How can I generate an Extended Validation self-signed certificate with CertEnroll?

Question

Due to a bug in Windows Azure, all self-signed client certificates used with the Azure REST API from a Windows 8 application must be specified as extended validation certificates.

To provide a better user experience, I am trying to generate this self-signed certificate on a remote server. I'm using the Certificate Enrolment API, a COM library distributed as part of Windows, as described in this answer to the question How to create a self-signed certificate using C#?

The code is largely the same, just slightly modified for my own use:

public static X509Certificate2 CreateSelfSignedCertificate(
    string cname,
    string friendlyName,
    string password)
{
    // create DN for subject and issuer
    var dn = new CX500DistinguishedName();
    dn.Encode("CN=" + cname, X500NameFlags.XCN_CERT_NAME_STR_NONE);

    // create a new private key for the certificate
    CX509PrivateKey privateKey = new CX509PrivateKey();
    privateKey.ProviderName = "Microsoft Base Cryptographic Provider v1.0";
    privateKey.MachineContext = true;
    privateKey.Length = 2048;
    privateKey.KeySpec = X509KeySpec.XCN_AT_SIGNATURE; // use is not limited
    privateKey.ExportPolicy 
        = X509PrivateKeyExportFlags.XCN_NCRYPT_ALLOW_PLAINTEXT_EXPORT_FLAG;
    privateKey.Create();

    // Use the stronger SHA512 hashing algorithm
    var hashobj = new CObjectId();
    hashobj.InitializeFromAlgorithmName(
        ObjectIdGroupId.XCN_CRYPT_HASH_ALG_OID_GROUP_ID,
        ObjectIdPublicKeyFlags.XCN_CRYPT_OID_INFO_PUBKEY_ANY, 
        AlgorithmFlags.AlgorithmFlagsNone,
        "SHA512");

    // Create the self signing request
    var cert = new CX509CertificateRequestCertificate();
    cert.InitializeFromPrivateKey(
        X509CertificateEnrollmentContext.ContextMachine,
        privateKey,
        string.Empty);

    cert.Subject = dn;
    cert.Issuer = dn; // the issuer and the subject are the same
    cert.NotBefore = DateTime.Now;
    cert.NotAfter = DateTime.Now.AddYears(50); 
    cert.HashAlgorithm = hashobj;

    var clientAuthenticationOid = new CObjectId();
    clientAuthenticationOid.InitializeFromValue("1.3.6.1.5.5.7.3.2");

    // Set up cert to be used for Client Authentication.
    var oids = new CObjectIds();
    oids.Add(clientAuthenticationOid);

    var eku = new CX509ExtensionEnhancedKeyUsage();
    eku.InitializeEncode(oids);

    cert.X509Extensions.Add((CX509Extension)eku);

    // Add the certificate policy.
    var policy = new CCertificatePolicy();
    policy.Initialize(clientAuthenticationOid);

    // THIS IS WRONG - NEEDS A DIFFERENT QUALIFIER
    var qualifier = new CPolicyQualifier();
    qualifier.InitializeEncode(
        "c0",
         PolicyQualifierType.PolicyQualifierTypeUserNotice);

    policy.PolicyQualifiers.Add(qualifier);

    var policies = new CCertificatePolicies();
    policies.Add(policy);

    var ecp = new CX509ExtensionCertificatePolicies();
    ecp.InitializeEncode(policies);

    cert.X509Extensions.Add((CX509Extension)ecp);

    cert.Encode();

    // Do the final enrolment process
    var enroll = new CX509Enrollment();
    enroll.InitializeFromRequest(cert); // load the certificate
    enroll.CertificateFriendlyName = friendlyName;
    string csr = enroll.CreateRequest(); // Output the request in base64

    // and install it back as the response
    enroll.InstallResponse(
        InstallResponseRestrictionFlags.AllowUntrustedCertificate,
        csr,
        EncodingType.XCN_CRYPT_STRING_BASE64,
        ""); // no password

    // output a base64 encoded PKCS#12 for import to .NET
    var base64encoded = enroll.CreatePFX(
        password,
        PFXExportOptions.PFXExportChainWithRoot);

    // instantiate the target class with the PKCS#12 data
    return new X509Certificate2(
        System.Convert.FromBase64String(base64encoded),
        password,
        X509KeyStorageFlags.Exportable);
}

I've found the ICertificatePolicy interface which appears to represent the right type of structure, but I can't deduce the right IPolicyQualifier to use. In my code, the qualifier is

For clarity, this where you configure the information in the Windows 8 Certificate Manager:

Which produces this property on the certificate:

My code currently produces this property:

Close, but not there yet.

Is there another way to load data into the IPolicyQualifier so that it produces the expected result, perhaps using the InitialiseDecode method?

Answer 1

If you want to set Client Authentication OID as "1.3.6.1.5.5.7.2" then I think you code does work as expected. How do you verify that the code does not have this value set?

I wrote the following code to save the generated certificate using your code:

X509Certificate2 xx = CreateSelfSignedCertificate("Avkash CNAME", "Test User Friendly Name", "xx");
byte[] bCertExported = xx.Export(X509ContentType.Pkcs12, "xx");
File.WriteAllBytes("c:\\Installbox\\test.pfx", bCertExported);

After I installed the certificate locally and checked it for Client Authentication OID, it does show as below: