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                                                     734
           CHAPTER 8                                                            Interactive Features



           Validation of a usage rights signature depends on whether the signature dictio-
           nary is referenced from the UR or UR3 entry in the permissions dictionary (See
           implementation note 142 in Appendix H):

           • UR: At the time of signing, the application computes the object digest over a
             subset of the PDF objects in the document; that is, the objects that are not
             modified, directly or indirectly, by permissible operations, as specified by the
             transform parameters dictionary. Appendix I describes the object digest com-
             putation. The calculated value of this digest is stored in the DigestValue entry
             in the signature reference dictionary (see Table 8.103). An application can com-
             pare this entry to its calculated object digest when validating. If the values are
             different, the signature is invalid.
             Note: The use of UR is not recommended because of the complex (and hence prone
             to errors) algorithm. Instead, the UR3 (see below) algorithm should be used.
           • UR3 (PDF 1.6): The     ByteRange entry in the signature dictionary (see Table
             8.102) is required to be present. First, the application verifies the byte range di-
             gest to determine whether the portion of the document specified by ByteRange
             corresponds to the state of the document at the time of signing. Next, the appli-
             cation examines the current version of the document to see whether there have
             been modifications to any objects that are not permitted by the transform pa-
             rameters.

                      TABLE 8.105 Entries in the UR transform parameters dictionary
KEY          TYPE        VALUE

Type         name        (Optional) The type of PDF object that this dictionary describes; if present, must be
                         TransformParams for a transform parameters dictionary.

Document     array       (Optional) An array of names specifying additional document-wide usage rights for
                         the document. The only defined value is FullSave, which permits a user to save the
                         document along with modified form and/or annotation data. (See implementation
                         note 143 in Appendix H.)

Msg          text        (Optional) A text string that can be used to specify any arbitrary information, such as
             string      the reason for adding usage rights to the document.

V            name        (Optional) The UR transform parameters dictionary version. The only valid value is
                         2.2. If an unknown version is present, no rights are enabled. (Note that this value is a
                         name object, not a number.) (See implementation note 145 in Appendix H.) Default
                         value: 2.2.

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