$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [198 B]                                                                                                           
Get:2 Upgrade tool [1,247 kB]                                                                                                                  
Fetched 1,247 kB in 0s (0 B/s)                                                                                                                 
authenticate 'xenial.tar.gz' against 'xenial.tar.gz.gpg' 
gpg exited 1
Debug information: 


gpg: Signature made Sat 16 Apr 2016 12:33:59 PM EDT using DSA key ID 437D05B5
gpg: /tmp/ubuntu-release-upgrader-42sisy6p/trustdb.gpg: trustdb created
gpg: BAD signature from "Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>"

Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.