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                                                    117
         SECTION 3.5                                                                      Encryption



KEY      TYPE        VALUE

V        number      (Optional but strongly recommended) A code specifying the algorithm to be used in en-
                     crypting and decrypting the document:
                        0    An algorithm that is undocumented and no longer supported, and whose use is
                             strongly discouraged.
                        1    Algorithm 3.1 on page 119, with an encryption key length of 40 bits; see below.
                        2    (PDF 1.4) Algorithm 3.1, but permitting encryption key lengths greater than 40
                             bits.
                        3    (PDF 1.4) An unpublished algorithm that permits encryption key lengths rang-
                             ing from 40 to 128 bits; see implementation note 22 in Appendix H.
                        4    (PDF 1.5) The security handler defines the use of encryption and decryption in
                             the document, using the rules specified by the CF, StmF, and StrF entries.
                     The default value if this entry is omitted is 0, but a value of 1 or greater is strongly rec-
                     ommended. (See implementation note 23 in Appendix H.)

Length   integer     (Optional; PDF 1.4; only if V is 2 or 3) The length of the encryption key, in bits. The value
                     must be a multiple of 8, in the range 40 to 128. Default value: 40.

CF       dictionary (Optional; meaningful only when the value of V is 4; PDF 1.5) A dictionary whose keys
                    are crypt filter names and whose values are the corresponding crypt filter dictionaries
                    (see Table 3.22). Every crypt filter used in the document must have an entry in this dic-
                    tionary, except for the standard crypt filter names (see Table 3.23).
                     Note: An attempt to redefine any of the standard names in Table 3.23 is ignored.

StmF     name        (Optional; meaningful only when the value of V is 4; PDF 1.5) The name of the crypt filter
                     that is used by default when decrypting streams. The name must be a key in the CF dic-
                     tionary or a standard crypt filter name specified in Table 3.23. All streams in the docu-
                     ment, except for cross-reference streams (see Section 3.4.7, “Cross-Reference Streams”)
                     or streams that have a Crypt entry in their Filter array (see Table 3.5), are decrypted by
                     the security handler, using this crypt filter.
                     Default value: Identity.

StrF     name        (Optional; meaningful only when the value of V is 4; PDF 1.5) The name of the crypt filter
                     that is used when decrypting all strings in the document. The name must be a key in the
                     CF dictionary or a standard crypt filter name specified in Table 3.23.

                     Default value: Identity.

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