I had been receiving errors like the following on Ubuntu 14.04 when trying to update my packages from synaptic:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32 and so on ...
This happens when you add too many trusted keyrings which GPG can’t handle. Not your fault either. Keyrings keep adding up as you add more and more PPAs. The solution is to remove the keyrings which you do not need. Try the following:
$ sudo cp -rvf etc/apt/trusted.gpg.d/ /etc/apt/trusted.gpg.d.backup $ cd etc/apt/trusted.gpg.d/ $ ls
Now start removing the .gpg files you no longer need. For example:
$ sudo rm linvinus-altyo.gpg*
If it’s not clear from the file name and you are wondering which package the PPA provides, Google the file name (without the .gpg extension) “+ PPA” and check the contents of the PPA online.
In case you have removed a necessary key, run the following:
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys [MISSING KEY] e.g. $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5
If everything goes fine, remove the backup directory:
$ sudo rm -rf /etc/apt/trusted.gpg.d.backup