Here is the original binary found on one of the many Qisda ES600 firmwares.
That executable is a trimmed-down copy of BusyBox 1.7.2 that only includes the following applets:
targzipuncompress
While the former one was wisely renamed to qdtar and slightly modified to skip the custom header, the latter ones seem to remain untouched.
I didn't bother to make a full static reverse engineering of the binary, as considering it a black box was enough for documentation purposes.
I've tried to request the source code of this binary, because BusyBox is licensed as GPLv2, and any derived work from it must follow the same license. Sadly, bq never answered my emails.
Sample output of running qdtar renamed as busybox:
BusyBox v1.7.2 (2010-10-27 14:58:03 CST) multi-call binary
Copyright (C) 1998-2006 Erik Andersen, Rob Landley, and others.
Licensed under GPLv2. See source distribution for full notice.
Usage: busybox [function] [arguments]...
or: [function] [arguments]...
BusyBox is a multi-call binary that combines many common Unix utilities into a single executable. Most people will create a
link to busybox for each function they wish to use and BusyBox
will act like whatever it was invoked as!
Currently defined functions:
gzip, qdtar, uncompress