Initial work on the Curve25519 field implementation

1 files changed, 159 insertions, 0 deletions

diff --git a/crypto/src/math/ec/custom/sec/Curve25519Field.cs b/crypto/src/math/ec/custom/sec/Curve25519Field.cs
new file mode 100644
index 000000000..be9878911
--- /dev/null
+++ b/crypto/src/math/ec/custom/sec/Curve25519Field.cs
@@ -0,0 +1,159 @@
+using System;
+using System.Diagnostics;
+
+namespace Org.BouncyCastle.Math.EC.Custom.Sec
+{
+    internal class Curve25519Field
+    {
+        // 2^255 - 2^4 - 2^1 - 1
+        internal static readonly uint[] P = new uint[]{ 0xFFFFFFED, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
+            0xFFFFFFFF, 0x7FFFFFFF };
+        private const int P7 = 0x7FFFFFFF;
+        private static readonly uint[] PExt = new uint[]{ 0x00000169, 0x00000000, 0x00000000, 0x00000000, 0x00000000,
+            0x00000000, 0x00000000, 0x00000000, 0xFFFFFFED, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
+            0xFFFFFFFF, 0x3FFFFFFF };
+        private const uint PInv = 0x13;
+
+        public static void Add(uint[] x, uint[] y, uint[] z)
+        {
+            Nat256.Add(x, y, z);
+            if (Nat256.Gte(z, P))
+            {
+                Nat256.AddWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+        }
+
+        public static void AddExt(uint[] xx, uint[] yy, uint[] zz)
+        {
+            Nat256.AddExt(xx, yy, zz);
+            if (Nat256.GteExt(zz, PExt))
+            {
+                Nat256.SubExt(zz, PExt, zz);
+            }
+        }
+
+        public static void AddOne(uint[] x, uint[] z)
+        {
+            Nat256.Copy(x, z);
+            Nat256.Inc(z, 0);
+            if (Nat256.Gte(z, P))
+            {
+                Nat256.AddWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+        }
+
+        public static uint[] FromBigInteger(BigInteger x)
+        {
+            uint[] z = Nat256.FromBigInteger(x);
+            if (Nat256.Gte(z, P))
+            {
+                Nat256.AddWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+            return z;
+        }
+
+        public static void Half(uint[] x, uint[] z)
+        {
+            if ((x[0] & 1) == 0)
+            {
+                Nat.ShiftDownBit(8, x, 0, z);
+            }
+            else
+            {
+                Nat256.Add(x, P, z);
+                Nat.ShiftDownBit(8, z, 0);
+            }
+        }
+
+        public static void Multiply(uint[] x, uint[] y, uint[] z)
+        {
+            uint[] tt = Nat256.CreateExt();
+            Nat256.Mul(x, y, tt);
+            Reduce(tt, z);
+        }
+
+        public static void Negate(uint[] x, uint[] z)
+        {
+            if (Nat256.IsZero(x))
+            {
+                Nat256.Zero(z);
+            }
+            else
+            {
+                Nat256.Sub(P, x, z);
+            }
+        }
+
+        public static void Reduce(uint[] xx, uint[] z)
+        {
+            Debug.Assert(xx[15] >> 30 == 0);
+
+            uint xx07 = xx[7];
+            Nat.ShiftUpBit(8, xx, 8, xx07, z);
+            uint c = Nat256.MulByWordAddTo(PInv, xx, z) << 1;
+            uint z07 = z[7];
+            z[7] = z07 & P7;
+            c += (z07 >> 31) - (xx07 >> 31);
+            Nat256.AddWord(c * PInv, z, 0);
+            if (Nat256.Gte(z, P))
+            {
+                Nat256.AddWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+        }
+
+        public static void Square(uint[] x, uint[] z)
+        {
+            uint[] tt = Nat256.CreateExt();
+            Nat256.Square(x, tt);
+            Reduce(tt, z);
+        }
+
+        public static void SquareN(uint[] x, int n, uint[] z)
+        {
+    //        assert n > 0;
+
+            uint[] tt = Nat256.CreateExt();
+            Nat256.Square(x, tt);
+            Reduce(tt, z);
+
+            while (--n > 0)
+            {
+                Nat256.Square(z, tt);
+                Reduce(tt, z);
+            }
+        }
+
+        public static void Subtract(uint[] x, uint[] y, uint[] z)
+        {
+            int c = Nat256.Sub(x, y, z);
+            if (c != 0)
+            {
+                Nat256.SubWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+        }
+
+        public static void SubtractExt(uint[] xx, uint[] yy, uint[] zz)
+        {
+            int c = Nat256.SubExt(xx, yy, zz);
+            if (c != 0)
+            {
+                Nat256.AddExt(zz, PExt, zz);
+            }
+        }
+
+        public static void Twice(uint[] x, uint[] z)
+        {
+            Nat256.ShiftUpBit(x, 0, z);
+            if (Nat256.Gte(z, P))
+            {
+                Nat256.AddWord(PInv, z, 0);
+                z[7] &= P7;
+            }
+        }
+    }
+}