Factor Wnaf out of EdDSA

1 files changed, 65 insertions, 0 deletions

diff --git a/crypto/src/math/ec/rfc8032/Wnaf.cs b/crypto/src/math/ec/rfc8032/Wnaf.cs
new file mode 100644
index 000000000..cc6e3704f
--- /dev/null
+++ b/crypto/src/math/ec/rfc8032/Wnaf.cs
@@ -0,0 +1,65 @@
+using System;
+using System.Diagnostics;
+
+using Org.BouncyCastle.Utilities;
+
+namespace Org.BouncyCastle.Math.EC.Rfc8032
+{
+    internal static class Wnaf
+    {
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+        internal static void GetSignedVar(ReadOnlySpan<uint> n, int width, Span<sbyte> ws)
+#else
+        internal static void GetSignedVar(uint[] n, int width, sbyte[] ws)
+#endif
+        {
+            Debug.Assert(2 <= width && width <= 8);
+
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+            Span<uint> t = n.Length <= 64
+                ? stackalloc uint[n.Length * 2]
+                : new uint[n.Length * 2];
+#else
+            uint[] t = new uint[n.Length * 2];
+#endif
+            {
+                uint c = 0U - (n[n.Length - 1] >> 31);
+                int tPos = t.Length, i = n.Length;
+                while (--i >= 0)
+                {
+                    uint next = n[i];
+                    t[--tPos] = (next >> 16) | (c << 16);
+                    t[--tPos] = c = next;
+                }
+            }
+
+            uint sign = 0U;
+            int j = 0, lead = 32 - width;
+
+            for (int i = 0; i < t.Length; ++i, j -= 16)
+            {
+                uint word = t[i];
+                while (j < 16)
+                {
+                    uint word16 = word >> j;
+
+                    int skip = Integers.NumberOfTrailingZeros((int)((sign ^ word16) | 0xFFFF0000U));
+                    if (skip > 0)
+                    {
+                        j += skip;
+                        continue;
+                    }
+
+                    int digit = ((int)word16 | 1) << lead;
+                    sign = (uint)(digit >> 31);
+
+                    ws[(i << 4) + j] = (sbyte)(digit >> lead);
+
+                    j += width;
+                }
+            }
+
+            Debug.Assert(sign == n[n.Length - 1] >> 31);
+        }
+    }
+}