Mac OS X Disk Image Verification

Question

Does anyone knows what exactly happens behind the scenes when Mac OS X verifies a disk image (.dmg) file? Is there any way to extend or customize the procedure?

EDIT: I would like to create a disk image that verifies that it does exactly what it should do and nothing more. For example, if I distribute some software that manages passwords, a malicious user could modify my package to send the passwords to an unwarranted third party. To the end user, the functionality would appear to be identical to my program, and they would never know the package was sabotaged. I would like to perform this verification at mount time.

Answer 1

To my knowledge, you cannot modify this procedure (unless you do some system hacks which I don't recommend). I believe it compares it with the internal checksum and makes sure that the disk's volume header is OK. It goes through all of the files to see if any of them are corrupted.

Answer 2

My understanding of dmg's is limited but as I understand it's essentially an osx specific archive format, similar to zips. One option would be to also distribute the checksum of your dmg. This isn't very useful as if an attacker can change the dmg a user downloads from your site they can also modify the checksum.

The functionality I believe you're looking for is codesigning. It's a cryptographic verification that an app hasn't been modified since it was signed by the author. There's a bit of a barrier to using this as you need a developer certificate from the apple developer program.

Apple's documentation on codesigning can be found here: https://developer.apple.com/library/mac/documentation/Security/Conceptual/CodeSigningGuide/Procedures/Procedures.html#//apple_ref/doc/uid/TP40005929-CH4-SW5