I'm a bit disturbed by what I've just seen. I saved a zip file encrypted (with AES, but I don't think it matters). To verify encryption I went inside the zip file and, as expected, when trying to peek into the file I was asked for a password. All good. I also noticed that a filename in the zip is misspelled so I corrected. I then sent this zip file to someone. To my surprise it arrived DECRYPTED, i.e. no password was required. Needless to say, this is disturbing, as it seems that under some circumstances DOpus doesn't preserve the encryption of a zip file that had been encrypted. Was it the action of peeking into the zip file (or renaming a file inside) that decrypted the zip file inside DOpus and left it in an unecrypted state?
Firstly, Zip encryption is on a per-file basis, not per-archive. There's no such thing as an encrypted zip archive, just a zip archive that can contain encrypted files.
Secondly, the Zip standard doesn't allow files to be renamed within an archive. When you rename a file in an archive what Opus actually does is remove the old file from the archive, and add it back under the new name.
Put these two things together and it explains what happened. Renaming the encrypted file caused it to be removed from the archive, and then re-added under the new name - this time without encryption.
Probably it should show some sort of warning in this instance - or, I guess re-encrypt the new name assuming we can work out how to feed the password through to the rename operation.