This thread says to:

Code:
apt-key adv --keyserver hkp://keys.gnupg.net --recv-keys 7D8D0BF6
and the documentation says to:

Code:
gpg --keyserver hkp://keys.gnupg.net --recv-key 7D8D0BF6
But neither of those work (output of last command shown):

Code:
root@kali:~# gpg --keyserver hkp://keys.gnupg.net --recv-key 7D8D0BF6
gpg: requesting key 7D8D0BF6 from hkp server keys.gnupg.net
gpgkeys: key 7D8D0BF6 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0