Add ec:msm:{Fixed,Variable}Base:msm_checked_len

Add the new methods for checked fixed and variable base multi-scalar
multiplication. They will return `None` if the arguments don't have the
same length.

This way, the users may confidently call this function without having to
check the length themselves.

Also, `ec:msm:{Fixed,Variable}BaseMSM` was renamed to
`ec:msm::{Fixed,Variable}Base` to avoid redundancy. The import path
contained `msm` several times, and now it contains only one incidence
for the type resolution and one for the function call.

Closes #319

CHANGELOG.md

@@ -10,12 +10,14 @@
 - [\#338](https://github.com/arkworks-rs/algebra/pull/338) (ark-ec) Add missing `UniformRand` trait bound to `GroupAffine`.
 - [\#338](https://github.com/arkworks-rs/algebra/pull/338) (workspace) Change to Rust 2021 edition.
 - [\#345](https://github.com/arkworks-rs/algebra/pull/345) (arc-ec, ark-serialize) Change the serialization format for Twisted Edwards Curves. We now encode the Y coordinate and take the sign bit of the X co-ordinate, the default flag is also now the Positive X value. The old methods for backwards compatibility are located [here](https://github.com/arkworks-rs/algebra/pull/345/files#diff-3621a48bb33f469144044d8d5fc663f767e103132a764812cda6be6c25877494R860)
+- [\#348](https://github.com/arkworks-rs/algebra/pull/348) (arc-ec) Rename `msm:{Fixed,Variable}BaseMSM:multi_scalar_mul` to `msm:{Fixed,Variable}:msm` to avoid redundancy.
 
 ### Features
 
 - [\#321](https://github.com/arkworks-rs/algebra/pull/321) (ark-ff) Change bigint conversions to impl `From` instead of `Into`.
 - [\#301](https://github.com/arkworks-rs/algebra/pull/301) (ark-ec) Add `GLVParameters` trait definition.
 - [\#312](https://github.com/arkworks-rs/algebra/pull/312) (ark-ec) Add `is_in_correct_subgroup_assuming_on_curve` for all `SWModelParameters`.
+- [\#348](https://github.com/arkworks-rs/algebra/pull/348) (ark-ec) Add `msm:{Fixed,Variable}Base:msm_checked_len`.
 
 ### Improvements
 

ec/src/msm/fixed_base.rs

@@ -5,9 +5,9 @@ use ark_std::{cfg_iter, cfg_iter_mut, vec::Vec};
 #[cfg(feature = "parallel")]
 use rayon::prelude::*;
 
-pub struct FixedBaseMSM;
+pub struct FixedBase;
 
-impl FixedBaseMSM {
+impl FixedBase {
     pub fn get_mul_window_size(num_scalars: usize) -> usize {
         if num_scalars < 32 {
             3
@@ -79,7 +79,9 @@ impl FixedBaseMSM {
         res
     }
 
-    pub fn multi_scalar_mul<T: ProjectiveCurve>(
+    // TODO use const-generics for the scalar size and window
+    // TODO use iterators of iterators of T::Affine instead of taking owned Vec
+    pub fn msm<T: ProjectiveCurve>(
         scalar_size: usize,
         window: usize,
         table: &[Vec<T::Affine>],

ec/src/msm/variable_base.rs

@@ -6,10 +6,30 @@ use crate::{AffineCurve, ProjectiveCurve};
 #[cfg(feature = "parallel")]
 use rayon::prelude::*;
 
-pub struct VariableBaseMSM;
+pub struct VariableBase;
 
-impl VariableBaseMSM {
-    pub fn multi_scalar_mul<G: AffineCurve>(
+impl VariableBase {
+    /// Optimized implementation of multi-scalar multiplication.
+    ///
+    /// Will return `None` if `bases` and `scalar` have different lengths.
+    ///
+    /// Reference: [`VariableBase::msm`]
+    pub fn msm_checked_len<G: AffineCurve>(
+        bases: &[G],
+        scalars: &[<G::ScalarField as PrimeField>::BigInt],
+    ) -> Option<G::Projective> {
+        (bases.len() == scalars.len()).then(|| Self::msm(bases, scalars))
+    }
+
+    /// Optimized implementation of multi-scalar multiplication.
+    ///
+    /// Will multiply the tuples of the diagonal product of `bases × scalars`
+    /// and sum the resulting set. Will iterate only for the elements of the
+    /// smallest of the two sets, ignoring the remaining elements of the biggest
+    /// set.
+    ///
+    /// ∑i (Bi · Si)
+    pub fn msm<G: AffineCurve>(
         bases: &[G],
         scalars: &[<G::ScalarField as PrimeField>::BigInt],
     ) -> G::Projective {

test-curves/src/bls12_381/tests.rs

@@ -3,7 +3,7 @@ use ark_ec::{models::SWModelParameters, AffineCurve, PairingEngine, ProjectiveCu
 use ark_ff::{Field, One, UniformRand, Zero};
 
 use crate::bls12_381::{g1, Fq, Fq2, Fq6, FqParameters, Fr, G1Affine, G1Projective};
-use ark_algebra_test_templates::{curves::*, fields::*, groups::*};
+use ark_algebra_test_templates::{curves::*, fields::*, groups::*, msm::*};
 use ark_std::rand::Rng;
 
 pub(crate) const ITERATIONS: usize = 5;
@@ -55,6 +55,11 @@ fn test_fq6() {
     frobenius_test::<Fq6, _>(Fq::characteristic(), 13);
 }
 
+#[test]
+fn test_g1_affine_curve() {
+    test_var_base_msm::<G1Affine>();
+}
+
 #[test]
 fn test_g1_projective_curve() {
     curve_tests::<G1Projective>();

test-curves/src/bn384_small_two_adicity/tests.rs

@@ -4,7 +4,7 @@ use ark_ff::{One, UniformRand, Zero};
 use ark_std::rand::Rng;
 
 use crate::bn384_small_two_adicity::{g1, Fq, FqParameters, Fr, G1Affine, G1Projective};
-use ark_algebra_test_templates::{curves::*, fields::*, groups::*};
+use ark_algebra_test_templates::{curves::*, fields::*, groups::*, msm::*};
 
 pub(crate) const ITERATIONS: usize = 5;
 
@@ -32,6 +32,11 @@ fn test_fq() {
     }
 }
 
+#[test]
+fn test_g1_affine_curve() {
+    test_var_base_msm::<G1Affine>();
+}
+
 #[test]
 fn test_g1_projective_curve() {
     curve_tests::<G1Projective>();

test-templates/src/msm.rs

@@ -1,4 +1,4 @@
-use ark_ec::{msm::VariableBaseMSM, AffineCurve, ProjectiveCurve};
+use ark_ec::{msm::VariableBase, AffineCurve, ProjectiveCurve};
 use ark_ff::{PrimeField, UniformRand, Zero};
 
 fn naive_var_base_msm<G: AffineCurve>(
@@ -27,7 +27,7 @@ pub fn test_var_base_msm<G: AffineCurve>() {
     let g = <G::Projective as ProjectiveCurve>::batch_normalization_into_affine(&g);
 
     let naive = naive_var_base_msm(g.as_slice(), v.as_slice());
-    let fast = VariableBaseMSM::multi_scalar_mul(g.as_slice(), v.as_slice());
+    let fast = VariableBase::msm(g.as_slice(), v.as_slice());
 
     assert_eq!(naive.into_affine(), fast.into_affine());
 }