Can not get signature

问题

I need sign data with certificate. My method to sign:

private static string CreateSignature(string signatureString)
    {
        SHA1CryptoServiceProvider sha1 = new SHA1CryptoServiceProvider();
        X509Certificate2 cert = new X509Certificate2(HttpContext.Current.Server.MapPath("~/eCommerceConnectCrfs/test-server.cert"), "password");
        RSACryptoServiceProvider rsaCryptoIPT = (RSACryptoServiceProvider)cert.PrivateKey;
        ASCIIEncoding encoder = new ASCIIEncoding();
        byte[] binData = encoder.GetBytes(signatureString);
        byte[] binSignature = rsaCryptoIPT.SignData(binData, sha1);
        return Convert.ToBase64String(binSignature);
    }

But field cert.PrivateKey is NULL. I have *.pem file with private key but I don't known how use it. Pls, help get signature!

来源：https://stackoverflow.com/questions/11184890/can-not-get-signature