Adding Key to Ubuntu

Sometime when you do apt-get update, ubuntu will complain about key not available. For example

...
Fetched 69.7kB in 50s (1,388B/s)
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9BF3BB4E5E17B5
$

To solve the error, you just need to install the key using this command

sudo apt-key adv --recv-keys --keryserver keyserver.ubuntu.com <public key number>

Change public key number with the one ubuntu complained. In this example it would be 5A9BF3BB4E5E17B5.

$sudo apt-key adv --recv-keys --keryserver keyserver.ubuntu.com 5A9BF3BB4E5E17B5
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --recv-keys --keyserver keyserver.ubuntu.com 5A9BF3BB4E5E17B5
gpg: requesting key 4E5E17B5 from hkp server keyserver.ubuntu.com
gpg: key 4E5E17B5: public key "Launchpad PPA for chromium-daily" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
$

Continue with update.