Хм...
Попробуйте эту команду
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com C5E6A5ED249AD24C
а затем эту:
gpg --export --armor C5E6A5ED249AD24C | sudo apt-key add -
стало вот что
Manage apt's list of trusted keys
apt-key add <file> - add the key contained in <file> ('-' for stdin)
apt-key del <keyid> - remove the key <keyid>
apt-key export <keyid> - output the key <keyid>
apt-key exportall - output all trusted keys
apt-key update - update keys using the keyring package
apt-key net-update - update keys using the network
apt-key list - list keys
apt-key finger - list fingerprints
apt-key adv - pass advanced options to gpg (download key)