One of those nasty file read/write issues

This is a discussion on One of those nasty file read/write issues within the C++ Programming forums, part of the General Programming Boards category; So, I've been working on this object-oriented xor file encryptor of mine (Yeah, I know, what's the point), and I've ...

  1. #1
    Kiss the monkey. CodeMonkey's Avatar
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    One of those nasty file read/write issues

    So, I've been working on this object-oriented xor file encryptor of mine (Yeah, I know, what's the point), and I've come to an almost-done roadblock. It seems that for text and bitmap files my algorith functions correctly. However, ecrypting and then decrypting a wav file yields a "wrong format" erro when I try to play it in winamp. Obviously, I messed up some bits. Unfortunately, I don't know which bits or why. I've included excerpts from functions that encode and decode files. Yes, I'm using ios::binary.

    Encoding:
    Code:
    //The actual encoding (writing)
     
     unsigned long int counter = 0;
     writer.open(encoded_path.c_str(),ios::binary);
     if(!writer) return CRYPT_FAIL;
     
     writer.write("XOR_CODEC_ENCRYPTED_FILE@",25); //write branding
     writer.write(orig_name.c_str(),orig_name.size());//write original file name
     writer.put('@');//separation character
     for(int y = 0; y < password.size(); y++) writer.put( (password[y] ^ PASSWORD_KEY) );
     writer.put('@'); //separation character
     writer.write(key.c_str(),key.size()); //write key
     writer.put('@'); //separation character
     
     char in_byte = '\0';
     cout << "Encrypting..." << endl;
     while( !((reader.get(in_byte)).eof()) )     // while get() does not set eof bit (get() into in_byte)
     {
      //ok, so this reads through input (with get) and writes char by char (using put)
      //    the xor-ed data. Modulus is used so counter can increase
      //    to whatever, while for key[index], index { [0,255]
      //    Thus, the key is effectively cycled through continuously
      writer.put( static_cast<char>(in_byte ^ key[counter % KEY_SIZE]) );
      counter++;
     }
     cout << "Encryption complete." << endl;
     reader.clear();//eof
     reader.seekg(0,ios::beg);//rewind
     //Note: as currently written, the encoder has a 4,294,967,295 byte limit ( max_val(unsigned long int) )
     encoded = true;
    Now, when the decoder is called, the seek pointer on the encoded file is already past my little header, so (theoretically) only the original contents of the file are written):
    Code:
     unsigned long int counter = 0;
     char in_byte = '\0';
     cout << "Decrypting..." << endl;
     while( !((reader.get(in_byte)).eof()) )     // while get() does not set eof bit (get() into in_byte)
     {
      //ok, so this reads through input (with get) and writes char by char (using put)
      //    the un-xor-ed data. Modulus is used so counter can increase
      //    to whatever, while for key[index], index { [0,255]
      //    Thus, the key is effectively cycled through continuously
      writer.put( static_cast<char>(in_byte ^ key[counter % KEY_SIZE]) );
      counter++;
     }
     cout << "Decryption complete." << endl;
     reader.clear();//eof
     reader.seekg(0,ios::beg);//rewind
    PS: I apologize in advance for using eof() a loop conditional, but I don't think that's the problem (correct me if I'm wrong).
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  2. #2
    and the hat of wrongness Salem's Avatar
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    1. Are the input and output files the same size?

    2. Prepare a 256 byte file with each byte set to 00, 01, 02 ... FF
    - then encrypt it and decrypt it back to another file.
    - then compare one set of 256 bytes with another set of 256 bytes.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    CSharpener vart's Avatar
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    have you tried the fc to understand which and how many bytes are broken?
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
    Kiss the monkey. CodeMonkey's Avatar
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    I will certainly try Salem's idea. What do you mean, vart? fc?
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  5. #5
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    >for(int y = 0; y < password.size(); y++) writer.put( (password[y] ^ PASSWORD_KEY) );
    Did you try decrypting the password, and see what it prints?

  6. #6
    and the hat of wrongness Salem's Avatar
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    fc is the command line compare tool

    fc /b oldfile newfile
    Will tell you the positions where corresponding bytes in two files differ.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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