sudo apt-get update
Get:1 http://security.ubuntu.com trusty-security InRelease [54.9 kB]           
Ign http://security.ubuntu.com trusty-security InRelease/lists/partial/security.ubuntu.com_ubuntu_dists_trusty-security_InRelease into data and signature failed
E: GPG error: http://security.ubuntu.com trusty-security InRelease: Clearsigned file isn't valid, got 'NODATA' (does the network require authentication?)