title: How to fix: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY
body: <p>I'm working on an embedded system on Debian 8 (to be upgraded).
  When I do <code>apt update</code>, I'm  getting the following:</p>
  <pre><code>...
  ...
  Hit http://deb.debian.org stable/contrib arm64 Packages
  Hit http://deb.debian.org stable/non-free arm64 Packages                       
  Fetched 116 kB in 19s (6011 B/s)                                               
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  109 packages can be upgraded. Run 'apt list --upgradable' to see them.
  W: GPG error: http://deb.debian.org stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9 NO_PUBKEY 605C66F00D6C9793
  </code></pre>
  <p>I tried:</p>
  <pre><code>apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 
  </code></pre>
  <p>but got:</p>
  <pre><code>Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --homedir /tmp/tmp.mXChDvLgjA --no-auto-check-trustdb --trust-model always --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-jessie-automatic.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-jessie-security-automatic.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-jessie-stable.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-squeeze-automatic.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-squeeze-stable.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-wheezy-automatic.gpg --keyring /etc/apt/trusted.gpg.d/debian-archive-wheezy-stable.gpg --keyserver hkp://p80.pool.sks-keyserver.net:80 --recv-keys 605C66F00D6C9793
  gpg: requesting key 0D6C9793 from hkp server p80.pool.sks-keyserver.net
  ?: p80.pool.sks-keyserver.net: Host not found
  gpgkeys: HTTP fetch error 7: couldn't connect: No such file or directory
  gpg: no valid OpenPGP data found.
  gpg: Total number processed: 0
  </code></pre>
  <p>I also tried:</p>
  <pre><code># gpg --keyserver pgp.mit.edu --recv-keys 648ACFD622F3D138 0E98404D386FA1D9 605C66F00D6C9793
  </code></pre>
  <p>which returned:</p>
  <pre><code>gpg: requesting key 22F3D138 from hkp server pgp.mit.edu
  gpg: requesting key 386FA1D9 from hkp server pgp.mit.edu
  gpg: requesting key 0D6C9793 from hkp server pgp.mit.edu
  gpg: /root/.gnupg/trustdb.gpg: trustdb created
  gpg: key 3CBBABEE: public key &quot;Debian Archive Automatic Signing Key (10/buster) &lt;ftpmaster@debian.org&gt;&quot; imported
  gpg: key 8DD47936: public key &quot;Debian Archive Automatic Signing Key (11/bullseye) &lt;ftpmaster@debian.org&gt;&quot; imported
  gpg: key 0D6C9793: public key &quot;Debian Stable Release Key (11/bullseye) &lt;debian-release@lists.debian.org&gt;&quot; imported
  gpg: no ultimately trusted keys found
  gpg: Total number processed: 3
  gpg:               imported: 3  (RSA: 3)
  </code></pre>
  <p>and running <code># apt update</code> after wards still gives me the <code>NO_PUBKEY</code> error. How do I get this fixed properly?
  My sources are the following:</p>
  <pre><code>cat /etc/apt/sources.list.d/multistrap-debian.list 
  deb [arch=arm64] http://deb.debian.org/debian stable main contrib non-free
  deb-src http://deb.debian.org/debian stable main contrib non-free
  </code></pre>