Cache-safety for EC lookup tables

- creation of cache-safe lookup tables delegated to ECCurve
- FixedPointCombMultiplier uses cache-safe lookup table
- FixedPointCombMultiplier avoids BigInteger.TestBit

1 files changed, 59 insertions, 0 deletions

diff --git a/crypto/src/math/ec/custom/sec/SecP256K1Curve.cs b/crypto/src/math/ec/custom/sec/SecP256K1Curve.cs
index 59e2cefb2..b3a5dd646 100644
--- a/crypto/src/math/ec/custom/sec/SecP256K1Curve.cs
+++ b/crypto/src/math/ec/custom/sec/SecP256K1Curve.cs
@@ -1,5 +1,6 @@
 using System;
 
+using Org.BouncyCastle.Math.Raw;
 using Org.BouncyCastle.Utilities.Encoders;
 
 namespace Org.BouncyCastle.Math.EC.Custom.Sec
@@ -11,6 +12,7 @@ namespace Org.BouncyCastle.Math.EC.Custom.Sec
             Hex.Decode("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F"));
 
         private const int SECP256K1_DEFAULT_COORDS = COORD_JACOBIAN;
+        private const int SECP256K1_FE_INTS = 8;
 
         protected readonly SecP256K1Point m_infinity;
 
@@ -71,5 +73,62 @@ namespace Org.BouncyCastle.Math.EC.Custom.Sec
         {
             return new SecP256K1Point(this, x, y, zs, withCompression);
         }
+
+        public override ECLookupTable CreateCacheSafeLookupTable(ECPoint[] points, int off, int len)
+        {
+            uint[] table = new uint[len * SECP256K1_FE_INTS * 2];
+            {
+                int pos = 0;
+                for (int i = 0; i < len; ++i)
+                {
+                    ECPoint p = points[off + i];
+                    Nat256.Copy(((SecP256K1FieldElement)p.RawXCoord).x, 0, table, pos); pos += SECP256K1_FE_INTS;
+                    Nat256.Copy(((SecP256K1FieldElement)p.RawYCoord).x, 0, table, pos); pos += SECP256K1_FE_INTS;
+                }
+            }
+
+            return new SecP256K1LookupTable(this, table, len);
+        }
+
+        private class SecP256K1LookupTable
+            : ECLookupTable
+        {
+            private readonly SecP256K1Curve m_outer;
+            private readonly uint[] m_table;
+            private readonly int m_size;
+
+            internal SecP256K1LookupTable(SecP256K1Curve outer, uint[] table, int size)
+            {
+                this.m_outer = outer;
+                this.m_table = table;
+                this.m_size = size;
+            }
+
+            public virtual int Size
+            {
+                get { return m_size; }
+            }
+
+            public virtual ECPoint Lookup(int index)
+            {
+                uint[] x = Nat256.Create(), y = Nat256.Create();
+                int pos = 0;
+
+                for (int i = 0; i < m_size; ++i)
+                {
+                    uint MASK = (uint)(((i ^ index) - 1) >> 31);
+
+                    for (int j = 0; j < SECP256K1_FE_INTS; ++j)
+                    {
+                        x[j] ^= m_table[pos + j] & MASK;
+                        y[j] ^= m_table[pos + SECP256K1_FE_INTS + j] & MASK;
+                    }
+
+                    pos += (SECP256K1_FE_INTS * 2);
+                }
+
+                return m_outer.CreateRawPoint(new SecP256K1FieldElement(x), new SecP256K1FieldElement(y), false);
+            }
+        }
     }
 }