Problems loading a pfx file programmatically

Question

I am trying to create a self-signed certificate and then reading it at some point.

This is the code for creating the pfx file: (source)

 public static void CreateSelfSignedCertificate(string subjectName)
        {
            string pathToCertificate = CommonHelper.MapPath($"path_to_certificate/{TokenSigningCertificateName}");

            if (!File.Exists(pathToCertificate))
            {
                // create DN for subject and issuer
                var dn = new CX500DistinguishedName();
                dn.Encode("CN=" + 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 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");

                // add extended key usage if you want - look at MSDN for a list of possible OIDs
                var oid = new CObjectId();
                oid.InitializeFromValue("1.3.6.1.5.5.7.3.1"); // SSL server
                var oidlist = new CObjectIds();
                oidlist.Add(oid);
                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
                cert.NotBefore = DateTime.Now;
                // this cert expires immediately. Change to whatever makes sense for you
                cert.NotAfter = DateTime.Now;
                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 = subjectName; // Optional: add a friendly name
                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);

                File.WriteAllText(pathToCertificate, base64encoded);
            }
        }

And this is the code for loading it:

public static X509Certificate2 GetTokenSigningCertificate()
        {
            string pathToCertificate = CommonHelper.MapPath($"path_to_certificate/{TokenSigningCertificateName}");

            X509Certificate2 certificate = new X509Certificate2(pathToCertificate, "");

            return certificate;
        }

The problem is I am getting:

An error occurred during encode or decode operation. System.Security.Cryptography.CryptographicException

at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr) at System.Security.Cryptography.X509Certificates.X509Utils._LoadCertFromFile(String fileName, IntPtr password, UInt32 dwFlags, Boolean persistKeySet, SafeCertContextHandle& pCertCtx) at System.Security.Cryptography.X509Certificates.X509Certificate.LoadCertificateFromFile(String fileName, Object password, X509KeyStorageFlags keyStorageFlags) at System.Security.Cryptography.X509Certificates.X509Certificate2..ctor(String fileName, String password)

Any ideas?

Answer 1

You seem to be exporting your PFX as base64. Unlike certificates, keys, PKCS#7 blobs, and PKCS#8 blobs, PKCS#12/PFX blobs have no defined PEM header. As a consequence, the PFX reading pipeline probably doesn't have a Base64-decode attached to it.

So the simple answer is likely to emit it with the binary encoding (and thus File.WriteAllBytes()) instead of base64.