[C#] Make the file unreadeable. (some kind of encryption)

Question

Hello there guys! I'm using encryption in my program, that decrypts files if it needs to use it, and encrypts it after it done with the file. But my files arouznd 100mb, and it takes more than a minute to crypt them.

My question is, is it possible, to make my file unreadable, without encrypting the whole file? So , for example, just encrypting some byte, or I dont know how to do it. I need to reduce my crypting time. Thanks a lot!

Edit1: Sorry, forgot. The file iam talking about, is MPQ files, that a game using (wow), to read from it model files, music , etc... so its not a text file. It can be opened by a special program (MPQ Editor). I edited some files, and I made my own MPQ file, and I want to protect it from other users, to open it with MPQ Editor. My program can open mpq files too. But the mpq file is created with this mpq editor thing. I just want to make it unreadeably to mpq editor. And if my program will use it, it should decrypt it, and if my program done with the MPQ file, it should encrypt it again, to block others to edit it.

Edit2: Ok, to make it clear. There is a program name world of warcraft. It reads game files from MPQ files, MPQ file is developped by Wow (Blizzard Co) , but It can be opened by some programs, editors. I've got an edited MPQ file, (replaced music) and I uploaded it to my server. My program downloads it to other players game folder, and they will got this change ingame, like me :) They will hear other musics too. (just example.) But I dont want them to edit / or see my MPQ file with any editor program, so I implented a simple encryption , decryption to my program. My MPQ file is uploaded encrypted to my server. When they start the game, my program decrypts the file (yes, they will be able to edit it now, but they dont know this :P , its basic security), so the game will reckognise the MPQ file, and it will load it , with the changes within it. When the game program closes, my program encrypts back the patch, so it will be unreadable again. I hope you understand the process. The thing is, my MPQ (patch) file is ~100mb big. It means, it takes ~1minute to encrypt / decrypt them. And this is what I want to replace, the crypting method. Must encrypt it with something better method, and decrypt it when it needed.

Answer 1

If by unreadable you mean that the data will not be plain text you can use binary Serialization/De-serialization.

Answer 2

Read an arbitrary number of bytes from the start and append them to the end, then for the backwards trek - reverse. This should stop an application from recognising the file as a valid type. This won't make text in a file unreadable though.

Answer 3

if it is just text - no. You will have a bad character in it, but it will still be readable. If it is a custom binary format, people would have to reverse engineer the file anyway, so there is no use.

Answer 4

You can use a poor mans encryption using xor

        int key = 83847; // whatever value you want
        string str = "This is my file";

        // "Encrypt"
        var sb = new StringBuilder(str.Length);
        foreach (var ch in str)
        {
            sb.Append((char)(ch ^ key));
        }

        Console.WriteLine(sb.ToString()); // Garbled text

        // "Decrypt"
        var sbReversed = new StringBuilder(str.Length);
        foreach (var ch in sb.ToString())
        {
            sbReversed.Append((char)(ch ^ key));
        }

        Console.WriteLine(sbReversed.ToString()); // "This is my file"

Answer 5

You can try to use the TripleDESCryptoServiceProvider.

Encrypt Text Using TripleDESCryptoServiceProvider | C# Snippet | SnippetBank.Net

Answer 6

Using DES (64-bit which is faster that rijndael) it takes me less than 5s to process a 112Mb File. Have you tried it ?

Nevertheless, you gave me a funny idea : Encrypt half of the block of your input file and keep the other half cleartext. (You'll get 2 files ... and you must forget about security obviously)

It cut down the processing time by ~40% and you can adjust the ratio encrypted/cleartext blocks to gain time :

        private static void EncryptData(String inName, String outName, byte[] desKey, byte[] desIV)
    {  
        //Create the file streams to handle the input and output files.
        FileStream fin = new FileStream(inName, FileMode.Open, FileAccess.Read);
        FileStream fout = new FileStream(outName, FileMode.OpenOrCreate, FileAccess.Write);
        FileStream foutclear = new FileStream(outName + ".clear", FileMode.OpenOrCreate, FileAccess.Write);
        fout.SetLength(0);

        //Create variables to help with read and write.
        byte[] bin = new byte[4096]; //This is intermediate storage for the encryption.
        long rdlen = 0;              //This is the total number of bytes written.
        long totlen = fin.Length;    //This is the total length of the input file.
        int len;                     //This is the number of bytes to be written at a time.

        System.Security.Cryptography.SymmetricAlgorithm des = SymmetricAlgorithm.Create("DES"); // Using DES      
        CryptoStream encStream = new CryptoStream(fout, des.CreateEncryptor(desKey, desIV), CryptoStreamMode.Write);
        Console.WriteLine("Encrypting...");

        //Read from the input file, then encrypt and write to the output file.
        while (rdlen < totlen)
        {
            len = fin.Read(bin, 0, 4096);
            encStream.Write(bin, 0, len);
            rdlen = rdlen + len;
            len = fin.Read(bin, 0, 4096);
            foutclear.Write(bin, 0, len);
            rdlen = rdlen + len;
        }

        encStream.Close();
        foutclear.Close();
        fout.Close();
        fin.Close();        
    }

Answer 7

I don't know anything about mpq files, but my first question would be, "how does the editor recognize that this is an mpq file?". If it is, for example, that there is a certain value in the first 3 bytes, just change those bytes and the editor will fail. Change them back and you can read it. Voila. If some other mechanism, same logic.

The point being, the file does not need to be encrypted at all - merely tweaked so that the mpq editor will not recognize it as a valid file for opening.