Here's what happens when I compare them. (The "$ " is the prompt by my shell; it comes before the command I typed; the other lines are the output of "diff".)
$ diff old.rar new.rar Binary files old.rar and new.rar differ $ diff -a old.rar new.rar [ lots of binary gibberish ] This is the documented behavior; I don't see any bug there. 'diff' is designed to compare text files, and doesn't do very well with binary files.
