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Re: [pdf-devel] Encryption filters

From: Ralph Giles
Subject: Re: [pdf-devel] Encryption filters
Date: Mon, 28 Jul 2008 12:21:23 -0700

On 28-Jul-08, at 1:37 PM, address@hidden wrote:

I dont fully understand. What is the difference between a "crypt filter dictionary" and a "filter dictionary"?

Not to step on the OP, but my take on this:

A standard stream can have a filter chain on it, and one of the filters in the chain can be a 'Crypt' filter. That's what was meant by a "filter dictionary" I think.

To actually know what algorithm to use in a particular 'Crypt' filter instance, you look at the corresponding DecodeParms entry. This is a dictionary, if present, with Type CryptFilterDecodeParms and a Name key. The value of that Name key indexes the CF dictionary in the global Encryption Dictionary. The values in the CF dictionary are "crypt filter dictionaries" which define the actual cipher, key length, authorization triggers and so on.

So the cipher used for 'Crypt' filter is a parameter, one specified one level of indirection away. The key must also be derived from the global encryption dictionary. So yes you need RC4 and AES stream elements, but like JPXDecode, their actual use requires information from outside the stream dictionary.

That's my understanding, anyway.


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