I created a 78GB zip file, with the Windows version of 7-zip, back in November. Today, for the first time, I tried unzipping it on my Linux desktop: a simple right-click and Extract Here and things looked good: all the file were there in all the right places, properly named and ready to go…

Unfortunately, when I then tried to do anything with the files produced by the extraction process, all failed to work: music and video files simply refused to play in VLC, Movie Player or anything else, for example.

I feared the worst: a corrupt zip file. I was about to shrug my shoulders, delete the zip file and chalk it up to experience when I thought, ‘let’s try to repair the zip!’ Nothing ventured, nothing gained after all… and if the repairs failed to work, I’d at least be no worse off than I was already.

Two programs suggested themselves, thanks to Google (both Windows-only, so run in a virtual machine).

First up: Disk Internals. Small download, apparently free, nice wizard point-and-click interface. A bit slow identifying anything capable of being recovered… and, in the end, it only output a couple of hundred files (out of over 3800 in the original zip) before falling over in a heap and crashing. None of them was usable.

So, second try: Object Fix Zip. Another small download, standard Windows installation and simple wizard interface. On its first run, it prompted me for a password to the zip file… and it was at that point I suddenly remembered that I had, indeed, password-protected the original zip file. I also suddenly realised that the Disk Internals program had never prompted me for a password… no wonder that first program had been unable to recover anything usable!

Even more interesting, Object Fix Zip prompted me for a password… and then kept prompting me for each file it encountered within the zip. It would accept the password I typed, extract a file from the zip, move onto the next file within the zip and then prompt me again. Then I noticed that (a) every file it appeared to extract was unusable and (b) I could type a password I knew to be rubbish… and the program would continue to extract, move on, re-prompt.

That in turn made me realise something: the Linux Extract Here menu option had never prompted me for a password at all… and its outputs were useless. The Disk Internals repair tool had not prompted me for a password either… its outputs were similarly useless. Object Fix was at least prompting me for a password … but its continual prompts regardless of what I typed in seemed to suggest that maybe I was providing an incorrect password all along.

So then the penny dropped: could the original idea that my zip file was corrupt be merely the result of the non-supply of a password, or the supply of a wrong password?

I re-ran Object Fix a couple of times more, each time supplying a different plausible password. And Lo! On one of these subsequent runs, it accepted the password and stopped asking for it after that. Clearly, when the right password was supplied, it only needed to be told it once. And, even better, the files being output by the Object Fix tool were (drum roll, please)… completely usable.

To cut a long story short, I then went back to my Linux desktop and used the Linux equivalent of 7zip (called p7zip, but invoked with the command 7za) to extract the file:

7za x /path/filename.zipghjghjgh

…and this program properly prompted me to supply a password. When I typed in a password I knew couldn’t possibly be correct, the program at least told me there was a problem:

CRC Failed in encrypted file. Wrong password?

The second part of that message is, at least, a good, strong clue as to what’s wrong, although you could get the wrong idea entirely if you paid any attention to that bit about CRC  failing. A genuinely corrupt file would generate warnings about CRC failures, after all!

So, finally, I re-ran 7za with what I knew by this time to be the correct password… and the whole thing extracted itself without a hitch. All files were usable, all 78GB of data were back safely from the brink!

In summary, if you think your zip file is corrupt, make sure it wasn’t originally password-protected. If it was password-protected, don’t expect Linux’s ‘extract here’ functionality to prompt you for the password -and expect its output to be garbage as a result. A program like p7zip will prompt you for a password appropriately, but may not be obvious about it if the password you supply turns out to be incorrect. And don’t give up on your zip until you’ve exhausted every conceivable password!