Decompress public key for stark about starknet-rs

[xJonathanLEI]

starknet-crypto does contain the logic of decompressing public keys as it needs to do that for verifying signatures. See this line:

starknet-rs/starknet-crypto/src/ecdsa.rs

Line 94 in bad876c

let full_public_key = EcPoint::from_x(*public_key);

However as of today, this functionality is an implementation detail and not exposed as part of the public API. If you need to use it, the best way to do that today would be to fork this repository and expose the function yourself.

[xJonathanLEI]

Optimistically closing this. Feel free to re-open if you have further questions on the topic.

[xJonathanLEI]

Hi @rhobro I noticed you just forked our library, and just want to remind you that we just discovered a critical bug which causes the decimal representation of FieldElement to be incorrect. This bug is currently tracked as #74.

So if you print out the coordinates you get, you'll be getting incorrect numbers. You might want to incorporate the fix once it's implemented (very soon).

[rhobro]

@xJonathanLEI thank you very much for the update. Please could you ping me on this issue when it's sorted? I'll then update the commits on the fork.

Thank you so much for letting me know.

[xJonathanLEI]

Sure thing :)

[xJonathanLEI]

The commit fixing the issue is on master now @rhobro

[rhobro]

That was rapid. Thanks man.

[rhobro]

I forked it and exposed just the logic to get the y coordinate which I found in EcPoint::from_x. It is amazing that it was the same logic I found on the starkware website.

Out of interest @xJonathanLEI, would it be possible to be able to do arithmetic with references to FieldElements rather than having to clone them to pass into a function?

[xJonathanLEI]

Yeah I'd expect the same logic to be used everywhere. I guess that means we have a correct implementation :)

As for using references for arithmetic, yes we will be implementing ops that take references alongside the existing ones. However, I'd say you shouldn't worry about this for performance reasons (I'm presuming you're trying to do it for performance reasons; please ignore me if that's not the case :)), as this will be taken care of by the Rust optimizer.

[rhobro]

I was trying to do it for performance reasons 😳. How can the rust optimiser solve it?

[xJonathanLEI]

Obviously you'd need to benchmark for your case yourself, but generally speaking, if ownership is taken without being used, the compiler might turn it into a reference instead, depending on a lot of factors including the size of the struct in question.

[rhobro]

Ok. Thanks for the info.