feat: add batch commit to trait (#366)

* feat: add batch_commit and default implementation to CommitmentEngineTrait

* feat: add batch_vartime_multiscalar_mul and default implementation to DlogGroup trait

* feat: implement batch_commit in hyperkzg

* fix: batch_commit should support using a slice of blinding factors

Author: jacobtrombetta

src/provider/hyperkzg.rs

@@ -278,6 +278,27 @@ where
     }
   }
 
+  fn batch_commit(
+    ck: &Self::CommitmentKey,
+    v: &[Vec<<E as Engine>::Scalar>],
+    r: &[<E as Engine>::Scalar],
+  ) -> Vec<Self::Commitment> {
+    assert!(v.len() == r.len());
+
+    let max = v.iter().map(|v| v.len()).max().unwrap_or(0);
+    assert!(ck.ck.len() >= max);
+
+    let h = <E::GE as DlogGroup>::group(&ck.h);
+
+    E::GE::batch_vartime_multiscalar_mul(v, &ck.ck[..max])
+      .iter()
+      .zip(r.iter())
+      .map(|(commit, r_i)| Commitment {
+        comm: *commit + (h * r_i),
+      })
+      .collect()
+  }
+
   fn derandomize(
     dk: &Self::DerandKey,
     commit: &Self::Commitment,
@@ -550,9 +571,10 @@ where
 
     // We do not need to commit to the first polynomial as it is already committed.
     // Compute commitments in parallel
-    let com: Vec<G1Affine<E>> = (1..polys.len())
-      .into_par_iter()
-      .map(|i| E::CE::commit(ck, &polys[i], &E::Scalar::ZERO).comm.affine())
+    let r = vec![E::Scalar::ZERO; ell - 1];
+    let com: Vec<G1Affine<E>> = E::CE::batch_commit(ck, &polys[1..], r.as_slice())
+      .iter()
+      .map(|i| i.comm.affine())
       .collect();
 
     // Phase 2

src/provider/traits.rs

@@ -3,6 +3,7 @@ use core::{
   fmt::Debug,
   ops::{Add, AddAssign, Sub, SubAssign},
 };
+use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
 use serde::{Deserialize, Serialize};
 
 /// A helper trait for types with a group operation.
@@ -48,6 +49,17 @@ pub trait DlogGroup:
   /// A method to compute a multiexponentation
   fn vartime_multiscalar_mul(scalars: &[Self::Scalar], bases: &[Self::AffineGroupElement]) -> Self;
 
+  /// A method to compute a batch of multiexponentations
+  fn batch_vartime_multiscalar_mul(
+    scalars: &[Vec<Self::Scalar>],
+    bases: &[Self::AffineGroupElement],
+  ) -> Vec<Self> {
+    scalars
+      .par_iter()
+      .map(|scalar| Self::vartime_multiscalar_mul(scalar, &bases[..scalar.len()]))
+      .collect::<Vec<_>>()
+  }
+
   /// Produce a vector of group elements using a static label
   fn from_label(label: &'static [u8], n: usize) -> Vec<Self::AffineGroupElement>;
 

src/traits/commitment.rs

@@ -5,6 +5,7 @@ use core::{
   fmt::Debug,
   ops::{Add, Mul, MulAssign},
 };
+use rayon::iter::{IndexedParallelIterator, IntoParallelRefIterator, ParallelIterator};
 use serde::{Deserialize, Serialize};
 
 /// A helper trait for types implementing scalar multiplication.
@@ -62,6 +63,19 @@ pub trait CommitmentEngineTrait<E: Engine>: Clone + Send + Sync {
   /// Commits to the provided vector using the provided generators and random blind
   fn commit(ck: &Self::CommitmentKey, v: &[E::Scalar], r: &E::Scalar) -> Self::Commitment;
 
+  /// Batch commits to the provided vectors using the provided generators and random blind
+  fn batch_commit(
+    ck: &Self::CommitmentKey,
+    v: &[Vec<E::Scalar>],
+    r: &[E::Scalar],
+  ) -> Vec<Self::Commitment> {
+    assert!(v.len() == r.len());
+    v.par_iter()
+      .zip(r.par_iter())
+      .map(|(v_i, r_i)| Self::commit(ck, v_i, r_i))
+      .collect()
+  }
+
   /// Remove given blind from commitment
   fn derandomize(
     dk: &Self::DerandKey,