Comment by knazarov | Hacker News Reader

Comment by knazarov | Hacker News Reader

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I'm not involved in the project in any way, but I can probably give an explanation.

This is a risc-v virtual machine that supports rv32im instruction set (the bare minimum + multiplication). You can compile and run programs there as you would on a usual microcontroller.

The "ZK" thing means that you can pass program code + data to this virtual machine, and as a result of execution get some output and a short sequence of bytes that allow the "other side" to verify that the result of program execution is correct without having to re-execute the program. This verification is computationally cheap. In order to do the verification the "other side" only needs that sequence of bytes and a hash of original code+data.

Blockchains use this in context of achieving "byzantine consensus", especially in cases where multiple systems that lack mutual trust are involved. Think for example about if you want to relay information that's been computed on one blockchain to the other. If both blockchains can prove their state transitions with such virtual machine, then it is possible to make a sort of trusted "event queue" between them. There are of course caveats that rollbacks can happen, so it's not a silver bullet.

Not sure how this applies to day-to-day software, but the thing that comes to mind is that it could serve some cases where TPM (hardware modules) are traditionally utilized. The TPM assumes you don't have means to break the hardware, and so it can attest to certain computation by signing the result with a baked-in key.

> pass program code + data to this virtual machine

If you have to pass the data, then how can this possibly help solve, as others have said, scenarios that prove something about data without revealing the data?

You don't _have_ to, I think the parent poster is just giving an example of how this can be used.

The "Zero Knowledge" part is that you can tell me "for this particular program code, I know an input that gives an output of 'foobar'" and I can be convinced that you're telling me the truth without seeing what that input actually is.

In order to verify the proof, you only need a "commitment" to the fact that you passed specific input data. This can be in the form of calculating a cryptographic hash.

Imagine that you represent a program and data as a flat byte array (which is the typical case for risc-v). Your program contains a prologue at the very start (0x0) that calculates the hash of the rest of the memory and checks that the hash is equal to some value hardcoded right after the prologue and if it doesn't match - then it panics. Then if you can prove that the VM followed all steps exactly as the hardware architecture prescribes, it's just impossible for the result to be any other way. What you need now is to only send to other side this prologue together with the hash. You don't have to reveal the rest of the memory.

Of course I'm simplifying a bit, but I hope the idea is clear.

How secure is this "ZK" thing? Can I use it for DRM? Trusted hardware is a pain to use.

Let's put it this way. If you can break it and are willing to commit crime - you can earn a lot of money. I'm personally not equipped to judge the level of security, because I'm not a cryptography researcher. What I understand is that the overall ZK cryptography space has been around for a long time, and basic properties are well researched. I tried to read the PLONK paper for entertainment purposes and it's quite easy to understand.

If you want to prove a program, you need to convert it to what is called "arithmetic circuits". This is a clever way of saying "a system of polynomial equations". It is as if you're converting the code to logic gates but instead here you use arithmetic: addition, raising to a certain power, etc. It is a process called "arithmetization".

The proof calculation involves folding this system of equations in various ways and collecting a "witness". Not sure if I'm explaining this correctly, but it's probably in the right direction at least.

The problem with all ZK proofs is that the last bit of calculating the witness is very computationally expensive. What takes milliseconds to run on the CPU can take many hours to prove. And a lot of research is focused on clever mathematical tricks that allow to speed up the proofs while keeping the risks of circuit compromise low enough. As you may guess, the more bleeding edge the research the less it is peer-reviewed.

So I'd say that "it depends".

As for DRM - probably not. At least not in a way that companies installing the DRM would want it to work. They likely want you to be unable to decrypt something at all outside of a particular chip, and watermark the video on top of that to be sure that you're not screen-capturing it.

UPD: there's a decent free into course into modern ZK cryptography: https://zkiap.com/

Proving performance is improving at a rapid pace though, like 10X/year. And converting to circuits is automatic now; there are compilers that can do it for arbitrary Rust code.

Zero Knowledge cryptography overheads are ridiculously too high to use for the thing DRM is usually used for: video.

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