Untrusted versions of packages

[Andrey Tataranovich <tataranovich@gmail.com>]

Hello everybody,

Recently I started to get following errors on my squeeze server while
using squeeze-lts.

# apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  apt apt-utils rsyslog tzdata
4 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 3375 kB of archives.
After this operation, 344 kB disk space will be freed.
Do you want to continue [Y/n]? 
WARNING: The following packages cannot be authenticated!
  apt tzdata apt-utils rsyslog
Install these packages without verification [y/N]? 
E: Some packages could not be authenticated

# cat /etc/apt/apt.conf.d/50squeeze-lts 
APT::Default-Release "squeeze-lts";

# cat /etc/apt/sources.list
deb http://http.debian.net/debian/ squeeze main contrib non-free
deb http://http.debian.net/debian squeeze-lts main contrib non-free

# apt-cache policy debian-archive-keyring
debian-archive-keyring:
  Installed: 2010.08.28+squeeze1
  Candidate: 2010.08.28+squeeze1
  Version table:
 *** 2010.08.28+squeeze1 0
        500 http://http.debian.net/debian/ squeeze/main i386 Packages
        100 /var/lib/dpkg/status

Could anybody explain why this error happens?

-- 
WBR, Andrey Tataranovich