Allow to overwrite default impl of msm in TwistedEdwards form (#567)

CHANGELOG.md

@@ -196,6 +196,7 @@
 - [\#357](https://github.com/arkworks-rs/algebra/pull/357) (`ark-poly`) Speedup division by vanishing polynomials for dense polynomials.
 - [\#445](https://github.com/arkworks-rs/algebra/pull/445) (`ark-ec`) Use 2-NAF for ate pairing in MNT4/6 curves.
 - [\#509](https://github.com/arkworks-rs/algebra/pull/509) (`ark-ff`, `ark-ff-macros`) Support prime fields with (64 * k)-bit modulus.
+- [\#567](https://github.com/arkworks-rs/algebra/pull/567) (`ark-ec`) Allow to overwrite the default implementation of the `msm` function for TwistedEdwards form provided by the `VariableBaseMSM` trait by a specialized version in `TECurveConfig`.
 
 ### Bugfixes
 

ec/src/models/twisted_edwards/group.rs

@@ -488,4 +488,8 @@ impl<P: TECurveConfig> ScalarMul for Projective<P> {
     }
 }
 
-impl<P: TECurveConfig> VariableBaseMSM for Projective<P> {}
+impl<P: TECurveConfig> VariableBaseMSM for Projective<P> {
+    fn msm(bases: &[Self::MulBase], bigints: &[Self::ScalarField]) -> Result<Self, usize> {
+        P::msm(bases, bigints)
+    }
+}

ec/src/models/twisted_edwards/mod.rs

@@ -4,7 +4,7 @@ use ark_serialize::{
 };
 use ark_std::io::{Read, Write};
 
-use crate::{AffineRepr, Group};
+use crate::{scalar_mul::variable_base::VariableBaseMSM, AffineRepr, Group};
 use num_traits::Zero;
 
 use ark_ff::fields::Field;
@@ -85,6 +85,16 @@ pub trait TECurveConfig: 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.
     /// If compressed, serializes y coordinate with a bit to encode whether x is positive.
     #[inline]