Ytukyg

This is a bit related to the answer to X509Certificate2 from store with private key.

It seems that when I want to use SHA256withRSA I can't use service provider directly from the certificate's PrivateKey - I need to create new crypto service provider:

  var bytes = new byte { 0, 1, 2, 3 };



  //_cert - X509Certificate2 with private key

  //csp1 is of type I need, but it won't work

  var csp1 = _cert.PrivateKey as RSACryptoServiceProvider;



  var cspParameters = new CspParameters

  {

    KeyContainerName = csp1.CspKeyContainerInfo.KeyContainerName,

    KeyNumber = csp1.CspKeyContainerInfo.KeyNumber == KeyNumber.Exchange ? 1 : 2,

  };



  var csp2 = new RSACryptoServiceProvider(cspParameters);



  //I can't use csp1 here - will throw "CryptographicException : Invalid algorithm specified."

  //I can use csp1 with "SHA1" though

  var signature = csp2.SignData(bytes, CryptoConfig.MapNameToOID("SHA256"));

I've found some information about this here:

https://blogs.msdn.microsoft.com/shawnfa/2008/08/25/using-rsacryptoserviceprovider-for-rsa-sha256-signatures/

But the above solution is taken from comments section and I don't really understand why I need to jump through hoops to use one of the common algorithms. So what I want to ask is:

• Why csp1 does not work with SHA256 exactly?


• Is it correct to create the csp2 like I did?


• Is there a better/newer way I can do it in .NET?

If needed the cert with private key can be generated as follows:

openssl req -x509 -sha256 -newkey rsa:2048 -keyout ./temp/key.key -out ./temp/crt.crt -days 10 –nodes

openssl pkcs12 -export -out .tempcert.pfx -inkey .tempkey.key –in .tempcrt.crt

It all depends on where your certificate is coming from. Like the MSDN comment says, if it's coming from Microsoft Base Cryptographic Provider, then it will not work with SHA256. This CSP came out with the first version of CryptoAPI back in 1996, and does not understand SHA256 because SHA256 was not around back then.

The way to check and handle this gracefully would be:

public byte SignData(RSACryptoServiceProvider csp, byte bytes)

{

    byte sig = null;

    if ((csp.CspKeyContainerInfo.ProviderType == PROV_RSA_FULL || csp.CspKeyContainerInfo.ProviderType == PROV_RSA_SCHANNEL) && !csp.CspKeyContainerInfo.HardwareDevice)

    {

        var cspParameters = new CspParameters

        {

            KeyContainerName = csp.CspKeyContainerInfo.KeyContainerName,

            KeyNumber = csp.CspKeyContainerInfo.KeyNumber == KeyNumber.Exchange ? 1 : 2,

        };

        using (var csp2 = new RSACryptoServiceProvider(cspParameters))

        {

            sig = csp2.SignData(bytes, CryptoConfig.MapNameToOID("SHA256"));

        }

    }

    else {

        sig = csp.SignData(bytes, CryptoConfig.MapNameToOID("SHA256"));

    }

    return sig;

}

FYI the CryptoAPI is being deprecated in favor of Cryptography API: Next Generation. One way to do what you want with CNG in C# would be with System.Security.Cryptography.Cng:

...

using (RSA rsa = new RSACng())

{

    byte signature = rsa.SignData(message, hashAlgorithm, paddingMode);

    ...

}