Changes

This question was given a fair amount of consideration. Ultimately, we decided to go with a custom file format because using libjar would have required a fair bit of hacking. Writing custom code was a simpler option, and it resulted in less code (mar_read.c is less than 300 lines of code). Moreover, the update system does not need a standard file format. The elements stored in the archive are bzip2 compressed binary diffs, generated using a variation of bsdiff. So, being able to unpack the archive file using standard tools wouldn't be very useful in and of itself.