+1 because it's a really neat idea, but your caveat applies more widely than you might think; most PDF content is compressed, for example, and MS Office documents in the 2007+ format are literally ZIP files. So in practice it might be more useful to go straight for per-format tests.

In reply to Re^2: Determine if a file is password protected by Porculus
in thread Determine if a file is password protected by slloyd

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