Installing / Updating from the Nuxeo repository requires to add the public key of the repo. See https://doc.nuxeo.com/display/ADMINDOC710/Installing+the+Nuxeo+Platform+on+Linux

Unfortunately the packages seem to be signed with a weak digest. This is what I get:

apt-get update
Holen:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [94,5 kB]
[...]
Paketlisten werden gelesen... Fertig
W: gpgv:/var/lib/apt/lists/apt.nuxeo.org_dists_xenial_Release.gpg: The repository is insufficiently signed by key 0F3BCCFE175E96EE42FB77DD5E2FAE6BDD1BF5E4 (weak digest)

IMHO the maintainer of the packages (Nuxeo) have to solve this issue. I am unsing Ubuntu 16.04 LTS.