Hi folks. Does this answer make sense to anyone? I may be too sleepy, but as of right now, it reads like gibberish. No offense intended to the author, of course.

Is that the right link? It's clearly not gibberish and it seems true to what I know of the PDF format, but I don't know whether it actually solves the issue in the question or not

@MichaelHomer It's the answer to the question above it. I'm not sure what you mean by "the right link". The question is "Encrypting PDF with qpdf and only user password".

Then no, it's not gibberish

I don't know about solving the issue. I probably need to hash it out with the author, if he has the patience. It still seems very strange to me.

I just want to make sure that the file that is supposed to be encrypted is actually encrypted. Is that so much to ask?

And I definitely don't want to see sentences like:

> Some packages, including qpdf, will try to see if the owner password might be empty even when the user password is not empty, and even though this doesn't really make much sense.

Ugh. I don't even know exactly what this means. Perhaps I should read it a few more times.

It sounds like it's not meaningfully possible to use PDF encryption without an owner password, since the key will just be in the clear

@MichaelHomer It sounds like you understand what is going on better than I do.

@FaheemMitha Out of curiosity I gave a look at the PDF specification. It looks like encrypted pdf files have an (always) unencrypted dictionary, one of whose entries, the "hashed owner password", is the user password encrypted using a key derived from the owner password and some fixed stuff. If the owner password is empty, programs can obtain the user password by decrypting that dictionary entry using the fixed (and known) stuff only, and then decrypt the document.

I believe the answer to your question would be: to use qpdf to simply encrypt a document, use the same password for user and owner. To specify further restrictions for an encrypted document, use different passwords for user and owner.

But I have never used `qpdf, I cannot be completely sure.

@fra-san Hi. So encryption is part of the PDF specification? Can you point me to the relevant section?

@fra-san I thought the point of a user password is that it makes the PDF unreadable. And then once the user password is given (if an owner password is empty), then there are not further restrictions on usage. I didn't want there to be restrictions on usage after the PDF has been opened using the user password.

As the answer you linked says, if the owner password is empty then the user password is stored in the clear

If you don't want to impose any extra restrictions... then just don't

@MichaelHomer Extra restrictions are not imposed automatically with the owner password, then?

You would be restricted from setting further restrictions if you didn't know the existing owner password

I think all of the others are optional, and obviously meaningless in practice in any case

@MichaelHomer Yes, because once you can view it, you basically have the thing, and can make a copy by some method. If only a screenshot.