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So, I've upgraded OSX to Sierra, but I need to downgrade (long story), and OSX won't let me because I didn't create a Time Machine backup before upgrading.

So I have downloaded the Install OS X El Capitan.app image using BitTorrent.

I assume Apple are wise enough to digitally sign their OS images, such that this is safe. But I would like to know for certain before proceeding.

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The original macOS installer apps are not distributed as images (i.e. dmg files). So you won't find a digitally signed macOS Installer.dmg, dmg signing is possible though: How to add codesigning to dmg file in mac.

However, you can check the codesigning of the macOS/OS X Installer.app and the sha sum of the InstallESD.dmg (which contains the files which will be installed to your disk later):

codesign -dv --verbose=4 /Volumes/InstallMavericks/Install\ OS\ X\ Mavericks.app

and 

shasum /Volumes/InstallMavericks/Install\ OS\ X\ Mavericks.app/Contents/SharedSupport/InstallESD.dmg

Compare the shasum result with the following list: Apple Installer Checksums. If the checksum is different the dmg is tampered.

The name of the mounted Mavericks.dmg (i.e. InstallMavericks) is just an example - so use the proper path to the macOS/OS X Installer.app.

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If you purchased/downloaded any version of Mac OS X from 10.7 to 10.11, you can always re-download it from the App Store on your computer.

After doing that, you can even create a bootable USB with the image.

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  • This doesn't answer the question!
    – klanomath
    Nov 29, 2016 at 16:34
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    @klanomath Perhaps not the question as asked, but as the ultimate goal of the OP is to install a previous version of Mac OS X, I'm offering a suggestion to help them accomplish that, in case they didn't know about that.
    – Zonker.in.Geneva
    Nov 29, 2016 at 16:39

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