Add pippenger under multiexp

crypto/multiexp/src/pippenger.rs

@@ -0,0 +1,79 @@
+use group::{ff::PrimeField, Group};
+
+fn prep<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> (Vec<Vec<u8>>, Vec<G>) {
+  let mut res = vec![];
+  let mut points = vec![];
+  for pair in pairs {
+    let p = res.len();
+    res.push(vec![]);
+    {
+      let mut repr = pair.0.to_repr();
+      let bytes = repr.as_mut();
+      if !little {
+        bytes.reverse();
+      }
+
+      res[p].resize(bytes.len(), 0);
+      for i in 0 .. bytes.len() {
+        res[p][i] = bytes[i];
+      }
+    }
+
+    points.push(pair.1);
+  }
+
+  (res, points)
+}
+
+pub(crate) fn pippenger<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> G {
+  let (bytes, points) = prep(pairs, little);
+
+  let mut res = G::identity();
+  for n in (0 .. bytes[0].len()).rev() {
+    for _ in 0 .. 8 {
+      res = res.double();
+    }
+
+    let mut buckets = [G::identity(); 256];
+    for p in 0 .. bytes.len() {
+      buckets[usize::from(bytes[p][n])] += points[p];
+    }
+
+    let mut intermediate_sum = G::identity();
+    for b in (1 .. buckets.len()).rev() {
+      intermediate_sum += buckets[b];
+      res += intermediate_sum;
+    }
+  }
+
+  res
+}
+
+pub(crate) fn pippenger_vartime<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> G {
+  let (bytes, points) = prep(pairs, little);
+
+  let mut res = G::identity();
+  for n in (0 .. bytes[0].len()).rev() {
+    if n != (bytes[0].len() - 1) {
+      for _ in 0 .. 8 {
+        res = res.double();
+      }
+    }
+
+    let mut buckets = [G::identity(); 256];
+    for p in 0 .. bytes.len() {
+      let nibble = usize::from(bytes[p][n]);
+      if nibble != 0 {
+        buckets[nibble] += points[p];
+      }
+    }
+
+    let mut intermediate_sum = G::identity();
+    for b in (1 .. buckets.len()).rev() {
+      intermediate_sum += buckets[b];
+      res += intermediate_sum;
+    }
+  }
+
+  res
+}