CHAPTER 8
734
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
•
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
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
array
(Optional)
The type of PDF object that this dictionary describes; if present, must be
TransformParams
for a transform parameters dictionary.
(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
(Optional)
A text string that can be used to specify any arbitrary information, such as
the reason for adding usage rights to the document.
(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
.
Document
Msg
text
string
name
V