Create X509Certificate2 from Cert and Key, without making a PFX file

Question

In the past I have been making secure TcpListener by exporting a PFX certificate with a password, but would like to know if this step could be skipped.

I'm not using commercial SSL certificates, and have a Root CA, that I use to issue server certificates. These server certificates require additional steps when hosting a TcpListener in C# (I guess because the CSR wasn't used)... but what if I do have the Private Key, and the Certificate that OpenSSL generates/uses.

sslCertificate = new X509Certificate2("myExportedCert.pfx", "1234");

So this is great, however I have to issue an openssl command to make a pfx file from the Certificate and the Private Key, then make up some password. Then include this password in my code.

I was wondering if this step was quite necessary. Is there a way to make up a X509Certificate2 from the Cert, and then apply the Private Key. The constructor arguments allow the Cert only part, but encrypting fails then because there is no private key.

Also, I don't want to rely on OpenSSL or IIS to export the pfx.... seems clumsy.

Ideally i would like:

sslCertificate = new X509Certificate2("myCert.crt");
sslCertificate.ApplyPrivateKey(keyBytes) // <= or "private.key" or whatever

sslStream.AuthenticateAsServer(sslCertificate, false, SslProtocols.Default, false);

Answer 1

There are a couple of different things you're asking for, with different levels of ease.

Attaching a private key to a certificate

Starting in .NET Framework 4.7.2 or .NET Core 2.0 you can combine a cert and a key. It doesn't modify the certificate object, but rather produces a new cert object which knows about the key.

using (X509Certificate2 pubOnly = new X509Certificate2("myCert.crt"))
using (X509Certificate2 pubPrivEphemeral = pubOnly.CopyWithPrivateKey(privateKey))
{
    // Export as PFX and re-import if you want "normal PFX private key lifetime"
    // (this step is currently required for SslStream, but not for most other things
    // using certificates)
    return new X509Certificate2(pubPrivEphemeral.Export(X509ContentType.Pfx));
}

on .NET Framework (but not .NET Core) if your private key is RSACryptoServiceProvider or DSACryptoServiceProvider you can use cert.PrivateKey = key, but that has complex side-effects and is discouraged.

Loading the private key

This one is harder, unless you've already solved it.

For the most part the answer for this is in Digital signature in c# without using BouncyCastle, but if you can move to .NET Core 3.0 things get a lot easier.

PKCS#8 PrivateKeyInfo

Starting in .NET Core 3.0 you can do this relatively simply:

using (RSA rsa = RSA.Create())
{
    rsa.ImportPkcs8PrivateKey(binaryEncoding, out _);
    // do stuff with the key now
}

(of course, if you had a PEM you need to "de-PEM" it, by extracting the contents between the BEGIN and END delimiters and running it through Convert.FromBase64String in order to get binaryEncoding).

PKCS#8 EncryptedPrivateKeyInfo

Starting in .NET Core 3.0 you can do this relatively simply:

using (RSA rsa = RSA.Create())
{
    rsa.ImportEncryptedPkcs8PrivateKey(password, binaryEncoding, out _);
    // do stuff with the key now
}

(as above, you need to "de-PEM" it first, if it was PEM).

PKCS#1 RSAPrivateKey

Starting in .NET Core 3.0 you can do this relatively simply:

using (RSA rsa = RSA.Create())
{
    rsa.ImportRSAPrivateKey(binaryEncoding, out _);
    // do stuff with the key now
}

(same "de-PEM" if PEM).