Graphics Programs Reference
In-Depth Information
CHAPTER 8
Encrypted Documents
PDF documents can be encrypted using a variety of industry-standard schemes which
have increased in complexity and security over the years, starting with PDF version 1.1.
The PDF standard provides, in addition, a general mechanism for encapsulating third-
party encryption and security policies.
Encryption applies, with a few exceptions, to streams and strings in the file, but does
not encrypt numbers or other PDF data types, nor does it encrypt the file as a whole.
Thus, the document's object structure remains visible to applications without the need
for decryption, but the substantive content of the document is safeguarded.
The more modern PDF encryption methods allow the file's XMP metadata stream
(
“XML Metadata” on page 93
) to be left unencrypted so it may be extracted and read
by programs which don't know how to open encrypted PDF files, or if the password is
not known.
Introduction
Due to the complexity of encrypted documents, it isn't possible to manually build an
example (as we have in other chapters), but we can use
pdftk
to process our standard
hello.pdf
file into an encrypted one,
encypted.pdf
:
pdftk hello.pdf output encrypted.pdf encrypt_40bit owner_pw fred
This creates the output file
encrypted.pdf
using the 40-bit RC4 method with an owner
password of “
fred
”. The
owner password
is the master password for the file. Someone
who has it can do anything with the file, including re-encrypting it or changing the
security settings. The
user password
allows the user to perform certain actions (view
the document, print the document etc.) defined by the owner when the file was
encrypted.
