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I generate a derived key for every document via KD=KDF∗KM∗filename where KDF and filename are known to the public. If I brute-force a document and get the correct KD, I know 3 out of 4 variables and therefore know KM. As a result, I’m able to derive every KD. The main problem here is, that you assume the existence of a function, which can calculate the ...


Yes, key derivation with known parameters can add to security. For a start, in the scenario of the question (understood as a real-world scenario, with PDF encryption used in step 4), when key derivation is used, you can hand one encrypted PDF document and its derived key to a person, and that won't enable her to decipher other enciphered documents; this ...

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