From 9278224f44d9e1237756758b3688595f3bc30639 Mon Sep 17 00:00:00 2001
From: Thorsten Sommer <service@sommer-engineering.com>
Date: Fri, 3 Jan 2020 20:25:34 +0100
Subject: [PATCH] Added extension for determine public key

---
 Ed25519/Extensions.cs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Ed25519/Extensions.cs b/Ed25519/Extensions.cs
index e5fb821..7e2ff16 100644
--- a/Ed25519/Extensions.cs
+++ b/Ed25519/Extensions.cs
@@ -103,5 +103,22 @@ namespace Ed25519
         {
             return data[index / 8] >> (index % 8) & 1;
         }
+
+        public static ReadOnlySpan<byte> ExtractPublicKey(this ReadOnlySpan<byte> privateKey)
+        {
+            var hash = privateKey.ComputeHash();
+            var a = Constants.TWO_POW_BIT_LENGTH_MINUS_TWO;
+            for (var i = 3; i < Constants.BIT_LENGTH - 2; i++)
+            {
+                var bit = hash.GetBit(i);
+                if (bit != 0)
+                {
+                    a += Constants.TWO_POW_CACHE[i];
+                }
+            }
+
+            var bigA = Constants.B.ScalarMul(a);
+            return bigA.EncodePoint();
+        }
     }
 }