Add Absorb trait bound on PCCommitment #143
Description
Summary
Using the PCS in a wider context will require absorbing the commitment for Fiat Shamir.
The current challenge is that Absorb is not implemented for AffineRepr, and the current pairing-based schemes in this repo have commitments of the form:
pub struct Commitment<E: Pairing>(
/// The commitment is a group element.
pub E::G1Affine,
);
We unfortunately can't add a blanket impl like:
impl<T> Absorb for T
where
T: AffineRepr,
{
fn to_sponge_bytes(&self, dest: &mut Vec<u8>) {
todo!()
}
fn to_sponge_field_elements<F: PrimeField>(&self, dest: &mut Vec<F>) {
todo!()
}
}
Rust complains "conflicting implementations of trait absorb::Absorb for type u8".
Option 1
We could add Absorb on AffineRepr in the ec crate. However, aside from creating a cyclic dependency from ec to crypto-primitives, I think this is not the best design.
Option 2
One solution is to further restrict G: AffineRepr + Absorb. This propagates into a lot of trait bounds in the poly-commit repo, though. The upside is that it doesn't require any upstream changes, and the two current implementors of AffineRepr which are the structs short_weierstrass::Affine and twisted_edwards::Affine already have Absorb implemented.
Option 3
Larger breaking refactor that makes use of the fact that most pairing computations (and certainly our implementations in ec::models) only ever support SW. The proposal is then to rip out the associated type G1Affine from Pairing, and restrict the class of pairings that can be done to SW-curves, so that we can have:
pub struct Commitment<P: SWCurveConfig>(
/// The commitment is a group element.
pub Affine<P>,
);
This almost works, as we still have IPA Commitment struct here in poly-commit which uses the trait AffineRepr (i.e. not an associated type from Pairing):
pub struct Commitment<G: AffineRepr> {
/// A Pedersen commitment to the polynomial.
pub comm: G,
...
}
And so mixing in Option 2 would need to happen anyway, but limited to one PCS.
Option 4
Any other ideas?