[bitcoin-dev] Secure Proof Of Stake implementation on Bitcoin

[Kenshiro []]

Hi ZmnSCPxj,

I'm based on the more evolved implementation of PoS that I know, which is PoS v3.0 and it's currently implemented in several coins:

http://earlz.net/view/2017/07/27/1904/the-missing-explanation-of-proof-of-stake-version

As far as I know the grinding attack is and old issue that is fixed in PoS v3.0.

>>>At least the proposed `assumeutxo` requires the operator to explicitly enable it, but I believe your "hardcoded checkpoints" cannot be disabled, much less disabled-by-default.

We don't trust the developers, the source code is public and anyone can check it. With the hardcoded checkpoints is exactly the same, they are in the source code repository and everyone can check them. The checkpoints are the easiest part to check. A user doesn't have any reason to remove the checkpoints, but as with anything in the source code, they could modify it to avoid the checkpoints (and become vulnerable to Long Range attacks doing it)

>>>Under the trust-minimization requirement of Bitcoin this is simply not acceptable.
As there is no way to trust-minimally heal from a network split (and every time a node is shut down, that is indistinguishable from a network split that isolates that particular node), this is not a trust-minimizing consensus algorithm.

The block explorer or other additional source of trust like a friend would only be required in the extreme situation that the network is under a 51% attack, and only by the nodes that are updating blocks in that moment. Updated nodes are fully protected, and under normal circumstances new nodes can just follow the longest chain as always. The other extreme situation that could cause a hard fork is that the network is splitted more than N blocks, which should require some social consensus to fix it. So N should be long enough, like a few hours of blocks or even 1 day.

>>> History rewrites are not the only attack possible.
The worst attack is a censorship attack, and a 99% staker can easily censor on the creation of new blocks.

I don't agree, history rewrite attacks are much worse than censorship because they can be used to steal funds from people. In PoS staking addresses are public, so maybe it should be possible to detect if some transaction in the mempool is repeatedly being ignored and what staking deposit is repeatedly ignoring transactions. After some time, a hard fork could burn the funds of the evil validator.

>>> Worse, under proof-of-stake it is often the case that stakers are awarded even more coin with which they can stake.

Sure, but in PoW the miners with more hash power earn more coins that can be used to purchase more miners. There is always the privilege of the rich guy, no matter if its PoW or PoS. The point is to design a protocol that don't allow the rich to destroy the network.


Let me put it in this way: NXT is a PoS coin that uses moving checkpoints with a market cap of 21 million dollars. If the current PoS protocols are so flawed, how can you explain that a coin with this market cap is not being attacked?

https://www.coingecko.com/en/coins/nxt

Another thing is that Ethereum itself is going to PoS next year, but with a different implementation that I'm proposing here.

Regards,

________________________________
From: ZmnSCPxj <ZmnSCPxj at protonmail.com>
Sent: Wednesday, July 17, 2019 1:00
To: Kenshiro \[\]; Bitcoin Protocol Discussion
Subject: Re: [bitcoin-dev] Secure Proof Of Stake implementation on Bitcoin

Good morning Kenshiro,


Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, July 16, 2019 8:33 PM, Kenshiro \[\] via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:

> Hi,
>
> After studying several Proof of Stake implementations I think it's not only an eco-friendly (and more ethical) alternative to Proof of Work, but correctly implemented could be 100% secure against all 51% history rewrite attacks. Over a "standard" PoS protocol like PoS v3.0, only 2 extra improvements are required:

Under the trust-minimization and uncensorability requirements that Bitcoin has, nothing is more efficient than proof-of-work.
The very idea of proof-of-stake labors under the assumption that unencumbered free-market payment for the consumption of energy is somehow not market-efficient despite the well-known phenomenon of the invisible hand, and believes that it is possible to get something for nothing.

Please re-examine your assumptions.

> - Hardcoded checkpoints:each Bitcoin Core release (each few months) should include a hardcoded checkpoint with the hash of the current block height in that moment. This simple measure protects the blockchain up to the last checkpoint, and prevents any Long-Range attack.

While this is a developer list and made up of developers who would be quite incentivized to agree to such a setup, note that this effectively trusts the developers.
At least the proposed `assumeutxo` requires the operator to explicitly enable it, but I believe your "hardcoded checkpoints" cannot be disabled, much less disabled-by-default.

> This extra rule could be connecting to a block explorer to download the hash of the current block height, or ask some trusted source like a friend and enter the hash manually. After being fully updated, the user can always check that he is in the correct chain checking with a block explorer.

Under the trust-minimization requirement of Bitcoin this is simply not acceptable.
As there is no way to trust-minimally heal from a network split (and every time a node is shut down, that is indistinguishable from a network split that isolates that particular node), this is not a trust-minimizing consensus algorithm.

>
> Someone could have 99% of the coins and still would be unable to use the coins to do any history rewrite attack. The attacker could only slow down the network not creating his blocks, or censor transactions in his blocks.

History rewrites are not the only attack possible.
The worst attack is a censorship attack, and a 99% staker can easily censor on the creation of new blocks.

Censorship attacks cannot be prevented except by ensuring that no single entity can claim 51% control of new block creation.
By ensuring this, we can ensure that at least some other entities are unlikely to keep a transaction out of the blockchain, and in particular that no entity can make a short-ranged history rewrite if a censored transaction *does* get into the blockchain from the efforts of another block producer.

This is trivial under proof-of-work, and is the cost we accept in order to achieve uncensorability, since it is non-trivial to acquire energy.
Under proof-of-stake it is difficult to impossible to determine if some single entity controls >51% of stakeable coins, and thus has no way to protect against censorship attack.
Worse, under proof-of-stake it is often the case that stakers are awarded even more coin with which they can stake.

Depending on the PoS implementation, stake-grinding may allow a 49% staker to achieve 51% and thereby the ability to censor transactions.


Regards,
ZmnSCPxj
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