Question

I’m making a enciphering/deciphering program using XTEA algorithm. The encipher/decipher functions work fine, but when I encipher a file and then decipher it, I get some extra characters in the end of the file:

--- Original file ---
QwertY

--- Encrypted file ---
»¦æŸS@±­

--- Deciphered from encrypted ---
QwertY  ß*tÞÇ

I have no idea why the ” ß*tÞÇ” appears in the end.
I will post some of my code, but not all of it since it would be too long. The encipher/decipher function takes 64 bits data and 128 bits key, and encipher/decipher the data to the same block size, which is again 64 bits (similar functions here). It can then be written to a new file.

    long data[2]; // 64bits
    ZeroMemory(data, sizeof(long)*2);
    char password[16];
    ZeroMemory(password, sizeof(char)*16);

    long *key;
    if(argc > 1)
    {
        string originalpath = argv[1];
        string finalpath;
        string eextension = "XTEA";
        string extension = GetFileExtension(originalpath);
        bool encipherfile = 1;

        if(extension.compare(eextension) == 0) // If extensions are equal, dont encipher file
        {
            encipherfile = 0;
            finalpath = originalpath;
            finalpath.erase(finalpath.length()-5, finalpath.length());
        }

        ifstream in(originalpath, ios::binary);
        ofstream out(finalpath, ios::binary);

        cout << "Password:" << endl;
        cin.get(password,sizeof(password));
        key = reinterpret_cast<long *>(password);

        while(!in.eof())
        {
            ZeroMemory(data, sizeof(long)*2);
            in.read(reinterpret_cast<char*>(&data), sizeof(long)*2); // Read 64bits from file

            if(encipherfile == 1)
            {
                encipher(data, key);
                out.write(reinterpret_cast<char*>(&data), sizeof(data));
                continue;
            }
            if(encipherfile == 0)
            {
                decipher(data, key);
                out.write(reinterpret_cast<char*>(&data), sizeof(data));
            }
        }

Answer 1

Check for eof immediately after your read, and if you get eof break out of the loop.

If you may have partial reads (i.e. it is possible to read fewer than all of the requested bytes), then you need also to call gcount to find out how many bytes you actually read, thus:

cin.read( ... )
if( cin.eof() )
    {
    streamsize bytesRead = cin.gcount();
    if( bytesRead > 0 )
       // process those bytes
    break;
    }