I have Lubuntu 16.04. My problem is similar to this questiuon:    sudo apt-get update    Get:1 http://it.archive.ubuntu.com/ubuntu xenial InRelease [247 kB]    Hit:2 http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease           Hit:3 http://it.archive.ubuntu.com/ubuntu xenial-updates InRelease                 Hit:4 http://it.archive.ubuntu.com/ubuntu xenial-backports InRelease               Hit:5 http://www.bchemnet.com/suldr debian InRelease                               Hit:6 http://archive.canonical.com xenial InRelease                                Get:7 http://security.ubuntu.com/ubuntu xenial-security InRelease [93,3 kB]        Fetched 340 kB in 6s (52,6 kB/s)                                                   Reading package lists... Done    W: There is no public key available for the following key IDs:    FB510D557CC3E840Non of answers provided is working:According to answer #1 of this question:sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840Executing: /tmp/tmp.oqit2axEyE/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.comgpgkeys: key FB510D557CC3E840 not found on keyservergpg: no valid OpenPGP data found.gpg: Total number processed: 0gpg: keyserver communications error: key not foundgpg: keyserver communications error: bad public keygpg: keyserver receive failed: bad public keyOr this one from the same question, answer #2:sudo apt-get install debian-keyring debian-archive-keyringReading package lists... DoneBuilding dependency tree       Reading state information... Donedebian-archive-keyring is already the newest version (2014.3).debian-keyring is already the newest version (2016.01.20).0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.Or this one from the same question, answer #3:sudo apt-get install debian-archive-keyringReading package lists... DoneBuilding dependency tree       Reading state information... Donedebian-archive-keyring is already the newest version (2014.3).0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.Or this one from the same question, answer #4:for n in `ls /var/lib/apt/lists/*gpg`; do echo $n ; gpg --list-packets $n | grep FB510D557CC3E840; donels: cannot access '/var/lib/apt/lists/*gpg': No such file or directoryOr this one from the same question, answer #5:sudo apt-get install debian-keyring debian-archive-keyringReading package lists... DoneBuilding dependency tree       Reading state information... Donedebian-archive-keyring is already the newest version (2014.3).debian-keyring is already the newest version (2016.01.20).0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.Another try, answer #1 of this question:gpg --keyserver keyserver.ubuntu.com:80 --recv-keys 7CC3E840gpg: requesting key 7CC3E840 from keyserver.ubuntu.com:80gpgkeys: no keyserver host providedgpg: keyserver internal errorgpg: keyserver receive failed: keyserver erroranswer number 2 of the same question:sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 7CC3E840 gpg: WARNING: unsafe ownership on configuration file `/home/username/.gnupg/gpg.conf'gpg: external program calls are disabled due to unsafe options file permissionsgpg: keyserver communications error: general errorgpg: keyserver receive failed: general errorto be sure there is no problem with permission:ls -l ~/.gnupg/gpg.conf-rw------- 1 username username 9398 mag 16 15:34 /home/username/.gnupg/gpg.confFrom last answer:apt-key adv --keyserver hkp://keyserver.ubuntu.com --recv-keys  7CC3E840Executing: /tmp/tmp.8iRzKu4RVN/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com --recv-keys 7CC3E840gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.comgpgkeys: key 7CC3E840 not found on keyservergpg: no valid OpenPGP data found.gpg: Total number processed: 0gpg: keyserver communications error: key not foundgpg: keyserver communications error: bad public keygpg: keyserver receive failed: bad public key
Problem with public key on apt-get update
bash;ubuntu;gpg
The missing key is the key for the Samsung Unified Linux Driver Repository. Following the instructions added to the website yesterday should fix things:wget http://www.bchemnet.com/suldr/pool/debian/extra/su/suldr-keyring_1_all.debsudo dpkg -i suldr-keyring_1_all.deb(assuming you trust the repository of course!).