diff --git a/crypto/src/math/ec/Mod.cs b/crypto/src/math/ec/Mod.cs
new file mode 100644
index 000000000..57f75ed12
--- /dev/null
+++ b/crypto/src/math/ec/Mod.cs
@@ -0,0 +1,115 @@
+using System;
+
+using Org.BouncyCastle.Utilities;
+
+namespace Org.BouncyCastle.Math.EC
+{
+ internal abstract class Mod
+ {
+ public static void Invert(uint[] p, uint[] x, uint[] z)
+ {
+ int len = p.Length;
+ if (Nat.IsOne(len, x))
+ {
+ Array.Copy(x, 0, z, 0, len);
+ return;
+ }
+
+ uint[] u = Nat.Copy(len, x);
+ uint[] a = Nat.Create(len);
+ a[0] = 1;
+
+ if ((u[0] & 1) == 0)
+ {
+ InversionStep(p, u, a);
+ }
+ if (Nat.IsOne(len, u))
+ {
+ Array.Copy(a, 0, z, 0, len);
+ return;
+ }
+
+ uint[] v = Nat.Copy(len, p);
+ uint[] b = Nat.Create(len);
+
+ for (;;)
+ {
+ if (Nat.Gte(len, u, v))
+ {
+ Subtract(p, a, b, a);
+ Nat.Sub(len, u, v, u);
+ if ((u[0] & 1) == 0)
+ {
+ InversionStep(p, u, a);
+ }
+ if (Nat.IsOne(len, u))
+ {
+ Array.Copy(a, 0, z, 0, len);
+ return;
+ }
+ }
+ else
+ {
+ Subtract(p, b, a, b);
+ Nat.Sub(len, v, u, v);
+ if ((v[0] & 1) == 0)
+ {
+ InversionStep(p, v, b);
+ }
+ if (Nat.IsOne(len, v))
+ {
+ Array.Copy(b, 0, z, 0, len);
+ return;
+ }
+ }
+ }
+ }
+
+ public static void Subtract(uint[] p, uint[] x, uint[] y, uint[] z)
+ {
+ int len = p.Length;
+ int c = Nat.Sub(len, x, y, z);
+ if (c != 0)
+ {
+ Nat.Add(len, z, p, z);
+ }
+ }
+
+ private static void InversionStep(uint[] p, uint[] u, uint[] x)
+ {
+ int len = p.Length;
+ int count = 0;
+ while (u[0] == 0)
+ {
+ Nat.ShiftDownWord(u, len, 0);
+ count += 32;
+ }
+
+ int zeroes = GetTrailingZeroes(u[0]);
+ if (zeroes > 0)
+ {
+ Nat.ShiftDownBits(u, len, zeroes, 0);
+ count += zeroes;
+ }
+
+ for (int i = 0; i < count; ++i)
+ {
+ uint c = (x[0] & 1) == 0 ? 0 : Nat.Add(len, x, p, x);
+ Nat.ShiftDownBit(x, len, c);
+ }
+ }
+
+ private static int GetTrailingZeroes(uint x)
+ {
+ // assert x != 0;
+
+ int count = 0;
+ while ((x & 1) == 0)
+ {
+ x >>= 1;
+ ++count;
+ }
+ return count;
+ }
+ }
+}
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