// Create message and signature on your end
string message = "Here is the license message";

var converter = new ASCIIEncoding();
byte[] plainText = converter.GetBytes(message);

var rsaWrite = new RSACryptoServiceProvider();
var privateParams = rsaWrite.ExportParameters(true);

// Generate the public key / these can be sent to the user.
var publicParams = rsaWrite.ExportParameters(false);

byte[] signature =
    rsaWrite.SignData(plainText, SHA1.Create());

// Verify from the user's side. Note that only the public parameters
// are needed.
var rsaRead = new RSACryptoServiceProvider();
rsaRead.ImportParameters(publicParams);
if (rsaRead.VerifyData(plainText,
                       SHA1.Create(),
                       signature))
{
    Console.WriteLine("Verified!");
}
else
{
    Console.WriteLine("NOT verified!");
}