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Add some clarification to encryption in the manual

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Per user suggestion in issue 351.
This commit is contained in:
Jay Berkenbilt 2019-08-23 21:26:47 -04:00
parent 2794bfb1a6
commit 53930d96d0
1 changed files with 32 additions and 0 deletions
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manual/qpdf-manual.xml
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@ -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>