Previous Next

                                                  118
        CHAPTER 3                                                                          Syntax



KEY     TYPE        VALUE

EFF     name        (Optional; meaningful only when the value of V is 4; PDF 1.6) The name of the crypt filter
                    that should be used by default when encrypting embedded file streams; it must corre-
                    spond to a key in the CF dictionary or a standard crypt filter name specified in
                    Table 3.23.
                    This entry is provided by the security handler. (See implementation note 24 in Appendix
                    H.) Applications should respect this value when encrypting embedded files, except for
                    embedded file streams that have their own crypt filter specifier. If this entry is not
                    present, and the embedded file stream does not contain a crypt filter specifier, the
                    stream should be encrypted using the default stream crypt filter specified by StmF.

        Unlike strings within the body of the document, those in the encryption
        dictionary must be direct objects. The contents of the encryption dictionary are
        not encrypted by the usual methods (the algorithm specified by the V entry).
        Security handlers are responsible for encrypting any data in the encryption
        dictionary that they need to protect.

        Note: Document creators have two choices if the encryption methods and syntax
        provided by PDF are not sufficient for their needs: they can provide an alternate se-
        curity handler or they can encrypt whole PDF documents themselves, not making
        use of PDF security.


  3.5.1 General Encryption Algorithm

        The following algorithms are used when encrypting data in a PDF file:
        • A proprietary encryption algorithm known as RC4. RC4 is a symmetric stream
          cipher: the same algorithm is used for both encryption and decryption, and the
          algorithm does not change the length of the data.
          Note: RC4 is a copyrighted, proprietary algorithm of RSA Security, Inc. Adobe
          Systems has licensed this algorithm for use in its Acrobat products. Independent
          software vendors may be required to license RC4 to develop software that encrypts
          or decrypts PDF documents. For further information, visit the RSA Web site at
          < http://www.rsasecurity.com > or send e-mail to < products@rsasecurity.com >.
        • The AES (Advanced Encryption Standard) algorithm (beginning with PDF
          1.6). AES is a symmetric block cipher: the same algorithm is used for both en-
          cryption and decryption, and the length of the data when encrypted is rounded
          up to a multiple of the block size, which is fixed in this implementation to al-

Previous Next