title: Apt rejects keyrings in `/etc/apt/trusted.gpg.d` on Ubuntu 18.04
body: <p>I am facing a problem on Ubuntu 18.04 (Bionic Beaver) with apt and the way it deals with trusted keys to authenticate repositories.</p>
  
  <p>On Ubuntu 14.04 we used to install the key that was used sign the repository of our software releases as keyring to <code>/etc/apt/trusted.gpg.d</code>. By this apt knows that the key is trusted.</p>
  
  <p>However, this seems to not work anymore on Ubuntu 18.04. If I do the same there, I get an error during updating:</p>
  
  <pre><code># apt-get update
  Hit:1 http://company.com/ubuntu-snapshot bionic InRelease                     
  Reading package lists... Done                                                     
  W: http://company.com/ubuntu-snapshot/dists/bionic/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg.d/company-keys.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
  </code></pre>
  
  <p>The obvious attempt to fix it by</p>
  
  <pre><code># sudo chown -v _apt /etc/apt/trusted.gpg.d/company-keys.gpg
  changed ownership of '/etc/apt/trusted.gpg.d/company-keys.gpg' from root to _apt
  </code></pre>
  
  <p>does not work, as <code>apt-get update</code> then yields:</p>
  
  <pre><code># apt-get update
  Hit:1 http://company.com/ubuntu-snapshot bionic     InRelease                    
  Reading package lists... Done                                                     
  W: http://company.com/ubuntu-snapshot/dists/bionic/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg.d/company-keys.gpg are ignored as the file has an unsupported filetype.
  </code></pre>
  
  <p>The key itself is valid, if I add it with the following line everything works as expected:</p>
  
  <pre><code> wget -O - http://company.com/key.gpg | sudo apt-key add -
  </code></pre>
  
  <p>The latter is unfortunately not an option for us since we want to deploy our own keys and also have the ability to change/revoke them.</p>
  
  <p>I could neither figure out why apt rejects the keyring in <code>/etc/apt/trusted.gpg.d</code>, nor could I find a changelog describing different expectations of apt for the new Ubuntu version. Would be very glad if you point to some resource to overcome this issue.</p>