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Diffstat (limited to 'manual')
| -rw-r--r-- | manual/qpdf-manual.xml | 32 |
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diff --git a/manual/qpdf-manual.xml b/manual/qpdf-manual.xml index 6e72456e..bd861efe 100644 --- a/manual/qpdf-manual.xml +++ b/manual/qpdf-manual.xml @@ -3490,6 +3490,38 @@ outfile.pdf</option> The only exception to this is that clear-text metadata will be preserved as clear-text if it is that way in the original file. </para> + <para> + One point of confusion some people have about encrypted PDF files + is that encryption is not the same as password protection. + Password protected files are always encrypted, but it is also + possible to create encrypted files that do not have passwords. + Internally, such files use the empty string as a password, and + most readers try the empty string first to see if it works and + prompt for a password only if the empty string doesn't work. + Normally such files have an empty user password and a non-empty + owner password. In that way, if the file is opened by an ordinary + reader without specification of password, the restrictions + specified in the encryption dictionary can be enforced. Most users + wouldn't even realize such a file was encrypted. Since qpdf always + ignores the restrictions (except for the purpose of reporting what + they are), qpdf doesn't care which password you use. QPDF will + allow you to create PDF files with non-empty user passwords and + empty owner passwords. Some readers will require a password when + you open these files, and others will open the files without a + password and not enforce restrictions. Having a non-empty user + password and an empty owner password doesn't really make sense + because it would mean that opening the file with the user password + would be more restrictive than not supplying a password at all. + QPDF also allows you to create PDF files with the same password as + both the user and owner password. Some readers will not ever allow + such files to be accessed without restrictions because they never + try the password as the owner password if it works as the user + password. Nonetheless, one of the powerful aspects of qpdf is that + it allows you to finely specify the way encrypted files are + created, even if the results are not useful to some readers. One + use case for this would be for testing a PDF reader to ensure that + it handles odd configurations of input files. + </para> </sect1> <sect1 id="ref.random-numbers"> <title>Random Number Generation</title> |