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          CHAPTER 3                                                                       Syntax



  3.3.9 Crypt Filter

          The Crypt filter (PDF 1.5) allows the document-level security handler (see
          Section 3.5, “Encryption”) to determine which algorithms should be used to
          decrypt the input data. The Name parameter in the decode parameters dictionary
          for this filter (see Table 3.12) specifies which of the named crypt filters in the
          document (see Section 3.5.4, “Crypt Filters”) should be used.

                            TABLE 3.12 Optional parameters for Crypt filters
KEY                 TYPE        VALUE

Type                name        (Optional) If present, must be CryptFilterDecodeParms for a Crypt filter de-
                                code parameter dictionary.

Name                name        (Optional) The name of the crypt filter that is to be used to decrypt this
                                stream. The name must correspond to an entry in the CF entry of the encryp-
                                tion dictionary (see Table 3.18) or one of the standard crypt filters (see
                                Table 3.23).
                                Default value: Identity.

          In addition, the decode parameters dictionary may include entries that are
          private to the security handler. Security handlers may use information from both
          the crypt filter decode parameters dictionary and the crypt filter dictionaries (see
          Table 3.22) when decrypting data or providing a key to decrypt data.

          Note: When adding private data to the decode parameters dictionary, security han-
          dlers should name these entries in conformance with the PDF name registry (see
          Appendix E, “PDF Name Registry”).


      3.4 File Structure

          The preceding sections describe the syntax of individual objects. This section
          describes how objects are organized in a PDF file for efficient random access and
          incremental update. A canonical PDF file initially consists of four elements (see
          Figure 3.2):

          • A one-line header identifying the version of the PDF specification to which the
            file conforms
          • A body containing the objects that make up the document contained in the file

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