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CHAPTER 8                                                            Interactive Features



  tion of the computation. Transform methods are described in detail in Section
  8.7.1, “Transform Methods.”

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-
  tion 3.4.5, “Incremental Updates”), the data corresponding to the byte range of the
  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-
  DP” on page 736.
• 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
  733 for information on how these signatures are created and validated.
• The Sig entry in the catalog of an FDF file (see “FDF Catalog” on page 713)
  specifies a signature dictionary.

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