C# RSA Encrypt Private Decrypt Public?

Here is the problem I am trying to solve. Let's say I was releasing some web software built on ASP.NET MVC and I only wanted the end user to be able to add X users to the system. The software would be hosted on their servers.

I want to include an encrypted file that when the user tries to add a new user, it goes out and reads from the file an encrypted string. When the website decodes it, the clear text will be the number of allowed users.

What is the best/simplest way on my end to encrypt to generate this string on my end then decode it back to clear text in my application? Obviously I want to ensure that the end user cannot be spinning up their own encrypted string and just replace mine. I don't want to worry about having to try and obfuscate my source so that they would not be able to see how I decode the string.

Is it possible to encrypt with a private rsa key, then decrypt it with the public one? I haven't had luck with that in the code below:

        var rsa = new RSACryptoServiceProvider();

        var pubicKey = rsa.ToXmlString(false);
        var privateKey = rsa.ToXmlString(true);

        var test = "this string needs to be encrypted then decrypted";

        var rsa2 = new RSACryptoServiceProvider();
        rsa2.FromXmlString(privateKey);

        var encryptedBytes = rsa2.Encrypt(Encoding.UTF8.GetBytes(test), false);
        var encryptedString = Convert.ToBase64String(encryptedBytes);

        var rsa3 = new RSACryptoServiceProvider();
        rsa3.FromXmlString(pubicKey);

        encryptedBytes = Convert.FromBase64String(encryptedString);

        var decryptedString = Encoding.UTF8.GetString(rsa3.Decrypt(encryptedBytes,                        false));      

You can use a signature strategy, where the private key is used to generate a signature that verifies that your message is authentic.

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

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

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, new SHA1CryptoServiceProvider());

// 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,
                       new SHA1CryptoServiceProvider(),
                       signature))
{
    Console.WriteLine("Verified!");
}
else
{
    Console.WriteLine("NOT verified!");
}

This example was largely copied from Microsoft's site:

• RSACryptoServiceProvider.SignData Method (Byte[], Object)

And here is web page that explains the concept:

• Using RSA for Signing Messages

I think what you are looking for is digital signature. It doesn't matter if the content is encrypted or not, since the user has the (public) key to decrypt it. All that matters is if the content's source is you. Since you have a config file I reckon it is XML, so you are looking for XMLDSIG.

You can easily achieve this using the SignedXml class in .Net. Then all you need to do is verify the signature when loading the config file. This method allows you to eaisly use any X509 certificates you may have. You can even embed the public key in the signed file, so the user does not need to install your cert (public key).

Your idea is correct, but I wonder about unforeseen consequences.

You state they will be running software on their servers, so this means they are hosting a service for themselves. But you also mention this service has to connect out tot he internet to add a user by validating with your server. What happens when the internet goes down or they want to have a secure system and a firewall blocks internet access to the servers? Will they completely lose their ability to function?

Just giving you a question to ask yourself :p

You don't use a public key to 'decrypt' the file. you can only decrypt the file with the private key.

In your case you could store the number of users as well a signature of the data which you create with your private key.

On the clients sever, you use your public key to verify the signature matches the data in the file (number of users)

However, it is possible that an advanced user could swap out your public key with their own and sign the file themselves.

As stated in your comment in the question, your approach is using a key from a client and a key from yourself. This will not work, as the prime numbers used in RSA are meant for use with only the corresponding private/public key.

What you need to do use your two keys and nothing from the client or the client's two keys and nothing of yours. For example,

1. You could sign it using your two keys by encrypting with your private key and allowing the client to decrypt using your public key.
2. You could encrypt it using your client's public key and have them decrypt is using their (the client's) private key.

Hope this helps!