Reduce single-bit extractions from scalars

2 files changed, 8 insertions, 5 deletions

diff --git a/crypto/src/math/ec/multiplier/FixedPointCombMultiplier.cs b/crypto/src/math/ec/multiplier/FixedPointCombMultiplier.cs

index 505832442..37e5b5c29 100644
--- a/crypto/src/math/ec/multiplier/FixedPointCombMultiplier.cs
+++ b/crypto/src/math/ec/multiplier/FixedPointCombMultiplier.cs
@@ -37,15 +37,17 @@ namespace Org.BouncyCastle.Math.EC.Multiplier
             int top = fullComb - 1;
             for (int i = 0; i < d; ++i)
             {
-                int secretIndex = 0;
+                uint secretIndex = 0;
 
                 for (int j = top - i; j >= 0; j -= d)
                 {
+                    uint secretBit = K[j >> 5] >> (j & 0x1F);
+                    secretIndex ^= secretBit >> 1;
                     secretIndex <<= 1;
-                    secretIndex |= (int)Nat.GetBit(K, j);
+                    secretIndex ^= secretBit;
                 }
 
-                ECPoint add = lookupTable.Lookup(secretIndex);
+                ECPoint add = lookupTable.Lookup((int)secretIndex);
 
                 R = R.TwicePlus(add);
             }
diff --git a/crypto/src/math/ec/rfc8032/Ed448.cs b/crypto/src/math/ec/rfc8032/Ed448.cs

index 0e56b12a8..c1c0788a7 100644
--- a/crypto/src/math/ec/rfc8032/Ed448.cs
+++ b/crypto/src/math/ec/rfc8032/Ed448.cs
@@ -971,8 +971,9 @@ namespace Org.BouncyCastle.Math.EC.Rfc8032
                     uint w = 0;
                     for (int t = 0; t < PrecompTeeth; ++t)
                     {
-                        uint tBit = (n[tPos >> 5] >> (tPos & 0x1F)) & 1U;
-                        w |= tBit << t;
+                        uint tBit = n[tPos >> 5] >> (tPos & 0x1F);
+                        w &= ~(1U << t);
+                        w ^= (tBit << t);
                         tPos += PrecompSpacing;
                     }