eneko@lap:~$ sudo apt-get update
[sudo] password for eneko:
Des:1 http://archive.ubuntu.com trusty InRelease
Ign http://extras.ubuntu.com trusty InRelease
100% [1 InRelease gpgv 12,6 kB] [Esperando las cabeceras] [Conectando a extras.Splitting up /var/lib/apt/lists/partial/archive.ubuntu.com_ubuntu_dists_trusty_InIgn http://archive.ubuntu.com trusty InRelease
E: Error de GPG: http://archive.ubuntu.com trusty InRelease: Clearsigned file isn't valid, got 'NODATA' (does the network require authentication?)