string secret = "My secret message";
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(512);  // Key bits length
/*
* Skip the loading part for the RSACryptoServiceProvider will generate
* random Private / Public keys pair, that you can save later with
* rsa.ToXmlString(true);
* 
string key = "private or public key as xml string";
rsa.FromXmlString(key);
*/
// Convert the string to byte array
byte[] secretData = Encoding.UTF8.GetBytes(secret);

// Encrypt it using the private key:
byte[] encrypted = rsa.PrivareEncryption(secretData);

// Decrypt it using the public key
byte[] decrypted = rsa.PublicDecryption(encrypted);
string decString = Encoding.UTF8.GetString(decrypted);  // And back to string
Assert.AreEqual("My secret message", decString);