Add a new scalar method to set and verity a private key

Author: Coding-Enthusiast

Src/Autarkysoft.Bitcoin/Cryptography/EllipticCurve/Scalar8x32.cs

@@ -296,6 +296,23 @@ public bool IsHigh
         }
 
 
+        /// <summary>
+        /// Creates a new instance of <see cref="Scalar8x32"/> using the given big-endian array
+        /// and reduces the result modulo curve order (n). 
+        /// Return value indicates validity of the result as a private key.
+        /// </summary>
+        /// <param name="data">Array to use</param>
+        /// <param name="res">Scalar</param>
+        /// <returns>True if value was non-zero and smaller than curve order; otherwise false</returns>
+        public static bool TrySetPrivateKey(ReadOnlySpan<byte> data, out Scalar8x32 res)
+        {
+            // secp256k1_scalar_set_b32_seckey
+            res = new Scalar8x32(data, out bool overflow);
+            Debug.Assert(res.Verify());
+            return !overflow && !res.IsZero;
+        }
+
+
         /// <summary>
         /// Multiply a scalar with the multiplicative inverse of 2
         /// </summary>

Src/Tests/Bitcoin/Cryptography/EllipticCurve/Scalar8x32Tests.cs

@@ -473,6 +473,32 @@ internal static Scalar8x32 CreateRandom(TestRNG rng)
             } while (true);
         }
 
+        /// <summary>
+        /// random_scalar_order
+        /// </summary>
+        private static Scalar8x32 RandomScalarOrder(TestRNG rng)
+        {
+            do
+            {
+                byte[] b32 = new byte[32];
+                rng.Rand256(b32);
+                Scalar8x32 num = new(b32, out bool overflow);
+                if (!overflow && !num.IsZero)
+                {
+                    return num;
+                }
+            } while (true);
+        }
+
+        /// <summary>
+        /// random_scalar_order_b32
+        /// </summary>
+        private static byte[] RandomScalarOrderB32(TestRNG rng)
+        {
+            Scalar8x32 num = RandomScalarOrder(rng);
+            return num.ToByteArray();
+        }
+
         // scalar_test(void)
         private static unsafe void ScalarTest(TestRNG rng)
         {
@@ -622,6 +648,27 @@ private static unsafe void ScalarTest(TestRNG rng)
             }
         }
 
+
+        // run_scalar_set_b32_seckey_tests
+        private static void SetB32SeckeyTests(TestRNG rng)
+        {
+            // Usually set_b32 and set_b32_seckey give the same result
+            Span<byte> b32 = RandomScalarOrderB32(rng);
+
+            Scalar8x32 s1 = new(b32, out _);
+            bool b = Scalar8x32.TrySetPrivateKey(b32, out Scalar8x32 s2);
+            Assert.True(b);
+            Assert.True(s1.Equals(s2));
+
+            b32.Clear(); // b32.Fill(0);
+            b = Scalar8x32.TrySetPrivateKey(b32, out _);
+            Assert.False(b);
+
+            b32.Fill(0xff);
+            b = Scalar8x32.TrySetPrivateKey(b32, out _);
+            Assert.False(b);
+        }
+
         [Fact]
         public void Libsecp256k1Tests() // run_scalar_tests
         {
@@ -633,6 +680,11 @@ public void Libsecp256k1Tests() // run_scalar_tests
                 ScalarTest(rng);
             }
 
+            for (int i = 0; i < Count; i++)
+            {
+                SetB32SeckeyTests(rng);
+            }
+
             // Check that the scalar constants secp256k1_scalar_zero and
             // secp256k1_scalar_one contain the expected values.
             // Note: these are tested in StaticPropTest()