GPG ERRO NO_PUBKEY: AVISO: Os seguintes pacotes não podem ser autenticados! [fechadas]

Acabei de receber alguns erros estranhos em relação às chaves GPG. Ou seja, eu acabei de adicionar ppa:ubuntu-toolchain-r/teste queria atualizar e, depois disso, obtive o seguinte:

W: GPG error: http://extras.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
W: GPG error: http://archive.canonical.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://qgis.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 030561BEDD45F6C3
W: GPG error: http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991
W: GPG error: http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991
W: GPG error: http://archive.ubuntu.com trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://archive.ubuntu.com trusty-updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://archive.ubuntu.com trusty-security Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

Tentei resolvê-lo de duas maneiras (fornecidas aqui )

for key in 40976EAF437D05B5 3B4FE6ACC0B21F32 A040830F7FAC5991 030561BEDD45F6C3 16126D3A3E5C1192  ; do sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys $key ; done

mas isso não ajudou (os mesmos erros vinham com as mesmas chaves), então tentei fazer isso:

sudo add-apt-repository ppa:webupd8team/y-ppa-manager
sudo apt-get update
sudo apt-get install y-ppa-manager

mas eu consegui isso que abriu meus olhos:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  launchpad-getkeys libvte-common libvte9 python-keyring python-launchpadlib
  python-lazr.restfulclient python-lazr.uri python-oauth python-secretstorage
  python-simplejson python-vte python-wadllib xclip yad
Suggested packages:
  python-gdata python-keyczar python-kde4 python-testresources
  python-secretstorage-doc
The following NEW packages will be installed:
  launchpad-getkeys libvte-common libvte9 python-keyring python-launchpadlib
  python-lazr.restfulclient python-lazr.uri python-oauth python-secretstorage
  python-simplejson python-vte python-wadllib xclip y-ppa-manager yad
0 upgraded, 15 newly installed, 0 to remove and 29 not upgraded.
Need to get 915 kB of archives.
After this operation, 4,515 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
WARNING: The following packages cannot be authenticated!
  libvte-common libvte9 python-keyring python-lazr.uri python-simplejson
  python-wadllib python-oauth python-lazr.restfulclient python-launchpadlib
  python-secretstorage python-vte xclip
Install these packages without verification? [y/N]

Comando de execução:

sudo apt-get update

Para cada chave de 16 caracteres relatada como ausente, execute o seguinte com a chave substituída:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ABCDEFGH12345678