CHAPTER 8
726
Interactive Features
tion of the computation. Transform methods are described in detail in Section
A PDF document may contain the following standard types of signatures:
•
One or more
document
(or
ordinary)
signatures. These signatures appear in sig-
nature form fields (see “Signature Fields” on page 695). The signature dictio-
nary corresponding to each signature is the value of the form field (as specified
by its
V
entry). The signature dictionary must contain a
ByteRange
entry repre-
senting a byte range digest, as described above. A signature is validated by re-
computing the digest and comparing it with the one stored in the signature.
Note:
If a signed document is modified and saved by incremental update (see Sec-
original signature is preserved. Therefore, if the signature is valid, it is possible to
recreate the state of the document as it existed at the time of signing.
•
At most one
MDP (modification detection and prevention)
signature (PDF
1.5),
also referred to as an
author
or
certifying
signature. The signature dictionary of
an MDP signature must be the value of a signature field and must contain a
ByteRange
entry. It may also be referenced from the
DocMDP
entry in the per-
missions dictionary (see Section 8.7.3, “Permissions”). The signature dictio-
nary must contain a signature reference dictionary (see Table 8.103) that has a
DocMDP
transform method. See “DocMDP” on page 731 for information on
how these signatures are created and validated.
A signature dictionary for an MDP or ordinary signature may also have a sig-
nature reference dictionary with a
FieldMDP
transform method; see “FieldM-
•
At most two
usage rights
signatures (PDF
1.5).
Its signature dictionary is refer-
enced from the
UR
or
UR3
(PDF
1.6)
entry in the permissions dictionary (not
from a signature field); see Table 8.107. The dictionary must contain a signa-
ture reference dictionary that has a
UR
transform method. See “UR” on page
•
The
Sig
entry in the catalog of an FDF file (see “FDF Catalog” on page 713)
specifies a signature dictionary.