How to fix “There are no public key available for the following key IDs” Error in Debian

http://blog.kovyrin.net/2006/11/28/debian-problem-apt-get-update/

Problem:

# apt-get update
......
Fetched 5562B in 13s (421B/s)
Reading package lists... Done
W: There are no public key available for the following key IDs:
A70DAF536070D3A1
W: You may want to run apt-get update to correct these problems
#

Answer1:

$ apt-key update

Answer2:

# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys XXXXXXXXXXXXXXXX
...
# apt-key add /root/.gnupg/pubring.gpg
...
# apt-get update
...