Allow to overwrite the default implementation of msm (#528)

Co-authored-by: mmagician <[email protected]>
Co-authored-by: Pratyush Mishra <[email protected]>

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Pending
 
+- [\#528](https://github.com/arkworks-rs/algebra/pull/528) (`ark-ec`) Allow to overwrite the default implementation of the `msm` function provided by the `VariableBaseMSM` trait by a specialized version in `SWCurveConfig`.
 - [\#521](https://github.com/arkworks-rs/algebra/pull/521) (`ark-poly`) Change `DensePolynomial::evaluate_over_domain` to not truncate terms higher than the size of the domain.
 
 ### Breaking changes

ec/src/models/short_weierstrass/group.rs

@@ -635,7 +635,11 @@ impl<P: SWCurveConfig> ScalarMul for Projective<P> {
     }
 }
 
-impl<P: SWCurveConfig> VariableBaseMSM for Projective<P> {}
+impl<P: SWCurveConfig> VariableBaseMSM for Projective<P> {
+    fn msm(bases: &[Self::MulBase], bigints: &[Self::ScalarField]) -> Result<Self, usize> {
+        P::msm(bases, bigints)
+    }
+}
 
 impl<P: SWCurveConfig, T: Borrow<Affine<P>>> core::iter::Sum<T> for Projective<P> {
     fn sum<I: Iterator<Item = T>>(iter: I) -> Self {

ec/src/models/short_weierstrass/mod.rs

@@ -6,7 +6,7 @@ use ark_std::io::{Read, Write};
 
 use ark_ff::fields::Field;
 
-use crate::{AffineRepr, Group};
+use crate::{scalar_mul::variable_base::VariableBaseMSM, AffineRepr, Group};
 
 use num_traits::Zero;
 
@@ -105,6 +105,16 @@ pub trait SWCurveConfig: super::CurveConfig {
         res
     }
 
+    /// Default implementation for multi scalar multiplication
+    fn msm(
+        bases: &[Affine<Self>],
+        scalars: &[Self::ScalarField],
+    ) -> Result<Projective<Self>, usize> {
+        (bases.len() == scalars.len())
+            .then(|| VariableBaseMSM::msm_unchecked(bases, scalars))
+            .ok_or(usize::min(bases.len(), scalars.len()))
+    }
+
     /// If uncompressed, serializes both x and y coordinates as well as a bit for whether it is
     /// infinity. If compressed, serializes x coordinate with two bits to encode whether y is
     /// positive, negative, or infinity.

test-templates/src/msm.rs

@@ -3,6 +3,7 @@ use ark_ec::{
     ScalarMul,
 };
 use ark_ff::{PrimeField, UniformRand};
+use ark_std::vec::Vec;
 
 fn naive_var_base_msm<G: ScalarMul>(bases: &[G::MulBase], scalars: &[G::ScalarField]) -> G {
     let mut acc = G::zero();