Tampered or corrupted update packages?

[Alimin Bijosono Oei <alimin_oei myrealbox com>]

Dear all,

I have encountered several Redhat update packages for my Psyche system 
with error message about the packages do not have a valid GPG signature 
and that they have been tampered with or corrupted. Because I am not 
sure about this I always choose No when asked whether I want to continue 
or not. One latest package that has the problem is 
glibc-common-2.3.2-4.80. What is wrong here and how to fix it?
Thanks for any help provided.

Alimin