tbf/Decrypt/Program.cs

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

namespace Decrypt
{
    /// <summary>
    /// See also:
    /// https://www.fluxbytes.com/csharp/encrypt-and-decrypt-files-in-c/
    /// </summary>
    class Program
    {
        static byte[] key = new byte[] { 83, 254, 105, 64, 184, 201, 195, 127, 52, 99, 77, 45, 252, 132, 163, 156 };
        static byte[] IV = 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];
            string intermediateFile = fileName + ".plain";  /// Intermediate file, output of cryptography decoding
            string backupFile = fileName + ".bak";

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

                bool successful = false;
                using (RijndaelManaged aes = new RijndaelManaged { Padding = PaddingMode.Zeros })
                {
                    using (FileStream fsCrypt = new FileStream(fileName, FileMode.Open))
                    {
                        using (FileStream fsOut = new FileStream(intermediateFile, FileMode.Create))
                        {
                            using (ICryptoTransform decryptor = aes.CreateDecryptor(key, IV))
                            {
                                using (CryptoStream cs = new CryptoStream(fsCrypt, decryptor, CryptoStreamMode.Read))
                                {
                                    int data;
                                    while ((data = cs.ReadByte()) != -1)
                                    {
                                        if (data != 0) fsOut.WriteByte((byte)data);
                                    }
                                    successful = true;
                                }
                            }
                        }
                    }
                }

                if (successful)
                {
                    /// File decryption successfuly completed => rotate files
                    if (File.Exists(backupFile)) File.Delete(backupFile);
                    File.Move(fileName, backupFile);
                    File.Move(intermediateFile, fileName);
                    return;
                }
                else
                {
                    Failed(fileName, intermediateFile, "");
                    return;
                }
            }
            catch (Exception e)
            {
                Failed(fileName, intermediateFile, e.Message);
                return;
            }
        }

        static void Failed(string fileName, string toBeDeleted, string message)
        {
            if (File.Exists(toBeDeleted))
            {
                File.Delete(toBeDeleted);
            }
            Console.WriteLine(string.Format("Decryption of file {0} failed: {1}", fileName, message));
            Console.ReadKey();
        }
    }
}