Represents the "CF" entry of the encryption dictionary object.

It is an optional entry defined as part of the PDF 1.5 specification.

It contains an Element.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 20, page 56.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 3.18, page 14.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 20, page 72.

The CF entry holds a dictionary of named crypt filter definitions. Each key is a crypt filter name and each value is a crypt filter dictionary specifying the algorithm and key length for that filter.

Crypt filters allow different parts of the document - strings, streams, and embedded files - to be encrypted with different algorithms or keys, or to be left unencrypted selectively.