When running apt update in Ubuntu MATE 16.04

[lah7]

Also, see @Wimpy's announcement:

Apt Security - Ubuntu MATE 16.04 action required

Debian and Ubuntu apt requirements are in the middle of a significant update. If you’re running Ubuntu MATE 16.04 you may need to take action! apt in Debian Unstable and Ubuntu 16.04 now requires that repositories be signed with RSA keys using SHA256 or higher. Some 3rd party repositories don’t meet these requirements. Currently the Google Music Manager, Google Talk Plugin, and SpiderOakONE repositories are affected and will cause graphical package and update managers to fail out …