In some windows OS machine I am getting cryptographic exception: Invalid provider type specified

Question

In most of the windows OS (Windows 2019 Server), below code works fine, but for some I am getting cryptographic exception: Invalid provider type specified while trying to access PrivateKey of self signed certificate.

What could be the issue?

Here is the code,

 var selfSignedCert = CreateSelfSignedCertificate("CN=localhost", "Agent Service");
        try
        {
            var rsaCsp = selfSignedCert.PrivateKey as RSACryptoServiceProvider;
            if (rsaCsp != null) Console.WriteLine(rsaCsp.KeySize);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Exception: {ex}");
        }

CreateSelfSignedCertificate Method :

  private static X509Certificate2 CreateSelfSignedCertificate(string subjectName, string friendlyName)
    {
        // create DN for subject and issuer
        var dn = new CX500DistinguishedName();
        dn.Encode(subjectName, 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 SHA-2 SHA256 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, "SHA256");

        var keyUsage = new CX509ExtensionKeyUsage();
        keyUsage.InitializeEncode(CertEnroll.X509KeyUsageFlags.XCN_CERT_DIGITAL_SIGNATURE_KEY_USAGE |
            CertEnroll.X509KeyUsageFlags.XCN_CERT_KEY_ENCIPHERMENT_KEY_USAGE);
        keyUsage.Critical = true;

        // add extended key usage if you want - look at MSDN for a list of possible OIDs
        // serverAuth (1.3.6.1.5.5.7.3.1) -- TLS Web server authentication
        // clientAuth(1.3.6.1.5.5.7.3.2)-- TLS Web client authentication
        var oidServerAuth = new CObjectId();
        oidServerAuth.InitializeFromValue("1.3.6.1.5.5.7.3.1");

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

        var oidlist = new CObjectIds
        {
            oidServerAuth,
            oidClientAuth
        };

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

        // Create the self signing request
        var cert = new CX509CertificateRequestCertificate();
        cert.InitializeFromPrivateKey(X509CertificateEnrollmentContext.ContextMachine, privateKey, "");
        cert.Subject = dn;
        cert.Issuer = dn; // the issuer and the subject are the same
        var hoursAgo = TimeSpan.FromHours(12);
        cert.NotBefore = DateTime.UtcNow.Subtract(hoursAgo);
        // this cert expires in twenty years. Change to whatever makes sense for you
        cert.NotAfter = DateTime.UtcNow.AddYears(20);
        cert.X509Extensions.Add((CX509Extension)keyUsage);
        cert.X509Extensions.Add((CX509Extension)eku); // add the EKU

        cert.HashAlgorithm = hashobj; // Specify the hashing algorithm
        cert.Encode(); // encode the certificate

        // Do the final enrollment process
        var enroll = new CX509Enrollment();
        enroll.InitializeFromRequest(cert); // load the certificate

        enroll.CertificateFriendlyName = friendlyName ?? subjectName;

        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 so we can import it back to the .Net security classes
        var base64encoded = enroll.CreatePFX("", // no password, this is for internal consumption
            PFXExportOptions.PFXExportChainWithRoot);

        // instantiate the target class with the PKCS#12 data (and the empty password)
        return new X509Certificate2(
            System.Convert.FromBase64String(base64encoded), "",
            // mark the private key as exportable (this is usually what you want to do)
            // mark private key to go into the Machine store instead of the current users store
            // mark key as persistent https://support.microsoft.com/en-us/kb/950090
            X509KeyStorageFlags.Exportable | X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet
        );
    }

Answer 1

Basically the exception is because the code is trying to access the private key of the self-signed certificate using the wrong type of provider, a problem that occurs on some operating systems. To resolve this exception, you can make a code modification to use the correct provider type when accessing the private key. Instead of using RSACryptoServiceProvider, you can use RSA from the System.Security.Cryptography namespace. Here is the modified code:

var privateKey = selfSignedCert.GetRSAPrivateKey();
if (privateKey != null)
{
     Console.WriteLine(privateKey.KeySize);
}

'GetRSAPrivateKey()' is used to get the private key of the self-signed certificate as an RSA object. you can verify that the private key is not null before accessing its KeySize property or performing other operations related to the private key.