Apple seems to be having trouble with the security certificate for the Mac App Store this week and, while they've issued a new one, not every Mac seems to have picked it. So, if you're trying to launch an app and all you get is the "damaged and can't be opened" popup, here's what you need to do.
Note: This also works when a Mac App Store app is reported as being damaged for other reasons.
How to fix a damaged app that won't open on the Mac
- Reboot your Mac.
- Open your Applications folder either by using the Finder or searching in Spotlight.
- Find the damaged app and delete it.
- Enter your Mac user password to authorize deletion, if requested.
- Launch the Mac App Store.
- Make sure you're logged in. (You may need to re-enter your password even if you were previously logged in.)
- Go to the app you just deleted, either in your Purchased tab or by searching for it.
- Click on Install to re-download it.
I only had to re-enter my password to get some apps up and running again. Others I had to delete and re-install. All are working for me again. Hopefully yours are too!