Joe.C

Error Msg :

W: GPG error: http://[UBUNTU SERVER ADDRESS] precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXXXXX

1. 제대로된 키를 가져와서 import 시키기

wget -O - http://[Repository which has a key]/[key_file] | sudo apt-key add - ; wget http://[Repository which has a key]/[key_file] ; gpg --import dpi_package_key ; rm -f ./dpi_package_key

2. 혹은...

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6AF0E1940624A220

에러를 만난 어떤 사람의 방법.....1

Run the following command - 
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6AF0E1940624A220This did not solve my problem. I got connection timed out.
Generally Ubuntu key-server uses port 11371 which may be blocked by tour firewall. So you can use 80 as the port - 
sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 6AF0E1940624A220 - See more at: http://opensourceforgeeks.blogspot.in/2013/04/w-gpg-error-httpppalaunchpadnet-precise.html#sthash.aQJUZnbW.dpuf

방법 2

   If the command are not doing it for you, you have to do it on your own. After a long research on this problem i finally decided that the best thing to do was manually download the key and add it.This is how i did it. Go to Ubuntu Key-server(keyserver.ubuntu.com). Here you can search your key. In my case it was 6AF0E1940624A220. Note while searching for key pre-append the key with 0x. So search for 0x6AF0E1940624A220 

Once you have copied the data to file. Save it. Then run the following command
sudo apt-key add key1 

You will get an "OK" response.

aniket@aniket-Compaq-610:~/Desktop$ sudo apt-key add key1
[sudo] password for aniket: 
OK

And you are done. Repeat the procedure of other keys that might be missing.
Then you can run
sudo apt-get update
and you will not get any error. Update happens smoothly. Do let me know if you still have any doubts.

(from "http://opensourceforgeeks.blogspot.in/2013/04/w-gpg-error-httpppalaunchpadnet-precise.html")

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1. https://www.petsymposium.org/2014/papers/Renkema.pdf