Faster const-time inversion

1 files changed, 21 insertions, 21 deletions

diff --git a/crypto/src/math/raw/Mod.cs b/crypto/src/math/raw/Mod.cs
index 49ac91351..d4d1f716d 100644
--- a/crypto/src/math/raw/Mod.cs
+++ b/crypto/src/math/raw/Mod.cs
@@ -67,13 +67,13 @@ namespace Org.BouncyCastle.Math.Raw
             Encode30(bits, m, 0, M, 0);
             Array.Copy(M, 0, F, 0, len30);
 
-            int eta = -1;
+            int delta = 0;
             int m0Inv32 = (int)Inverse32((uint)M[0]);
             int maxDivsteps = GetMaximumDivsteps(bits);
 
             for (int divSteps = 0; divSteps < maxDivsteps; divSteps += 30)
             {
-                eta = Divsteps30(eta, F[0], G[0], t);
+                delta = Divsteps30(delta, F[0], G[0], t);
                 UpdateDE30(len30, D, E, t, m0Inv32, M);
                 UpdateFG30(len30, F, G, t);
             }
@@ -298,38 +298,38 @@ namespace Org.BouncyCastle.Math.Raw
             }
         }
 
-        private static int Divsteps30(int eta, int f0, int g0, int[] t)
+        private static int Divsteps30(int delta, int f0, int g0, int[] t)
         {
-            int u = 1, v = 0, q = 0, r = 1;
+            int u = 1 << 30, v = 0, q = 0, r = 1 << 30;
             int f = f0, g = g0;
 
             for (int i = 0; i < 30; ++i)
             {
                 Debug.Assert((f & 1) == 1);
-                Debug.Assert((u * f0 + v * g0) == f << i);
-                Debug.Assert((q * f0 + r * g0) == g << i);
+                Debug.Assert(((u >> (30 - i)) * f0 + (v >> (30 - i)) * g0) == f << i);
+                Debug.Assert(((q >> (30 - i)) * f0 + (r >> (30 - i)) * g0) == g << i);
 
-                int c1 = eta >> 31;
+                int c1 = delta >> 31;
                 int c2 = -(g & 1);
 
-                int x = (f ^ c1) - c1;
-                int y = (u ^ c1) - c1;
-                int z = (v ^ c1) - c1;
+                int x = f ^ c1;
+                int y = u ^ c1;
+                int z = v ^ c1;
 
-                g += x & c2;
-                q += y & c2;
-                r += z & c2;
+                g -= x & c2;
+                q -= y & c2;
+                r -= z & c2;
 
-                c1 &= c2;
-                eta = (eta ^ c1) - (c1 + 1);
+                c2 &= ~c1;
+                delta = (delta ^ c2) - (c2 - 1);
 
-                f += g & c1;
-                u += q & c1;
-                v += r & c1;
+                f += g & c2;
+                u += q & c2;
+                v += r & c2;
 
                 g >>= 1;
-                u <<= 1;
-                v <<= 1;
+                q >>= 1;
+                r >>= 1;
             }
 
             t[0] = u;
@@ -337,7 +337,7 @@ namespace Org.BouncyCastle.Math.Raw
             t[2] = q;
             t[3] = r;
 
-            return eta;
+            return delta;
         }
 
         private static int Divsteps30Var(int eta, int f0, int g0, int[] t)