Mac OS high sierra app is damaged

Different Solution: I had the same problem, resolution was to set the time reasonably accurately. I had just replaced the battery and the hard disk with an SSD. So I didn't have a bootable system, and the time was set to Jan 1 2000. You can set the time without even re-booting using the Terminal app in the Utilities menu. Once the terminal window comes up, enter the command "date -u". This will show you what the system clock is set to in UST. If it's wrong enter "date -u mmddHHMMyyyy". mm=month, dd=day, HH=hour, MM=minutes, cc=century, yyyy=year-in-4-digits. Yes, it would be hard to come up with a stupider format for the date-and-time. But it fixed the problem for me.

Okay so since you do have another MacBook... Download MacOS High Sierra there, make a bootable USB (follow that link) then boot from the USB. (This is done by pressing the options key and the power button together.

Afterwards you should have an option "Install MacOS High Sierra", press that and then choose your Mac drive. Wait for the process to finish and you should be good to go.

A backup is always recommend since this is just a workaround

https://9to5mac.com/2017/08/02/how-to-create-bootable-macos-high-sierra-usb-inst all-drive-video/

Try a fresh install

The first priority is for you to make an external backup of your existing system, at least with Time Machine, and preferably with a bootable clone as well.

It is not necessary, nor desirable, to alter the installer in any way and SIP is an important protection against malware.

I got the 19Mb download too. It initiates a full 5.1Gb download when run, which took 25 minutes itself, before the installation started. There was also a Firmware update followed by a restart for the installation proper, so the whole business took two hours.

A clean install is not required and your user account will stay in place.

I updated Macos High Sierra last week and only nightmares for the whole media department. one Mac Pro Died after 2 hours of update and several iMacs have permission issues and a lot more, not good Apple, this doesn't s happen with windows, this didn't happen years ago wit apple but happens now more and more, but looks like apple is becoming more and more like windows in a way, because I can still use windows for the media department after any update,

My case is a little stranger, I just used 2 USB sticks to do a fresh install of High Sierra on 11 out of 14 Mac mini's (late 2012) and the 11 worked out just fine.

And now suddenly on the last 3 Mac Mini's the installer on the USB sticks got damaged, ***?

Re: This copy of the Install macOS High Sierra.app application is damaged and can't be used to install macOS

Or give this a try, maybe it works

sudo softwareupdate -ia only installs software updates and will never update the OS itself.

That's exactly how I updated the last couple OS updates in High Sierra... SORRY

Problem resolved, the error message "This copy of the Install macOS High Sierra.app application is damaged and can't be used to install macOS" does NOT necessarily indicate that anything is wrong with the actual installation media.

No instead this little gem is Apple's way of telling you that either:

1) You are installing on non authentic Apple hardware

2) Your Apple hardware needs a firmware update, which requires you to first install the original OS X version that came with your hardware.

In the case of those 3 2012 Mac Mini's that would not install High Sierra for me, I had to install Mountain Lion first, after that I could erase the drive again and do a clean install of High Sierra with no problems.

Thank you Apple for the quality of your 'error' messages.

Try from terminal and see if you can there:

sudo softwareupdate -ia