sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys $Key

Where $key == the gpg key id. The key id is shown in the error message you get when running apt-get update after adding a new repository. For example:

GPG error: http://download.virtualbox.org karmic Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY DCF9F87B6DFBCBAE

执行命令：sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 6DFBCBAE

出现以下错误提示：
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 6DFBCBAE
gpg: requesting key 6DFBCBAE from hkp server keyserver.ubuntu.com
gpg: keyserver timed out
gpg: keyserver receive failed: keyserver error

DCF9F87B6DFBCBAE is the key ID for the virtualbox karmic repository.

解决办法，执行如下命令：
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys DCF9F87B6DFBCBAE