Is it possible to programmatically generate an X509 certificate using only C#?

Question

We're trying to generate an X509 certificate (including the private key) programmatically using C# and the BouncyCastle library. We've tried using some of the code from this sample by Felix Kollmann but the private key part of the certificate returns null. Code and unit test are as below:

using System;
using System.Collections;
using Org.BouncyCastle.Asn1;
using Org.BouncyCastle.Asn1.X509;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Prng;
using Org.BouncyCastle.Math;
using Org.BouncyCastle.Security;
using Org.BouncyCastle.X509;

namespace MyApp
{
    public class CertificateGenerator
    {
        /// <summary>
        /// 
        /// </summary>
        /// <remarks>Based on <see cref="http://www.fkollmann.de/v2/post/Creating-certificates-using-BouncyCastle.aspx"/&gt;&lt;/remarks&gt;
        /// <param name="subjectName"></param>
        /// <returns></returns>
        public static byte[] GenerateCertificate(string subjectName)
        {
            var kpgen = new RsaKeyPairGenerator();

            kpgen.Init(new KeyGenerationParameters(new SecureRandom(new CryptoApiRandomGenerator()), 1024));

            var kp = kpgen.GenerateKeyPair();

            var gen = new X509V3CertificateGenerator();

            var certName = new X509Name("CN=" + subjectName);
            var serialNo = BigInteger.ProbablePrime(120, new Random());

            gen.SetSerialNumber(serialNo);
            gen.SetSubjectDN(certName);
            gen.SetIssuerDN(certName);
            gen.SetNotAfter(DateTime.Now.AddYears(100));
            gen.SetNotBefore(DateTime.Now.Subtract(new TimeSpan(7, 0, 0, 0)));
            gen.SetSignatureAlgorithm("MD5WithRSA");
            gen.SetPublicKey(kp.Public);

            gen.AddExtension(
                X509Extensions.AuthorityKeyIdentifier.Id,
                false,
                new AuthorityKeyIdentifier(
                    SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(kp.Public),
                    new GeneralNames(new GeneralName(certName)),
                    serialNo));

            gen.AddExtension(
                X509Extensions.ExtendedKeyUsage.Id,
                false,
                new ExtendedKeyUsage(new ArrayList() { new DerObjectIdentifier("1.3.6.1.5.5.7.3.1") }));

            var newCert = gen.Generate(kp.Private);
            return DotNetUtilities.ToX509Certificate(newCert).Export(System.Security.Cryptography.X509Certificates.X509ContentType.Pkcs12, "password");
        }
    }
}

Unit test:

using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace MyApp
{
    [TestClass]
    public class CertificateGeneratorTests
    {
        [TestMethod]
        public void GenerateCertificate_Test_ValidCertificate()
        {
            // Arrange
            string subjectName = "test";

            // Act
            byte[] actual = CertificateGenerator.GenerateCertificate(subjectName);

            // Assert
            var cert = new X509Certificate2(actual, "password");
            Assert.AreEqual("CN=" + subjectName, cert.Subject);
            Assert.IsInstanceOfType(cert.PrivateKey, typeof(RSACryptoServiceProvider));
        }
    }
}

Answer 1

Just to clarify, an X.509 certificate does not contain the private key. The word certificate is sometimes misused to represent the combination of the certificate and the private key, but they are two distinct entities. The whole point of using certificates is to send them more or less openly, without sending the private key, which must be kept secret. An X509Certificate2 object may have a private key associated with it (via its PrivateKey property), but that's only a convenience as part of the design of this class.

In your first BouncyCastle code example, newCert is really just the certificate and DotNetUtilities.ToX509Certificate(newCert) is built from the certificate only.

Considering that the PKCS#12 format requires the presence of a private key, I'm quite surprised that the following part even works (considering you're calling it on a certificate which can't possibly know the private key):

.Export(System.Security.Cryptography.X509Certificates.X509ContentType.Pkcs12,
    "password");

(gen.Generate(kp.Private) signs the certificate using the private key, but doesn't put the private key in the certificate, which wouldn't make sense.)

If you want your method to return both the certificate and the private key you could either:

• Return an X509Certificate2 object in which you've initialized the PrivateKey property
• Build a PKCS#12 store and returns its byte[] content (as if it was a file). Step 3 in the link you've sent explains how to build a PKCS#12 store.

Returning the byte[] (DER) structure for the X.509 certificate itself will not contain the private key.

If your main concern (according to your test case) is to check that the certificate was built from an RSA key-pair, you can check the type of its public key instead.