Awkward GPG error with Ubuntu Server running “sudo apt-get update”

[GabMus]

I just installed raring 32 bit server on an old machine, it seemed to work fine, but when I try to run "sudo apt-get update" I get this error (I'm translating from italian):

E: GPG Error: http://security.ubuntu.com raring-security Release: The following signatures were not valid: NODATA 1 NODATA 2
This is just awkward, I never experienced something like this, what should I do?
PS: I can't install any package because they are not recognised (i.e. if i try to install openjdk-6-jdk it tells me that the package cannot be found)

[dino99]

try:

sudo apt-get clean
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get clean
sudo apt-get update

[GabMus]

try:

sudo apt-get clean
cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get clean
sudo apt-get update
I already tried, but it doesn't work :\

[oldos2er]

Try
sudo apt-key update

[GabMus]

Try
sudo apt-key update
I receive this output:

gpg: key 437D05B5: "Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>" not changedgpg: key FBB75451: "Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>" not changed
gpg: key C0B21F32: "Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>" not changed
gpg: key EFE21092: "Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>" not changed
gpg: Total number processed: 4
gpg: unchanged: 4




And sudo apt-get update still returns the same error :(

[oldos2er]

Can you install and remove packages ok? If so I wouldn't worry about the error.

[GabMus]

Can you install and remove packages ok? If so I wouldn't worry about the error.
I can't, it is like the repos are empty. If I try to install something it says that the package doesn't exist.

[oldos2er]

Can you post your sources.list?
cat /etc/apt/sources.list