In a note to developers Apple Developer Relations apologized for the recent Mac App Store app signing snafu that made some apps quit working. The glitch caused users to see a false "damaged" error when opening certain apps.
In some cases users had to delete apps and re-download them. Apple says the problem was due to an update of the Mac App Store signing certificate.
A copy of the note from Apple was posted to Twitter by developer Donald Southard, Jr. It reads: “In anticipation of the expiration of the old Mac App Store certificate, we issued a new certificate in September. The new certificate used the stronger SHA-2 hashing algorithm in accordance with current recommended industry practice, where the old certificate had used the SHA-1 hashing algorithm.”
Apple adds that most of the issues are now resolved, though “some apps may still experience receipt verification failure if their receipt checking code makes incorrect assumptions about the certificate. Apple says developers should make sure that their code adheres to the Receipt Validation Programming Guide and check that all receipt validation issues are resolved.