Say someone else posts an encrypted file online, and challenges people to decrypt it.
Say I download the file headers, and I manage to determine the password. (Maybe it was a made-up word that only me and a few old friends know, maybe it was emailed to me secretly by the person who encrypted it, maybe it just happens to be "apple" and I am the only person who thought to try a dictionary brute force.)
Upon decrypting the headers, I determine that the full file, which I don't download but could decrypt, most likely contains a complete and highly unauthorized copy of the hit 2004 film Shrek 2.
Do I have any legal obligations with regard to the encryption key? Can I go out and publicly say "The password to that file that so-and-so posted is 'apple', and definitely delete it right away if you downloaded it because it contains a complete and highly unauthorized copy of the hit 2004 film Shrek 2"? Or do I have to keep the password secret under some theory of liability for enabling something-or-other? Or may I disclose the password but not the identity of the file, which would help people find it?