weird behavior of symmetrically encrypted file

[Tobias]

Hi there,

I have a symmetrically encrypted gpg file I want to decrypt. It seems I
have forgotten the passphrase, because none of the ones I'd usually use
for such a file will work. So I wrote a python script that tries to find
the correct passphrase by brute force. I didn't put much hope in it, but
hey, better to do something else other than agonize about the correct
passphrase, right?

So imagine my surprise when after only a couple of hours my script says
it has found the correct passphrase. I try it and hooray, gpg doesn't
throw the "decrpytion failed: bad key" message. Unfortunately, it still
doesn't decrypt the file. It simply does nothing at all.
.
# gpg --no-use-agent --passphrase "3ity" usbkey-howto.txt.gpg
gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
gpg: WARNING: message was not integrity protected
# ls usbkey-howto.txt
ls: cannot access usbkey-howto.txt: No such file or directory

Why do I get a passphrase ("3ity") which I can't remember having ever
used in my life? Why does gpg regard it as correct but still not decrypt
my file? And apart from these somewhat academical questions: Is there a
way I can use the half-correct passphrase to refine (which means, speed
up) my search for the truly correct one? If I can use it to
significantly reduce the set of possible passphrases, it may save me
some decades worth of blind guessing.

I'm using GnuPG 1.4.9 on Ubuntu 9.04. The encrypted file is attached for
reference. Glad for any help!


-- 
Liebe Grüße
Tobias



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