Helper methods divided in separate controllers.

source/TestAuthorityCore/Controllers/DerToolsController.cs

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+using System;
+using System.IO;
+using System.Net.Mime;
+using System.Text;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Org.BouncyCastle.OpenSsl;
+using Org.BouncyCastle.X509;
+
+namespace TestAuthorityCore.Controllers
+{
+    /// <summary>
+    /// Provides some useful tools like certificateName conversion.
+    /// </summary>
+    [Route("api/tools")]
+    public class DerToolsController : Controller
+    {
+        [HttpPost("der-to-pem")]
+        public IActionResult ConvertCertificateToPem([FromForm] IFormFile file, string certificateName = "certificate.crt")
+        {
+            using (var streamReader = new StreamReader(file.OpenReadStream()))
+            {
+                X509Certificate certificate = new X509CertificateParser().ReadCertificate(streamReader.BaseStream);
+                string certificateString = ConvertCertificateToPem(certificate);
+                byte[] result = Encoding.ASCII.GetBytes(certificateString);
+                return File(result, MediaTypeNames.Application.Octet, certificateName);
+            }
+        }
+
+        private static string ConvertCertificateToPem(X509Certificate certificate)
+        {
+            var generator = new MiscPemGenerator(certificate);
+
+            string certificateString;
+            using (var textWriter = new StringWriter())
+            {
+                var writer = new PemWriter(textWriter);
+                writer.WriteObject(generator);
+                writer.Writer.Flush();
+                certificateString = textWriter.ToString();
+            }
+
+            if (string.IsNullOrWhiteSpace(certificateString))
+            {
+                throw new InvalidOperationException();
+            }
+
+            return certificateString;
+        }
+    }
+}

source/TestAuthorityCore/Controllers/PemToolsController.cs

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+using System;
+using System.IO;
+using System.Linq;
+using System.Net.Mime;
+using System.Text;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Org.BouncyCastle.OpenSsl;
+using Org.BouncyCastle.Pkcs;
+
+namespace TestAuthorityCore.Controllers
+{
+    public class PemToolsController : Controller
+    {
+        [HttpPost("pfx-to-certificate")]
+        public IActionResult GetCertificateFromPfx([FromForm] IFormFile file, string password, string certificateName = "certificate.crt")
+        {
+            using (var streamReader = new StreamReader(file.OpenReadStream()))
+            {
+                var store = new Pkcs12Store(streamReader.BaseStream, password.ToCharArray());
+                string firstAlias = store.Aliases.OfType<string>().FirstOrDefault();
+                if (string.IsNullOrWhiteSpace(firstAlias))
+                {
+                    throw new InvalidOperationException("Unable to find any certificateName in PFX store");
+                }
+
+                X509CertificateEntry certificateEntry = store.GetCertificate(firstAlias);
+
+                string certificateString = ConvertToPemFormat(certificateEntry.Certificate);
+                byte[] result = Encoding.ASCII.GetBytes(certificateString);
+                return File(result, MediaTypeNames.Application.Octet, certificateName);
+            }
+        }
+
+        [HttpPost("pfx-to-key")]
+        public IActionResult GetKeyFromPfx([FromForm] IFormFile file, string password, string filename = "certificate.key")
+        {
+            using (var streamReader = new StreamReader(file.OpenReadStream()))
+            {
+                var store = new Pkcs12Store(streamReader.BaseStream, password.ToCharArray());
+                string firstAlias = store.Aliases.OfType<string>().FirstOrDefault();
+                if (string.IsNullOrWhiteSpace(firstAlias))
+                {
+                    throw new InvalidOperationException("Unable to find any certificateName in PFX store");
+                }
+
+                AsymmetricKeyEntry key = store.GetKey(firstAlias);
+                string convertedKey = ConvertToPemFormat(key.Key);
+
+                byte[] result = Encoding.ASCII.GetBytes(convertedKey);
+                return File(result, MediaTypeNames.Application.Octet, filename);
+            }
+        }
+
+        private static string ConvertToPemFormat(object input)
+        {
+            var generator = new MiscPemGenerator(input);
+
+            string certificateString;
+            using (var textWriter = new StringWriter())
+            {
+                var writer = new PemWriter(textWriter);
+                writer.WriteObject(generator);
+                writer.Writer.Flush();
+                certificateString = textWriter.ToString();
+            }
+
+            if (string.IsNullOrWhiteSpace(certificateString))
+            {
+                throw new InvalidOperationException();
+            }
+
+            return certificateString;
+        }
+    }
+}