How to I find out the size of a GZIP section embedded in firmware?

Question

I am currently analyzing firmware images which contain many different sections, one of which is a GZIP section.

I am able to know the location of the start of the GZIP section using magic number and the GZIPInputStream in Java.

However, I need to know the compressed size of the gzip section. GZIPInputStream would return me the uncompressed file size.

Is there anybody who has an idea?

Answer 1

You can count the number of byte read using a custom InputStream. You would need to force the stream to read one byte at a time to ensure you don't read more than you need.

---

You can wrap your current InputStream in this

class CountingInputStream extends InputStream {
    final InputStream is;
    int counter = 0;

    public CountingInputStream(InputStream is) {
        this.is = is;
    }

    public int read() throws IOException {
        int read = is.read();
        if (read >= 0) counter++;
        return read;
    }
}

and then wrap it in a GZIPInputStream. The field counter will hold the number of bytes read.

To use this with BufferedInputStream you can do

 InputStream is = new BufferedInputStream(new FileInputStream(filename));
 // read some data or skip to where you want to start.

 CountingInputStream cis = new CountingInputStream(is);
 GZIPInputStream gzis = new GZIPInputStream(cis);
 // read some compressed data
 cis.read(...);

 int dataRead = cis.counter;