I'm trying to verify that storing a .dmg file stays encrypted while only the mounted file system is unencrypted (see here). My thought was to create a .dmg containing a text file, make a copy of the .dmg, then track file changes with cmp -b myvol.dmg myvol2.dmg | less - N after various actions; my hope was that the .dmg wouldn't change until I unmounted the volume, at which point it would be updated and re-encrypt the file. It looks like I was wrong, though.

Both .dmg files remain identical after mounting and opening the (identical) text files, but can be seen to differ by at least several thousand bytes after a single character change. Apparently the .dmg is diffusely (good!) updating to reflect changes in the file system, but unfortunately is a bit of a null result in my experiment.

My Question: How can I verify that unencrypted information isn't stored in the .dmg file? Is it simply enough to say that a .dmg is not directly readable, and can't be mounted without the password? I'm an encryption novice who wants to verify things directly.


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