signature with SHA256

Question

I have a smartcard and I need to sign a file with this. That is a big problem as I see in stackover.

I couldnt use RSACryptoServiceProvider, bkz it doesnt support RSA-SHA256 alogrithm.

At First I used CAPICOM.dll , like code bellow,

SignedData sed = new SignedData();
sed.Content = "a"; // data to sign
Signer ser = new Signer();
ser.Certificate = cc;
string singnn = sed.Sign(ser, false, CAPICOM_ENCODING_TYPE.CAPICOM_ENCODE_BASE64);

But there isnt a public key to validate my signature value,, I couldnt get a validate key from capicom.dll.

And after ,

I used X509Certificate2 , and RSACryptoServiceProvider like code below,

        X509Certificate2 certificate = new X509Certificate2();
        // Access Personal (MY) certificate store of current user
        X509Store my = new X509Store(StoreName.My, StoreLocation.CurrentUser);
        my.Open(OpenFlags.ReadOnly);

        // Find the certificate we'll use to sign            
        RSACryptoServiceProvider csp = null;
        foreach (X509Certificate2 cert in my.Certificates)
        {
            if (cert.Subject.Contains(certSubject))
            {
                // We found it. 
                // Get its associated CSP and private key
                certificate = cert;
                csp = (RSACryptoServiceProvider)cert.PrivateKey;
            }
        }
        if (csp == null)
        {
            throw new Exception("No valid cert was found");
        }

        // Hash the data
        SHA1Managed sha1 = new SHA1Managed();
        UnicodeEncoding encoding = new UnicodeEncoding();
        byte[] data = encoding.GetBytes(text);
        byte[] hash = sha1.ComputeHash(data);

        //byte[] data = Encoding.UTF8.GetBytes(text);
        //HashAlgorithm sha = new SHA256Managed();
        //byte[] hash = sha.TransformFinalBlock(data, 0, data.Length);

        string key = csp.ToXmlString(false);
        // Sign the hash
        csp.PersistKeyInCsp = true;
        byte[] response = csp.SignData(data, CryptoConfig.MapNameToOID("SHA1"));
        string signbase64 = Convert.ToBase64String(response);

It works , but I need to sign with RSA-SHA256 algorithm. When I changw hash algorithm like this

byte[] response = csp.SignData(data, CryptoConfig.MapNameToOID("SHA256"));

I m getting an

error : "unspecified error".

Thats my problem, What is the sollution , or which library should I use ??

Thanks for any advice..

Mensuration

Answer 1

RSACryptoServiceProvider does work with SHA2-based signatures, but you have to invest some effort into it.

When you use a certificate to get your RSACryptoServiceProvider it really matters what's the underlying CryptoAPI provider. By default, when you create a certificate with 'makecert', it's "RSA-FULL" which only supports SHA1 hashes for signature. You need the new "RSA-AES" one that supports SHA2.

So, you can create your certificate with an additional option: -sp "Microsoft Enhanced RSA and AES Cryptographic Provider" (or an equivalent -sy 24) and then your code would look like (in .NET 4.0):

var rsa = signerCertificate.PrivateKey as RSACryptoServiceProvider;
//
byte[] signature = rsa.SignData(data, CryptoConfig.CreateFromName("SHA256"));

If you are unable to change the way your certificate is issued, there is a semi-ligitimate workaround that is based on the fact that by default RSACryptoServiceProvider is created with support for SHA2. So, the following code would also work, but it is a bit uglier: (what this code does is it creates a new RSACryptoServiceProvider and imports the keys from the one we got from the certificate)

var rsa = signerCertificate.PrivateKey as RSACryptoServiceProvider;
// Create a new RSACryptoServiceProvider
RSACryptoServiceProvider rsaClear = new RSACryptoServiceProvider();
// Export RSA parameters from 'rsa' and import them into 'rsaClear'
rsaClear.ImportParameters(rsa.ExportParameters(true));
byte[] signature = rsaClear.SignData(data, CryptoConfig.CreateFromName("SHA256"));

Hope you find this helpful.

Distinguishing

For the second workaround (importing keys from certificate from the certificate into a new csp), how will that work for hardware keys? Are those keys exportable? – Virile

Hardware keys are not exportable – Knot