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Appendix I: Computation of Object Digests

Appendix I. Computation of Object Digests

This appendix describes the algorithm for computing object digests (discussed in Section 8.7, “Digital Signatures”). The computation uses a hashing method, specified by the DigestMethod entry of the signature reference dictionary (see Table 8.99). Its value can be SHA1 for the Secure Hash Algorithm 1 (SHA-1) or MD5 for the MD5 message-digest algorithm; see the Bibliography. Both algorithms operate on an arbitrary-length stream of bytes to produce a digest of fixed length (16 bytes for MD5, 20 bytes for SHA-1).

The following sections describe how the stream of bytes to be digested is generated, starting with a specific object within a PDF file. A PDF object is digested by recursively traversing the object hierarchy beginning with the given object. Objects encountered during the traversal are categorized as basic PDF types, described in Section I.1, “Basic Object Types,” or more complex types, described in Section I.2, “Selective Computation.” Each object is digested as it is processed. Not all objects may be included, depending on the transform method and parameters (see Section 8.7.1, “Transform Methods”) that are being used.


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