.Net Programmatically Sign PKCS#10 Request with Bouncy Castle

Question

We have a valid PKCS#10 Certificate Request generated on the Client using CertEnroll.

Now we need to sign it and return the result to the Client, where CertEnroll will handle the local Certificate Store stuff.

This is a B2B application and the root signing certificate will be self-generated or we can use our existing Thawte SSL cert.

The Server (2008) does not have Active Directory running and we don't want to create a stand-alone signing infrastructure/service for this unless absolutely necessary. There is no need for revocation etc. - we want to do it programmatically.

I would be happy to use the BouncyCastle Library however the C# lib lacks any meaningful documentation and while the original Java docs are admittedly similar the C# implementation is different enough to have left me more than a little confused.

Is anyone aware of (or have) sample C# (or VB) code or a known-relaible link to same, using BouncyCastle or for that matter the native .Net classes ?

Any assistance in getting this thing done would be greatly appreciated!

Answer 1

This proved an interesting exercise to say the least :-)

We used both BouncyCastle and the .Net certificate objects. The solution still has !much! room for improvement, but it does in fact work.

The guts of it (Cert Gen) are below. When we get here the CSR has already been generated on the client and and when we leave the resulting certificate is installed by Client-side code (see this blog for client-side workings.)

Again, I am not offering this as a Finished Product but hopefully of some value to those facing the same task. Happy Encrypting :-)

        // Jul 10, 2012 see
        // http://social.technet.microsoft.com/Forums/en-NZ/winserversecurity/thread/45781b46-3eb7-4715-b877-883bf0dc2ae7
        // instaed of CX509CertificateRequestPkcs10 csr = new CX509CertificateRequestPkcs10(); use:
        IX509CertificateRequestPkcs10 csr = (IX509CertificateRequestPkcs10)Activator.CreateInstance(Type.GetTypeFromProgID("X509Enrollment.CX509CertificateRequestPkcs10"));
        csr.InitializeDecode(csrText, EncodingType.XCN_CRYPT_STRING_BASE64);
        csr.CheckSignature(Pkcs10AllowedSignatureTypes.AllowedKeySignature);

        //get Bouncy CSRInfo Object
        Trace.Write("get Bouncy CSRInfo Object");
        Byte[] csrBytes = Convert.FromBase64String(csrText);
        CertificationRequestInfo csrInfo = CertificateTools.GetCsrInfo(csrBytes);
        SubjectPublicKeyInfo pki = csrInfo.SubjectPublicKeyInfo;

        //pub key for the signed cert
        Trace.Write("pub key for the signed cert");
        AsymmetricKeyParameter publicKey = PublicKeyFactory.CreateKey(pki);

        // Build a Version1 (No Extensions) Certificate
        DateTime startDate = DateTime.Now;
        DateTime expiryDate = startDate.AddYears(100);
        BigInteger serialNumber = new BigInteger(32, new Random());

        Trace.Write("Build a Version1 (No Extensions) Certificate");
        X509V1CertificateGenerator certGen = new X509V1CertificateGenerator();
        string signerCN = ConfigurationManager.AppSettings["signerCN"].ToString();
        X509Name dnName = new X509Name(String.Format("CN={0}", signerCN));
        X509Name cName = new X509Name(csr.Subject.Name);
        certGen.SetSerialNumber(serialNumber);
        certGen.SetIssuerDN(dnName);
        certGen.SetNotBefore(startDate);
        certGen.SetNotAfter(expiryDate);
        certGen.SetSubjectDN(cName);
        certGen.SetSignatureAlgorithm("SHA1withRSA");
        certGen.SetPublicKey(publicKey);

        //get our Private Key to Sign with
        Trace.Write("get our Private Key to Sign with");
        X509Store store = new X509Store(StoreLocation.LocalMachine);
        store.Open(OpenFlags.ReadOnly);
        string signerThumbprint = ConfigurationManager.AppSettings["signerThumbprint"].ToString();
        X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
        X509Certificate2Collection fcollection = (X509Certificate2Collection)collection.Find(X509FindType.FindByThumbprint, signerThumbprint, false);
        X509Certificate2 caCert = fcollection[0];

        Trace.Write("Found:" + caCert.FriendlyName);
        Trace.Write("Has Private " + caCert.HasPrivateKey.ToString());
        Trace.Write("Key Size " + caCert.PrivateKey.KeySize.ToString());

        //Get our Signing Key as a Bouncy object
        Trace.Write("Get our Signing Key as a Bouncy object from key ");
        AsymmetricCipherKeyPair caPair = DotNetUtilities.GetKeyPair(caCert.PrivateKey);

        //gen BouncyCastle object
        Trace.Write("gen BouncyCastle object");
        Org.BouncyCastle.X509.X509Certificate cert = certGen.Generate(caPair.Private);

        //convert to windows type 2 and get Base64 String
        Trace.Write("convert to windows type 2 and get Base64 String");
        X509Certificate2 cert2 = new X509Certificate2(DotNetUtilities.ToX509Certificate(cert));
        byte[] encoded = cert2.GetRawCertData();
        string certOutString = Convert.ToBase64String(encoded);

        //output to the page (hidden)
        Certificate.Value = certOutString;