signature with SHA256

rsa, rsacryptoserviceprovider, sha1, sha256, signature

Solution

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.

Problem

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..

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