You can look at the sample FuelSDK C# application available on Github. It does not do encryption by itself but it's easy to change the source code to do encryption/ decryption in C#. Something along these lines (may vary based on encryption technique you want to use)
E.g.
Encode
public static string Base64Encode(string plainText) {
var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);
return System.Convert.ToBase64String(plainTextBytes);
}
Decode
public static string Base64Decode(string base64EncodedData) {
var base64EncodedBytes = System.Convert.FromBase64String(base64EncodedData);
return System.Text.Encoding.UTF8.GetString(base64EncodedBytes);
}
There are encryption methods available in AmpScript as well but for a C# application, it might be the simplest solution to use custom methods such as above.