Get Private Key from BouncyCastle X509 Certificate? C#

Question

Normally when I grab an X509Certificate2 out of my keystore I can call .PrivateKey to retrieve the cert's private key as an AsymmetricAlgorithm. However I have decided to use Bouncy Castle and its instance of X509Certificate only has a getPublicKey(); I cannot see a way to get the private key out of the cert. Any ideas?

I get the an X509Certificate2 from my Windows-MY keystore then use:

//mycert is an X509Certificate2 retrieved from Windows-MY Keystore
X509CertificateParser certParser = new X509CertificateParser();
X509Certificate privateCertBouncy = certParser.ReadCertificate(mycert.GetRawCertData());
AsymmetricKeyParameter pubKey = privateCertBouncy.GetPublicKey();
//how do i now get the private key to make a keypair?

Is there anyway to convert a AsymmetricAlgorithm(C# private key) to a AsymmetricKeyParameter(bouncycastle private key)?

Answer 1

Don't know BouncyCastle that much but it seems to me that the simple thing to do is to recreate the key based on the key parameters.

public static AsymmetricKeyParameter TransformRSAPrivateKey(
    AsymmetricAlgorithm privateKey)
{
    RSACryptoServiceProvider prov = privateKey as RSACryptoServiceProvider;
    RSAParameters parameters = prov.ExportParameters(true);
    
    return new RsaPrivateCrtKeyParameters(
        new BigInteger(1,parameters.Modulus),
        new BigInteger(1,parameters.Exponent),
        new BigInteger(1,parameters.D),
        new BigInteger(1,parameters.P),
        new BigInteger(1,parameters.Q),
        new BigInteger(1,parameters.DP),
        new BigInteger(1,parameters.DQ),
        new BigInteger(1,parameters.InverseQ));
}

You can call the code by using

AsymmetricKeyParameter bouncyCastlePrivateKey = 
    TransformRSAPrivateKey(mycert.PrivateKey);

Obviously this assumes that the certificate includes a RSA Key but the same result can be achieved for DSA with DSACryptoServiceProvider and DSAParameters.