Associate a private key with the X509Certificate2 class in .net

Question

I'm working on some code that creates a X509certificate and a public/private key pair. The public key is added to the certificate and it is sent to an CA which signs it.

The returned certificate is then accessed through the System.Security.Cryptography.X509Certificates.X509Certificate2 class. Now I want to use this certificate to initiate a secure connection with other clients. Therefore I use the SslStream class. To start the SSL Handshake I use this method:

server.AssociatedSslStream.AuthenticateAsServer(
                        MyCertificate,                      // Client Certificate
                        true,                               // Require Certificate from connecting Peer
                        SslProtocols.Tls,                   // Use TLS 1.0
                        false                               // check Certificate revocation
                    );

This method requires that the private key is associated with the certificate. Of course the certificate returned by the CA does not contain a private key. But it is stored as .key file on the harddrive. The X509Certificate2 class has a property called PrivateKey which I guess will associate a private key with the certificate, but I can't find a way to set this property.

Is there any way I can associate the private key with the .net X509 class?

Answer 1

You can save yourself the hassle of copy-pasting all that code and store the private key next to the certificate in a pfx/pkcs#12 file:

openssl pkcs12 -export -in my.cer -inkey my.key -out mycert.pfx

You'll have to supply a password, which you have to pass to the constructor of X509Certificate2:

X509Certificate2 cert = new X509Certificate2("mycert.pfx","password");

Answer 2

For everyone else with the same problem, I found a neat little piece of code that let's you do exactly that:

http://www.codeproject.com/Articles/162194/Certificates-to-DB-and-Back

byte[] certBuffer = Helpers.GetBytesFromPEM(publicCert, PemStringType.Certificate);
byte[] keyBuffer  = Helpers.GetBytesFromPEM(privateKey, PemStringType.RsaPrivateKey);

X509Certificate2 certificate = new X509Certificate2(certBuffer, password);

RSACryptoServiceProvider prov = Crypto.DecodeRsaPrivateKey(keyBuffer);
certificate.PrivateKey = prov;

EDIT: The code for the Helper method (which otherwise requires a codeproject login) is as follows:

public static byte[] GetBytesFromPEM(string pemString, PemStringType type)
{
    string header; string footer;
    switch (type)
    {
        case PemStringType.Certificate:
            header = "-----BEGIN CERTIFICATE-----";
            footer = "-----END CERTIFICATE-----";
            break;
        case PemStringType.RsaPrivateKey:
            header = "-----BEGIN RSA PRIVATE KEY-----";
            footer = "-----END RSA PRIVATE KEY-----";
            break;
        default:
            return null;
    }

    int start = pemString.IndexOf(header) + header.Length;
    int end = pemString.IndexOf(footer, start) - start;
    return Convert.FromBase64String(pemString.Substring(start, end));
}