Added certificate helper

Author: eocron

Eocron.Algorithms/Certificates/CertificateHelper.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Linq;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+
+namespace Eocron.Algorithms.Certificates
+{
+    public static class CertificateHelper
+    {
+        public static X509Certificate2 FindByThumbprint(string thumbprint)
+        {
+            if (string.IsNullOrWhiteSpace(thumbprint))
+                throw new ArgumentException("Thumbprint cannot be null or whitespace.", nameof(thumbprint));
+            using var store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
+            store.Open(OpenFlags.ReadOnly);
+
+            return store.Certificates.Find(X509FindType.FindByThumbprint, thumbprint, false).Single();
+        }
+
+        public static string ExportPrivateKeyToPem(X509Certificate2 cert)
+        {
+            if (!cert.HasPrivateKey)
+                throw new InvalidOperationException("No private key found in certificate: " + cert.Subject);
+
+            var rsa = cert.GetRSAPrivateKey();
+            if (rsa is RSACng rsaCng)
+            {
+                try
+                {
+                    var parameters = rsaCng.ExportParameters(true);
+                    using var rsaTemp = RSA.Create();
+                    rsaTemp.ImportParameters(parameters);
+                    return rsaTemp.ExportPkcs8PrivateKeyPem();
+                }
+                catch (Exception ex)
+                {
+                    throw new InvalidOperationException("Private key can't be exported: " + cert.Subject, ex);
+                }
+            }
+
+            return rsa.ExportPkcs8PrivateKeyPem();
+
+        }
+    }
+}