$ apt-key adv --verify /var/lib/apt/lists/gb.archive.ubuntu.com_ubuntu_dists_xenial_InRelease
Executing: /tmp/tmp.hn8nF0WW0g/gpg.1.sh --verify
/var/lib/apt/lists/gb.archive.ubuntu.com_ubuntu_dists_xenial_InRelease
gpg: Signature made Wed 27 Jan 2016 07:19:32 GMT using DSA key ID 437D05B5
gpg: Good signature from "Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>"
gpg: WARNING: Using untrusted key!
gpg: Signature made Wed 27 Jan 2016 07:19:32 GMT using RSA key ID C0B21F32
gpg: Good signature from "Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>"
gpg: WARNING: Using untrusted key!