EVM: Reduce rustbn-wasm Bundle Size

[Holger Drewes]

Part of #3446

While not being directly suggested in the above meta issue from @roninjin10 this issue is inspired by the write-up and particularly by again looking at the "shocking" graph included 😂 pointing out the large and unproportional share of the rustbn-wasm dependency heaping up onto the EVM bundle size.

When looking a bit into the Rust code from the underlying bn Curve library from Parity it very much seems that only a small part of this code is used to realize the precompile and there should be a potential pretty large gain (substantially > 50% reduction still seems to be conservative) by removing these code parts from the build.

While this might sound a bit frightening on first read after having a closer look I would estimate that this might not be as complicated as expected. At the same time benefits would be pretty substantial and lasting on our side (basically completely a one-time thing). So we should give this minimally a small PoC by forking the bn package, delete some first-round-innocent code parts and see if we manage to point the depedency in Cargo.toml to the fork and see if we can build and integrate still successfully.

One preparatory work might be that this library still has the ethereum-bn128 as an intermediary library, which would (?) create the somewhat unnecessary need to fork as well, so we should consider to directly go to the bn lib and remove this intermediate chain part by integrating the Rust glue code over here.

Looking at the Rust glue code from ethereum-bn128 lib.rs more or less reveals the following methods and objects from the bn library being used:

bn ::
  AffineG1 ::
    new()
    from_jacobinian()
  Fq::
    fromSlice()
    zero()
  Group::
    [ Not sure yet ]
  G1::
    zero()

On the bn side all this resides in the main lib.rs file. When looking at this file there seems to be very distinct code parts (fields?), these seems to be not used at all, so to name:

FR
AffineG2
G2
Gt

So these seems to be likely candidates to be thrown out, including dependents in e.g. the fields subfolder.

Special note to @acolytec3: this issue can but does not need to be tackled within the next 4 weeks or the next 24 hours. 🤣

Another note: in contrast to other size optimizations regarding tree shaking this issue is not breaking and can be theoretically tackled "stand alone".

[acolytec3]

One alternative might be to explore the BN254 implementation in the mcl-wasm library. We're using this one for the EIP 2537 precompiles already so if I can map the public API methods that we currently consume on rustbn-wasm to equivalents in mcl-wasm, we might be able to remove this dep entirely.

[Holger Drewes]

One alternative might be to explore the BN254 implementation in the mcl-wasm library. We're using this one for the EIP 2537 precompiles already so if I can map the public API methods that we currently consume on rustbn-wasm to equivalents in mcl-wasm, we might be able to remove this dep entirely.

Interesting, didn't know this might be an option 👀

[Will Cory]

Wow removing the dep entirely would be a giant win!

[Holger Drewes]

@acolytec3 is this "just" - let's call it - "mathmatical intuition" that this might be a possible replacement with mcl-wasm, or did you read something about this that the one curve can be replaced by the other? Without having any clue ;-) I am just noticing that the 254 from the BN254 is not even the double of BN128 (so: 128 + 128 = 256) which might mean nothing (again: I have no clue) but that would be somewhat my intuition-of-the-clueless that this might work if the one is somewhat in the divisible realm of the other?

[acolytec3]

@acolytec3 is this "just" - let's call it - "mathmatical intuition" that this might be a possible replacement with mcl-wasm, or did you read something about this that the one curve can be replaced by the other? Without having any clue ;-) I am just noticing that the 254 from the BN254 is not even the double of BN128 (so: 128 + 128 = 256) which might mean nothing (again: I have no clue) but that would be somewhat my intuition-of-the-clueless that this might work if the one is somewhat in the divisible realm of the other?

I "think" it's the same thing, despite the weird naming variance. Here's the original EIP and then here's the definition of the curve in ark-works which is the modern, canonical set of ZK crypto libraries for Rust and it looks like they're defining the same curve. I'm going to have to just put some inputs in and see if it gives the same outputs along the way.

[Holger Drewes]

Interesting. I would have never considered THIS base line. 😅

[Paul Miller]

bn254 pairings are now available in noble.

The naive EVM implementation could be copy-pasted from the test file

[Holger Drewes]

Cool (actually: super-cool 🤩)! I will try an integration today or tomorrow and give you some feedback!

[Holger Drewes]

Closed by #3564