Repository navigation
PDF20 Spec 14 File Identifiers
Part of: PDF20-Spec-14-Document
Navigation: PDF20-Spec-14-Metadata | PDF20-Spec-Index | PDF20-Spec-14-Page-Piece
14.4 File identifiers PDF file identifiers shall be defined by the ID entry in a PDF file’s trailer dictionary (see 7.5.5, "File trailer"). The value of this entry shall be an array of two byte strings. The first byte string shall be a permanent identifier based on the contents of the PDF file at the time it was originally created and shall not change when the PDF file is updated. The second byte string shall be a changing identifier based on the PDF file’s contents at the time it was last updated (see 7.5.6, "Incremental updates"). When a PDF file is first written, both identifiers shall be set to the same value. If the first identifier in the reference matches the first identifier in the referenced file’s ID entry, and the last identifier in the reference matches the last identifier in the referenced file’s ID entry, it is very likely that the correct and unchanged PDF file has been found. If only the first identifier matches, a different version of the correct PDF file has been found. PDF writers should attempt to ensure the uniqueness of file identifiers. This may be achieved by computing them by means of a message digest algorithm such as MD5 (described in Internet RFC 1321), using the following information: •
The current time;
•
A string representation of the PDF file’s location;
•
The size of the PDF file in bytes.
Navigation: PDF20-Spec-14-Metadata | PDF20-Spec-Index | PDF20-Spec-14-Page-Piece