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                                                 698
          CHAPTER 8                                                          Interactive Features



KEY            TYPE         VALUE

DigestMethod   array        (Optional; PDF 1.7) An array of names indicating acceptable digest algorithms
                            to use while signing. The valid values are SHA1, SHA256, SHA384, SHA512 and
                            RIPEMD160. The default value is implementation-specific.

                            Note: This property is only applicable if the digital credential signing contains
                            RSA public/private keys. If it contains DSA public/ private key, the digest algo-
                            rithm is always SHA1 and this attribute is ignored.

V              real         (Optional) The minimum required capability of the signature field seed value
                            dictionary parser. A value of 1 specifies that the parser must be able to recog-
                            nize all seed value dictionary entries specified in PDF 1.5. A value of 2 specifies
                            that it must be able to recognize all seed value dictionary entries specified in
                            PDF 1.7 and earlier.
                            The Ff entry indicates whether this is a required constraint.
                            Note: The PDF Reference fifth edition (PDF 1.6) and earlier, erroneously indi-
                            cates that the V entry is of type integer. This entry is of type real.

Cert           dictionary   (Optional) A certificate seed value dictionary (see Table 8.84) containing infor-
                            mation about the certificate to be used when signing.

Reasons        array        (Optional) An array of text strings that specifying possible reasons for signing
                            a document. If specified, the reasons supplied in this entry replace those used
                            by viewer applications. The Ff entry specifies whether one of the reasons in the
                            array must be used in the signature.
                            • If the Reasons array is provided and the Ff entry indicates that Reasons is a
                              required constraint, one of the reasons in the array must be used for the sig-
                              nature dictionary; otherwise, signing must not take place. If the Ff entry in-
                              dicates Reasons is an optional constraint, one of the reasons in the array can
                              be chosen or a custom reason can be provided.
                            • If the Reasons array is omitted or contains a single 0-character length string
                              and the Ff entry indicates that Reasons is a required constraint, the Reason
                              entry must be omitted from the signature dictionary (see Table 8.102).

MDP            dictionary   (Optional; PDF 1.6) A dictionary containing a single entry whose key is P and
                            whose value is an integer between 0 and 3. A value of 0 defines the signature as
                            an ordinary (non-author) signature (see Section 8.7, “Digital Signatures”). The
                            values 1 through 3 are used for author signatures and correspond to the value
                            of P in a DocMDP transform parameters dictionary (see Table 8.104).
                            If this entry is not present or does not contain a P entry, no rules are defined
                            regarding the type of signature or its permissions.

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