Are Mac OS images (eg. "OS X El Capitan.app") digitally signed? Safe to install from torrent?

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.