===== BADSIG ===== 

Error:
<code>
http://httpredir.debian.org squeeze-lts Release: The following signatures were invalid: BADSIG 8B48AD6246925553 Debian Archive Automatic Signing Key (7.0/wheezy) <ftpmaster@debian.org>
</code>

<code bash>
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get update
</code>

This will rebuild the cache.

Error:
<code>
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 13B00F1FD2C19886
</code>

<code bash>sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key 13B00F1FD2C19886</code>