Error "invalid distance code" with large backups

[josvos]

ab-decrypt works fine for small .ab files, but for large .ab files, e.g. a backup of 317 MB, I get at some point:

 zlib.error: Error -3 while decompressing data: invalid distance code

The partly written tar output file is ok for the part it already wrote (284 MB in case of the 317 MB backup), but it is incomplete.

This is with ab-decrypt 1.0.0 (pip install in a Pythoin 3.9.12 venv).

[joernheissler]

Hi,
can you try to supply a parameter to decompressobj? Perhaps try all values from range(-15, 48) in a loop and see which ones work. Or try 0 first.

[josvos]

Hi,

Values 0 and 15 give the same error as with no value (and produce the same tarball). All other values produce an empty tarball and give one of these errors:

zlib.error: Error -3 while decompressing data: invalid stored block lengths
zlib.error: Error -3 while decompressing data: invalid window size
zlib.error: Error -3 while decompressing data: incorrect header check

[joernheissler]

Is the backup encrypted? If not, please do encrypt it.
For larger backups (4-5 GiB) I get Invalid padding bytes which sounds like either an interrupted transfer or a bug in android.

[josvos]

Is the backup encrypted?

Yes, it is.

[joernheissler]

I'm unable to reproduce the issue.

You could modify the code to not decompress the backup, i.e. it outputs compressed data. Then try using this java program to decompress your backup:

import java.util.zip.Inflater;
import java.util.zip.InflaterOutputStream;
import java.io.BufferedOutputStream;

public class Inflate
{
    public static void main(String[] args) throws Exception
    {
        Inflater inflater = new Inflater();
        InflaterOutputStream stream = new InflaterOutputStream(System.out, inflater);
        byte[] buf = new byte[4096];
        for (;;) {
            int len = System.in.read(buf);
            if (len < 0) {
                break;
            }
            stream.write(buf, 0, len);
        }
        stream.flush();
        stream.close();
    }
}

If that gives the same error I assume there's a transmission error (CBC encryption mode wouldn't detect this!) or a bug in your android version.
But if it works the bug might be in my python program or somewhere in python or zlib. But to investigate further, I'd still need some way to reproduce this.