Fix https://github.com/serai-dex/serai/issues/5

crypto/multiexp/src/lib.rs

@@ -0,0 +1,55 @@
+use ff::PrimeField;
+use group::{Group, GroupEncoding, ScalarMul};
+
+// An implementation of Straus, with a extremely minimal API that lets us add other algorithms in
+// the future. Takes in a list of scalars and points with a boolean for if the scalars are little
+// endian encoded or not
+pub fn multiexp_vartime<F: PrimeField, G: Group + GroupEncoding + ScalarMul<F>>(
+  scalars: &[F],
+  points: &[G],
+  little: bool
+) -> G {
+  let mut tables = vec![];
+  // dalek uses 8 in their impl, along with a carry scheme where values are [-8, 8)
+  // Moving to a similar system here did save a marginal amount, yet not one significant enough for
+  // its pain (as some fields do have scalars which can have their top bit set, a scenario dalek
+  // assumes is never true)
+  tables.resize(points.len(), [G::identity(); 16]);
+  for p in 0 .. points.len() {
+    let mut accum = G::identity();
+    for i in 1 .. 16 {
+      accum += points[p];
+      tables[p][i] = accum;
+    }
+  }
+
+  let mut nibbles = vec![];
+  nibbles.resize(scalars.len(), vec![]);
+  for s in 0 .. scalars.len() {
+    let mut repr = scalars[s].to_repr();
+    let bytes = repr.as_mut();
+    if !little {
+      bytes.reverse();
+    }
+
+    nibbles[s].resize(bytes.len() * 2, 0);
+    for i in 0 .. bytes.len() {
+      nibbles[s][i * 2] = bytes[i] & 0b1111;
+      nibbles[s][(i * 2) + 1] = (bytes[i] >> 4) & 0b1111;
+    }
+  }
+
+  let mut res = G::identity();
+  for b in (0 .. nibbles[0].len()).rev() {
+    for _ in 0 .. 4 {
+      res = res.double();
+    }
+
+    for s in 0 .. scalars.len() {
+      if nibbles[s][b] != 0 {
+        res += tables[s][nibbles[s][b] as usize];
+      }
+    }
+  }
+  res
+}