See below the Apple information release on this issue:
How will customers be affected by the certificate renewal?
Users may receive a notification that your Mac app is damaged if it utilizes receipt validation to request a new receipt from Apple. They can resolve this issue by restarting their Mac or updating to OS X El Capitan (v10.11.2) or later.
Mac App Store customers running OS X Snow Leopard (v10.6.8) will be unable to purchase new apps or run previously purchased apps that utilize receipt validation until they install the Mac App Store Update for OS X Snow Leopard which is available via OS X Software Update.
If continue to get the "damaged and can't be opened" popup, please try the following:
- 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 and find the app you just deleted from the 'Purchases' tab on the Mac App Store
- Click on Install to re-download it.