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As such, it doesn't make sense to provide a user password and not an owner password, as this would mean that not providing a password is more restrictive than providing a password.

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In older PDF encryption formats, the file was encrypted by an encryption key that is derived from the user password, and the user password is actually stored in the file encrypted with the owner password. PDF viewers would first try to see if the supplied password is the user password. If so, they can retrieve the key and decrypt the file. If not, they check to see if it's the owner password. If so, they can retrieve the user password, and from there, they can retrieve the key. Newer encryption formats encrypt the actual key independently with both the user password and the owner password, and as such, it is not possible to retrieve the user password with the owner password.īecause of this behavior, some applications will try to open an encrypted password trying the empty string as the user password, and if that fails, they will prompt for a password. 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. That would explain the behavior you are seeing. I believe that older versions of Adobe Acrobat will not allow admin access to the file at all if the user and owner password match or if the user password is set and the owner password is empty because they only attempt to validate the owner password if the supplied password doesn't work as the user password. Hopefully this explains the behavior you are seeing. Indeed, perhaps it would be a good idea if I started with my use case. I just want to encrypt a PDF, preferably with a single password.

Once it is decrypted, I don't want further restrictions. And of course, if I'm using encryption, I want encryption which cannot be easily broken. My understanding from my earlier reading (possibly incorrect), was that the user password is used to encrypt the file so that it could not be read.

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The owner password was required if one wanted to set restrictions on other usages of the files, like printing.Įven the QPDF manual says that either or both of the user/owner passwords can be used. From Section 3.4 'Encryption Options' of the QPDF manual:Įither or both of the user password and the owner password may be empty strings. There is no indication that just using the user password won't do any actual encryption. I've found what information available about PDF encryption quite confusing. Someone on U&L chat referred me to the PDF standard at Section 7.6, at around 15 pages, appears to be the relevant section. In any case, I find aspects of this to be quite nonsensical. I have other specific questions about your answer, but perhaps the above will help to indicate where I'm coming from, to start with? But this doesn't make sense, because once the file has been opened, the contents are available, and one can take a For example, that once the file has been opened for reading, other restrictions can potentially be made. You're correct that the whole PDF encryption thing is pretty nonsensical. The very existence of tools like qpdf shows how trivial it is to defeat the permissions. Owner password differences is left completely up to the PDF reading software. Maybe if PDF were a proprietary file format and there were no open source readers, this would provide some meager measure of security by obscurity, but certain with qpdf, or any number of other open source PDF readers, having two different passwords is of minimal value.

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If you're in the commercial software space, there can be some differences.

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