using System;
using System.IO;
using System.Security.Cryptography;

namespace Decode
{
    class Program
    {
        static byte[] cryptoKey = new byte[] { 235, 7, 236, 58, 24, 225, 97, 102, 231, 1, 121, 12, 177, 145, 87, 200,
                                               102, 85, 152, 73, 230, 203, 101, 169, 242, 213, 228, 219, 239, 157, 245, 156 };

        static byte[] cryptoIV = new byte[] { 189, 23, 137, 56, 204, 241, 242, 118, 28, 89, 9, 198, 224, 111, 186, 125 };

        static void Main(string[] args)
        {
            if ((args.Length < 1) || !File.Exists(args[0]))
            {
                Console.WriteLine("Usage:  Decode <file>");
                Console.ReadKey();
                return;
            }

            string fileName = args[0];

            try
            {
                using (StreamReader reader = new StreamReader(fileName))
                {
                    if (reader.ReadLine().StartsWith("<?xml version="))
                    {
                        Console.WriteLine(string.Format("File {0} was not encrypted.", fileName));
                        Console.ReadKey();
                        return;
                    }
                }

                bool successful = false;
                Stream stream = File.OpenRead(fileName);
                using (BinaryReader binaryReader = new BinaryReader(stream))
                {
                    int length = (int)(new FileInfo(fileName).Length);

                    /// Load encrypted binary data into an array
                    byte[] encrypted = new byte[length];
                    int bytesRead = binaryReader.Read(encrypted, 0, length);
                    if (bytesRead != length)
                    {
                        Console.WriteLine(string.Format("Failed to read file {0}.", fileName));
                        Console.ReadKey();
                        return;
                    }

                    using (RijndaelManaged myRijndael = new RijndaelManaged())
                    {
                        /// Create a decryptor to perform the stream transform.
                        ICryptoTransform decryptor = myRijndael.CreateDecryptor(cryptoKey, cryptoIV);

                        /// Create the streams used for decryption.
                        using (MemoryStream msDecrypt = new MemoryStream(encrypted))
                        {
                            using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                            {
                                using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                                {
                                    using (TextWriter writer = new StreamWriter(fileName + ".plain"))
                                    {
                                        writer.Write(srDecrypt.ReadToEnd());
                                        successful = true;
                                    }
                                }
                            }
                        }
                    }
                }

                if (successful)
                {
                    /// Decoding the file was successfuly completed => rotate files
                    if (File.Exists(fileName + ".bak")) File.Delete(fileName + ".bak");
                    File.Move(fileName, fileName + ".bak");
                    File.Move(fileName + ".plain", fileName);
                    return;
                }
                else
                {
                    Failed(fileName);
                    return;
                }
            }
            catch (Exception)
            {
                Failed(fileName);
                return;
            }
        }

        static void Failed(string fileName)
        {
            if (File.Exists(fileName + ".plain"))
            {
                File.Delete(fileName + ".plain");
            }
            Console.WriteLine(string.Format("Decoding file {0} failed.", fileName));
            Console.ReadKey();
        }
    }
}