The enhancement was requested and approved a year ago. No progress yet. A right method is to compare rasters pdfwrite(NoEncryption)->ppmraw and pdfwrite (WithEncryption)->ppmraw.
Created attachment 1348 [details] test_encryption.bat My script for testing the pdf encryption (actually a part of).
*** Bug 687259 has been marked as a duplicate of this bug. ***
Maybe this can be tested, too.