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                                                       116
            CHAPTER 3                                                                            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
            Handler.” Entries for public-key security handlers are described in Section 3.5.3,
            “Public-Key Security Handlers.”

                         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).

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