Change AVX2 backend to use width-8 tables

src/backend/avx2/edwards.rs

@@ -22,7 +22,7 @@ use subtle::ConditionallyAssignable;
 use subtle::Choice;
 
 use edwards;
-use scalar_mul::window::{LookupTable, NafLookupTable5};
+use scalar_mul::window::{LookupTable, NafLookupTable5, NafLookupTable8};
 
 use traits::Identity;
 
@@ -307,6 +307,19 @@ impl<'a> From<&'a edwards::EdwardsPoint> for NafLookupTable5<CachedPoint> {
     }
 }
 
+impl<'a> From<&'a edwards::EdwardsPoint> for NafLookupTable8<CachedPoint> {
+    fn from(point: &'a edwards::EdwardsPoint) -> Self {
+        let A = ExtendedPoint::from(*point);
+        let mut Ai = [CachedPoint::from(A); 64];
+        let A2 = A.double();
+        for i in 0..63 {
+            Ai[i + 1] = (&A2 + &Ai[i]).into();
+        }
+        // Now Ai = [A, 3A, 5A, 7A, 9A, 11A, 13A, 15A, ..., 127A]
+        NafLookupTable8(Ai)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

src/backend/avx2/scalar_mul/vartime_double_base.rs

@@ -19,7 +19,7 @@ use backend::avx2::constants::BASEPOINT_ODD_LOOKUP_TABLE;
 /// Compute \\(aA + bB\\) in variable time, where \\(B\\) is the Ed25519 basepoint.
 pub fn mul(a: &Scalar, A: &EdwardsPoint, b: &Scalar) -> EdwardsPoint {
     let a_naf = a.non_adjacent_form(5);
-    let b_naf = b.non_adjacent_form(5);
+    let b_naf = b.non_adjacent_form(8);
 
     // Find starting index
     let mut i: usize = 255;