apt-get update failed. GPG ...The following signatures are invalid: NODATA 1 NODATA 2 How to fix?

Question

linux

Hello.
One day on Ubuntu-server 12.04 apt-get update stopped running
Here is the output of apt-get update

[email protected]:/# apt-get update
Получено:1 http://ru.archive.ubuntu.com precise Release.gpg [90 B]
Получено:2 http://extras.ubuntu.com precise Release.gpg [90 B]
Получено:3 http://ru.archive.ubuntu.com precise-updates Release.gpg [90 B]
Получено:4 http://extras.ubuntu.com precise Release [90 B]                     
Получено:5 http://ru.archive.ubuntu.com precise-backports Release.gpg [90 B]   
Получено:6 http://ru.archive.ubuntu.com precise Release [90 B]
Получено:7 http://ru.archive.ubuntu.com precise-updates Release [90 B]
Получено:8 http://ru.archive.ubuntu.com precise-backports Release [90 B]
Игн http://extras.ubuntu.com precise Release        
E: Ошибка GPG: http://extras.ubuntu.com precise Release: Следующие подписи неверные: NODATA 1 NODATA 2

# deb cdrom:[Ubuntu-Server 12.04.4 LTS _Precise Pangolin_ - Release amd64 (20140204)]/ dists/precise/main/binary-i386/
# deb cdrom:[Ubuntu-Server 12.04.4 LTS _Precise Pangolin_ - Release amd64 (20140204)]/ dists/precise/restricted/binary-i386/
#deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)]/ precise main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://ru.archive.ubuntu.com/ubuntu/ precise main restricted
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://ru.archive.ubuntu.com/ubuntu/ precise-updates main restricted
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://ru.archive.ubuntu.com/ubuntu/ precise universe
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise universe
deb http://ru.archive.ubuntu.com/ubuntu/ precise-updates universe
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://ru.archive.ubuntu.com/ubuntu/ precise multiverse
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://ru.archive.ubuntu.com/ubuntu/ precise-updates multiverse
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://ru.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://ru.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse

#deb http://security.ubuntu.com/ubuntu precise-security main restricted
#deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
#deb http://security.ubuntu.com/ubuntu precise-security universe
#deb-src http://security.ubuntu.com/ubuntu precise-security universe
#deb http://security.ubuntu.com/ubuntu precise-security multiverse
#deb-src http://security.ubuntu.com/ubuntu precise-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main
#deb http://drbl.sourceforge.net/drbl-core drbl stable

apt-get clean didn't help,
commenting out the repository doesn't help either.
please help.
/var/lib/apt/lists tried to clean. did not help

Answer 1

Maybe it will help:
ubuntuforums.org/showthread.php?t=1983220

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 176A5C84ED67C9ED