Fix apt-get update “signatures couldn’t be verified because the public key is not available”

There are times when running apt-get update in Ubuntu will result in error messages such as the following:

[dev@dev ~]$ sudo apt-get update
Ign http://security.ubuntu.com trusty-security InRelease
Get:1 http://security.ubuntu.com trusty-security Release.gpg [933 B]
...
Fetched 21.9 MB in 14s (1,537 kB/s)
Reading package lists... Done
W: GPG error: http://security.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
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

Looking at the error above, apt is telling us that the following keys are missing: 40976EAF437D05B5 . Notice that these are listed multiple times. Each unique key will only need to be added once.

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32

Now run sudo apt-get update . The error should disappear now.