Package cannot be authenticated?

[Behrang Saeedzadeh]

When I try install a package via Synaptic I get a message that it cannot be 
authenticated although that I'm downloading it from the ubuntu repositories. 
Is this normal? For example, I see this error message when I want to install 
partimage or linux-686.


Best Regards,
-- 

Behrang Saeedzadeh
http://www.jroller.com/page/behrangsa
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