Thanks to kelalaka for pointing out the Black Hat talk. For pdfs with more pages (and hence larger string and streams sections) the entropy should be closer to 8-bits. Known for being a magazine that upholds its promises to play a role in the greater literary community, Entropy will be greatly missed. Therefore if the input file is quite short or has a lot of object structures that are neither stream nor strings, the entropy will be closer to this figure. However, the talk also says that PDF encryption only encrypts some of the objects in PDF's Carousel Object Structure and the other objects are unencrypted, largely human readable, markup language which might have a bytewise entropy of only 4-5 bits (English language words have a bytewise entropy in this range). For lengthy plaintexts this should remove bytewise structure and the bytewise entropy should tend towards 8-bits for larger texts. UPDATE: According to the description of PDF encryption in this Black Hat talk 256-bit AES-CBC is used. Without more detail on the input and encryption method, there is a limit to what can be said. It's also possible that your encryption product adds unencrypted metadata to a file and if there's a large amount of metadata relative to the file size and the metadata is of a very stereotyped form (such as English readable ASCII) then this will have less entropy than the pdf format. There are many forms of encryption that still preserve some level of information about the input (the encrypted version of a bitmap under a block cipher using ECB is a good example). forEach((c) => console.log(c.As bmm60 comments, its not clear that the entropy has gone down. Let url = ` // Submit the request and process the response Please include where you’re submitting, how much it costs, and your PayPal account. So if you’re looking to submit but cannot cover the fees, email with FEE SUPPORT in the subject line. Entropy Magazine: Where to Submit Last updated November 9, 2019.
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