Bdtyktl

Execute the following commands in terminal

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>

where <PUBKEY> is your missing public key for repository, e.g. 8BAF9A6F.

Then update

sudo apt-get update

ALTERNATE METHOD:

sudo gpg --keyserver pgpkeys.mit.edu --recv-key  <PUBKEY>

sudo gpg -a --export <PUBKEY> | sudo apt-key add -

sudo apt-get update

Note that when you import a key like this using apt-key you are telling the system that you trust the key you're importing to sign software your system will be using. Do not do this unless you're sure the key is really the key of the package distributor.

It happens when you don't have a suitable public key for a repository.

To solve this problem use this command:

gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv 9BDB3D89CE49EC21

which retrieves the key from ubuntu key server. And then this:

gpg --export --armor 9BDB3D89CE49EC21 | sudo apt-key add -

which adds the key to apt trusted keys.

The solution can be found here & here & here.

You need to get and import the key.

To get the key from a PPA, visit the PPA's Launchpad page. On every PPA page at Launchpad you will find this link (2), after clicking on 'Technical details about this PPA' (1):

Follow it and click on the key ID link (3):

Save the page, this is your key file.

Now it's time to import it:

• 
  Applications > Software Center,


• 
  Edit > Software sources...,


• Enter your password,


• Go to the Authentication tab and click on Import Key File..., finally


• Select the saved key file and click on OK.

That's it.

apt can only handle 40 keys in /etc/apt/trusted.gpg.d . 41 keys and you will get the GPG error "no public key found" even if you go through all the steps to add the missing key(s).

Check to see if there are any unused keys in this file from ppa(s) you no longer use. If all are in use, consider removing some ppa(s) along with the corresponding keyfiles in /etc/apt/trusted.gpg.d

Furthermore, using

sudo apt-key adv

Is considered a security risk and is not recommended as you are "undermining the whole security concept as this is not a secure way of recieving keys for various reasons (like: hkp is a plaintext protocol, short and even long keyids can be forged, …)". http://ubuntuforums.org/showthread.php?t=2195579

I believe the correct way to add missing keys (for example 1ABC2D34EF56GH78) is

gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv 1ABC2D34EF56GH78

gpg --export --armor 1ABC2D34EF56GH78 | sudo apt-key add -

There is a tiny script packaged in the WebUpd8 PPA which I'll link as a single .deb download so you don't have to add the whole PPA - which automatically imports all missing GPG keys.

Download and install Launchpad-getkeys (ignore the ~natty in its version, it works with all Ubuntu versions from Karmic all the way to Oneiric). Once installed, open a terminal and type:

sudo launchpad-getkeys

If you're behind a proxy, things are a bit more complicated so see this for more info

I faced the same issue while installing Heroku. The link below solved my problem -

http://naveenubuntu.blogspot.in/2011/08/fixing-gpg-keys-in-ubuntu.html

After fixing the NO_PUBKEY issue, the below issue remained

W: GPG error: xhttp://toolbelt.heroku.com ./ Release: The following signatures were invalid: BADSIG C927EBE00F1B0520 Heroku Release Engineering <release@heroku.com>

To fix it I executed the following commands in terminal:

sudo -i  

apt-get clean  

cd /var/lib/apt  

mv lists lists.old  

mkdir -p lists/partial  

apt-get clean  

apt-get update  

Source - Link to solve it

Make sure you have apt-transport-https installed:

dpkg -s apt-transport-https > /dev/null || bash -c "sudo apt-get update; 

sudo apt-get install apt-transport-https -y" 

Add repository:

curl https://repo.skype.com/data/SKYPE-GPG-KEY | sudo apt-key add - 

echo "deb [arch=amd64] https://repo.skype.com/deb stable main" | sudo tee /etc/apt/sources.list.d/skype-stable.list 

Install Skype for Linux:

sudo apt-get update 

sudo apt-get install skypeforlinux -y

Source: https://community.skype.com/t5/Linux/Skype-for-Linux-Beta-signatures-couldn-t-be-verified-because-the/td-p/4645756

More generally, the following method should work for every repository. First of all search, with eventual help of a search engine, for a text on the program provider's website looking like the following:

-----BEGIN PGP PUBLIC KEY BLOCK-----

Version: GnuPG v1.4.1 (GNU/Linux)

[...]

-----END PGP PUBLIC KEY BLOCK-----

Such a text is for example displayed on http://deb.opera.com. Copy the passage, paste it in an empty file that you create on your desktop. This results in the key file.

Then continue with the importation of the key:

• Applications > Sofware Center


• Edit > Sofware sources..., enter password


• Authentication tab, click on 'Import Key File...'


• Select the saved key file and click on 'Ok'.

You may now remove the previously created key file.

Good! I finaly found the way!

I've tested all method's to fix GPG error NO_PUBKEY and nothing working for me.

I've deleted the entire contents of the folder /etc/apt/trusted.gpg.d

cd /etc/apt/trusted.gpg.d

sudo rm -R *

sudo apt-get update

And I use the Y-PPA-Manager method because I'm too lazy to create all pubkey's manually (too many): http://www.unixmen.com/fix-w-gpg-error-no_pubkey-ubuntu/

run sudo apt-get update again and finaly all work great now! Tanks!

Based Source : post #17 on https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1263540

I had the same problem with DynDNS's Updater client.

Turns out it was just expired keys.

Reinstalling the software (downloading a new .deb from the website, then using Software Centre to reinstall) fixed the problem.

Error message for reference:

W: GPG error: http://cdn.dyn.com stable/ Release: The following signatures were invalid: KEYEXPIRED 141943.......