W: GPG error: http://dl.google.com stable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991

W: GPG error: http://deb.opera.com stable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY E585066A30C18A2B

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 52A794126E3AB2D3

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 83FBA1751378B444

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 00A6F0A3C300EE8C

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 531EE72F4C9D234C

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 7FB8BEE0A1F196A8

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY A7E13D78E4A4F4F4

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 464AD83D4631BBEA

W: GPG error: http://download.virtualbox.org precise InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 6AF0E1940624A220

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 991E6CF92D9A3C5B

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY C2518248EEA14886

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY C2518248EEA14886

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY C2518248EEA14886

W: GPG error: http://mirrors.dotsrc.org precise-getdeb Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY A8A515F046D7E7CF

W: GPG error: http://linux.dropbox.com precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY FC918B335044912E

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/precise/Release Unable to find expected entry ‘estricted/binary-i386/Packages’ in Release file (Wrong sources.list entry or malformed file)

here is the solution:

apt-key adv –keyserver keyserver.ubuntu.com –recv-keys A040830F7FAC5991 A8A515F046D7E7CF E585066A30C18A2B 52A794126E3AB2D3 83FBA1751378B444 00A6F0A3C300EE8C 531EE72F4C9D234C 7FB8BEE0A1F196A8 A7E13D78E4A4F4F4 464AD83D4631BBEA 6AF0E1940624A220 991E6CF92D9A3C5B C2518248EEA14886 C2518248EEA14886 C2518248EEA14886 54422A4B98AB5139 FC918B335044912E

Hi, I tried installing Dropbox on Ubuntu 12.04 LTS using Ubuntu Software Centre but it didn’t due to unmet dependencies. So I uninstalled it. However, every time I run apt-get update I get the error:

GPG error: http://linux.dropbox.com precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY FC918B335044912E

I tried the solution you’ve provided but I still get the same error.