GPG Error while running apt-get

Question

Possible Duplicate:
What is the easiest way to resolve apt-get BADSIG GPG errors?

I am trying to update some R packages. For that I am running sudo apt-get update command. I am running Ubuntu 10.04 on VM Player, host is Windows XP SP3 and I am not getting BADSIG error. When I run that command I get the following error:

Fetched 491B in 7s (64B/s)

Reading package lists... Done

> W: GPG error: http://streaming.stat.iastate.edu lucid/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9

ubuntu@ubuntu-desktop:

I tried many things like using running some command like these:

gpg --keyserver keyserver.ubuntu.com --recv D67FC6EAE2A11821

gpg --export --armor D67FC6EAE2A11821 | sudo apt-key add -

OR

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 313D312748A22A95; gpg --export --armor 313D312748A22A95 | sudo apt-key add -

OR

Like in this question.

None of this is working. Can anyone help me out?

(Also, I am not much familiar with Ubuntu and this forum. If the tags are not perfect, please re-tag the question.)

EDIT 1:

After trying Bruno's answer:

ubuntu@ubuntu-desktop:~$ gpg --keyserver keyserver.ubuntu.com --recv E084DAB9
gpg: requesting key E084DAB9 from hkp server keyserver.ubuntu.com
gpgkeys: key E084DAB9 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
ubuntu@ubuntu-desktop:~$ 

EDIT 2:

I tried as per Bruno's second comment, still no luck:

Edited the file `/etc/sudoers as:

# /etc/sudoers
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the man page for details on how to write a sudoers file.
#

Defaults    env_reset
Defaults    env_keep=https_proxy     # (I added his line, without this comment)
# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL) ALL

# Allow members of group sudo to execute any command after they have
# provided their password
# (Note that later entries override this, so you might need to move
# it further down)
%sudo ALL=(ALL) ALL
#
#includedir /etc/sudoers.d

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

After modifying this file, I re-started the machine. Then ran the following commands and got the errors:

ubuntu@ubuntu-desktop:~$ export http_proxy=http://abc-xyz.com:8080; export https_proxy=$http_proxy
ubuntu@ubuntu-desktop:~$ sudo -E apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9
[sudo] password for ubuntu: 
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv-keys E084DAB9
gpg: requesting key E084DAB9 from hkp server keyserver.ubuntu.com
gpgkeys: key E084DAB9 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
ubuntu@ubuntu-desktop:~$ 

I also tried the info given here. This script can handle the proxy issue by itself. But when I run this command: sudo add-apt-repository ppa:nilarimogard/webupd8 , I get following error:

Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv 1DB29AFFF6C70907B57AA31F531EE72F4C9D234C
gpg: requesting key 4C9D234C from hkp server keyserver.ubuntu.com
gpgkeys: HTTP fetch error 7: couldn't connect to host
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
ubuntu@ubuntu-desktop:~$ 

which is kind of chicken-egg problem. Since this also needs to cross the same proxy. :)