CHAPTER 3
116
Syntax
The
SubFilter
entry specifies the syntax of the encryption dictionary contents. It
allows interoperability between handlers; that is, a document may be decrypted
by a handler other than the preferred one (the
Filter
entry) if they both support
the format specified by
SubFilter
.
The
V
entry, in specifying which algorithm to use, determines the length of the
encryption key, on which the encryption (and decryption) of data in a PDF file is
based. For
V
values 2 and 3, the
Length
entry specifies the exact length of the
encryption key. In PDF 1.5, a value of 4 for
V
permits the security handler to use
its own encryption and decryption algorithms and to specify
crypt filters
to use
on specific streams (see Section 3.5.4, “Crypt Filters”).
The remaining contents of the encryption dictionary are determined by the
security handler and may vary from one handler to another. Entries for the
standard security handler are described in Section 3.5.2, “Standard Security
TABLE 3.18 Entries common to all encryption dictionaries
KEY
TYPE
VALUE
Filter
name
(Required)
The name of the preferred
security handler
for this document. Typically, it is
the name of the security handler that was used to encrypt the document. If
SubFilter
is
not present, only this security handler should be used when opening the document. If it
is present, consumer applications can use any security handler that implements the for-
mat specified by
SubFilter
.
Standard
is the name of the built-in password-based security handler. Names for other
security handlers can be registered by using the procedure described in Appendix E.
Note:
The definition of this entry has been clarified since the previous version of this docu-
ment.
SubFilter
name
(Optional; PDF 1.3)
A name that completely specifies the format and interpretation of
the contents of the encryption dictionary. It is needed to allow security handlers other
than the one specified by
Filter
to decrypt the document. If this entry is absent, other se-
curity handlers should not be allowed to decrypt the document.
Note:
This entry was introduced in PDF 1.3 to support the use of public-key cryptography
in PDF files (see Section 3.5.3, “Public-Key Security Handlers”); however, it was not incor-
porated into the
PDF Reference
until the fourth edition (PDF 1.5).