Friday, May 23, 2014

Decoding HTML pages with Content-Encoding : deflate

All web servers do not implement zlib protocol the same way when they return data with Content-Encoding set to deflate. Some servers return a zlib header as specified in RFC 1950, but some return the compressed data alone.

Java Inflator class can be used to deal with both cases, but first we must check for the header. The first two bytes denote the header and it is a simple check :

    static boolean isZlibHeader(byte[] bytes) {
        //deal with java stupidity : convert to signed int before comparison
        char byte1 = (char)(bytes[0] & 0xFF);
        char byte2 = (char)(bytes[1] & 0xFF);
        return byte1 == 0x78 && (byte2 == 0x01 || byte2 == 0x9c || byte2 == 0xDA);

    private void inflateToFile(byte[] encBytes, int offset, int size, BufferedOutputStream f) throws IOException {
        Inflater inflator = new Inflater(true);
        inflator.setInput(encBytes, isZlibHeader(encBytes) ? offset+2 : offset, isZlibHeader(encBytes) ? size-2 : size);
        byte[] buf = new byte[4096];
        int nbytes = 0;
        do {
            try {
                nbytes = inflator.inflate(buf);
                if (nbytes > 0) {
            } catch (DataFormatException e) {
                //handle error
        } while (nbytes > 0);

An example URL that had to be processed this way : Here is the Wireshark capture, showing the Content-Encoding set to deflate as well as the de-chunked header (the first 2 bytes "78 9c") at the lower bottom pane of the display:

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