In early November, Apple updated the Mac App Store’s signing certificate, and switched from SHA-1 to SHA-2 hashing.

This inadvertently caused a problem when launching some apps purchased through the Mac App Store, where the user would receive an alert stating the application “is damaged and can’t be opened” and “delete the app and download it again from the App Store”.

Apple has since reverted to using SHA-1 hashing for their certificate, and plans to include a more permanent fix in future OS X updates. Rebooting should reload the certificate cache, and fix this issue for anyone currently experiencing it.