I did a little digging and it looks like we could write the encryption dictionary for the PDF at the beginning of the file if we add the following to our PDFax specification:
[[ "The size of the ﬁle in bytes" should be removed from the list of input information at the end of section 9.3 (pg. 581) in the PDF 1.4 reference. ]] We may recommend something else be added in its place - TBD.
Since the encryption key algorithm specifies that the 'ID' entry in the document's trailer be added to the MD5 hash (step 5 of Algorithm 3.2 on pg. 78) we would have to calculate the ID at the beginning of document creation (but could be cached until doc close) and hence would preclude us from using the document size in the ID.
Other than that, all information to generate the encryption dictionary is known at doc start time. This would allow us to also add this to our specification:
[[For an encrypted PDF, a copy of the encryption dictionary must be placed at the beginning of the document soon after the '/PDFax' tag. Details TBD.]]
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