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Topic: Acrobat 5.0 Encryption v2r3 question
Conf: (P-PDF) Developers, Msg: 51245
From: alfzli
Date: 5/29/2002 04:44 PM

Currently I'm reading through the new tech notes "Portable Document Format: Changes from version 1.3 to 1.4", but I am confused by the following description on page 17:

Algorithm 4 Computing the U (user) value in the encryption dictionary

2. If R is 2, then pass to step 3 the 32-byte padding string shown in Algorithm 3.2 step 1. If R is 3, then initialize an MD5 hash function, update the hash with the output of step 1, update the hash with the 16-byte first element of the document's file identifier, producing a 16-byte result to pass to step 3.

I have two questions:

1. If R is 3, is the "U" entry in the encryption dictionary really only 16-byte long? Isn't it supposed to be 32-byte?

2. How should I understand "initalize an MD5 hash function, ..."? Do I pass the MD5 function two scalar values: encryption key (variable length) and file ID (16-byte)?

I really appreciate if someone could help clarify this.

Zhigang Li

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