Most compression schemes work by finding patterns in your data that can be in some way factored out, and thanks to the encryption now there are none; the data is completely incompressible. You should always compress before encrypting. That is what I was thinking on a level 9 question but you indicated the answer was: "Data should be compressed first before encrypting the data." Which is correct the level 9 question or the level 10 question? This question seems problematic in the sense that it asks "What is more secure"; however, the heart of the question instead seems to be "what will achieve the best balance of both security and file compression". Strongly suggest revising question. No way "You should never do both" is the correct answer. Compression programs include encryption as an option for pete's sake!