Insecure update error - SUPublicDSAKeyFile
The error comes from the Sparkle Update system (SU). It is a freely available system for integration by developers into apps to provide online updating of their apps.

The error message you see could come from any of your third party apps that are using Sparkle (it is very popular). 

The error message indicates that an important file is missing from the application's bundle. This usually indicates developer error (i.e. the program you've downloaded was "flawed" to begin with) - or it could be caused by you manually deleting the file.

I would start with Activity Monitor to see which third party apps you're running, and simply try closing down app after app until you're left with the one responsible for this popup.