguous in some contexts but not in the context of cryptographic data structures, and Merkle trees are in fact formally defined.

You can visualize how immudb Merkle tree grows as you insert data on https://play.codenotary.com

> there isn't anything original about them

Cool, so let's say we're using this to track financial transactions.

Server A has an Immudb at some state x and server B has an Immudb at some state y. Which one is correct, how do I decide?

It's only the actually-useful bits of a "blockchain" without the planet-cooking proof-of-waste consensus algorithm brute-forcing sha256 over and over again.

And also without the useful "automatically achieve consensus between untrusted parties" bit.

This is insufficient. The strongest guarantee you can get without consensus is that the state of the DB you see on the client is/was a correct state at some point, it doesn't provide freshness/rollback attack prevention, aka that the state you see is in fact the latest one.

Keeping track of the "HEAD" hash on the clients is what consensus protocols achieve. You can also achieve it with trusted counters like the one SGX provides (depends on Intel ME so not exactly recommended, also most probably switched off in cloud environments). Alternative is an implementation of something like https://dl.acm.org/doi/10.5555/3241189.3241289.

You can of course say that it's the clients' responsibility to do this, but in practice they won't and they'll implicitly trust the server state.

Having said this, the project does look promising, we may end up using it in a confidential compute setting where clients can verify the server code running, and we'll add rollback protection on top

> aka that the state you see is in fact the latest one.

This is an impossible guarantee. Suppose the state that is sent to you from the server needs some time to get to you. meanwhile the state on the server could have changed. You don't even need a remote server to have this issue. Your thread (where you see the latest state) is put to sleep for a while (sheduler, os, ...) It wakes up. Is the state it observes still the latest? That's impossible to know. The only thing you can do is to refuse future updates if the state they were built upon is not the current state of the database.

That's what I meant. If you have many transactions building on a state hash X racing to be committed, only one of them will succeed.

With crypto protocols in general "guarantees" are always prefixed with "if the protocol completed successfully, then ...". For example authenticated DH + e2e encryption guarantees that you will send data to the intended participant only. But an attacker can still disrupt the network packets, so the true guarantee is "if the protocol completed successfully, then you have sent the data to the intended participant only".

Same thing here, you cannot of course guarantee the "latest state", if we want to go into the extreme, one could even argue that actually time doesn't work like that because of relativity/speed of light limitations:D. What you can guarantee is that if your commit protocol succeeded, then it updated the latest state at the time of consensus/monotonic counter update.

  > That's what I meant.

Good. Sorry to be picky, but wording is important here and you don't wanna know how often I failed to convince people of the impossibility of exactly that guarantee.

  >  time doesn't work like that because of relativity/speed of light.

You're right.

you can do something quite simple like posting a tweet or inserting something into a public chain, like Etherium. Then follow that back to the private immudb hash.

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