Best way to initiate RSACryptoServiceProvider from x509Certificate2?

Question

What is the best way to initate a new RSACryptoServiceProvider object from an X509Certificate2 I pulled out of a key store? The certificate is asso

Answer 1

The recommended way is to use RSA base class and call certificate.GetRSAPrivateKey().

RSA publicKeyProvider = certificate.GetRSAPrivateKey();

Since .NET 4.6, casting to RSACryptoServiceProvider as suggested by @blowdart is no longer recommended. This is even more an issue now since there are several versions of .NET (such as .NET Core).

By casting to RSACryptoServiceProvider that way, there is a good chance you might get this cast exception (depending on the platform and libraries used):

Unable to cast object of type 'System.Security.Cryptography.RSACng' to type 'System.Security.Cryptography.RSACryptoServiceProvider'

The reason is the actual implementation could be different from each platform, on Windows RSACng is used.

Here is a link that describes this issue (look for answer by Jeremy Barton).

Answer 2

RSACryptoServiceProvider publicKeyProvider = 
    (RSACryptoServiceProvider)certificate.PublicKey.Key;

and

RSACryptoServiceProvider privateKeyProvider = 
    (RSACryptoServiceProvider)certificate.PrivateKey;

The key property on the public or private key property of the certificate is of type AsymmetricAlgorithm.

Answer 3

Blowdart's answer is indeed correct. However, for clarity I should point out that if you want your RSACryptoServiceProvider instance to contain both the public and private keys of the X509 certificate (assuming the certificate does have a private key). Check the certificate's HasPrivateKey property.

RSACryptoServiceProvider rsa;
if (cert.HasPrivateKey)
    rsa = (RSACryptoServiceProvider)cert.PrivateKey;
else
    rsa = (RSACryptoServiceProvider)cert.PublicKey.Key;

In the case of RSA when only the public key is present the RSA Parameters will be only Exponent and Modulus, all others will be null; If on the other hand the private key is present the RSA Parameters will contain D, DP, DQ, Exponent, InverseQ, Modulus, P and Q.