Puppet users - Jul 2013 - Getting the following error with repo key authentication on puppet client

I am getting the following error while running "puppet agent -t" on
client
side..how do I enable repository keys on puppet master to get rid off this 
error.

info: Applying configuration version ''1375302925''
err: /Stage[main]/Nrpe/Package[nagios-plugins-basic]/ensure: change from 
purged to present failed: Execution of ''/usr/bin/apt-get -q -y -o 
DPkg::Options::=--forc.
Building dependency tree...
Reading state information...
The following extra packages will be installed:
  nagios-plugins-common
Suggested packages:
  nagios3 icinga
The following NEW packages will be installed:
  nagios-plugins-basic nagios-plugins-common
0 upgraded, 2 newly installed, 0 to remove and 44 not upgraded.
Need to get 1204 kB of archives.
After this operation, 2654 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
  nagios-plugins-common nagios-plugins-basic
E: There are problems and -y was used without --force-yes

Regards,
Chittaranjan

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