GnuPG can not read some pgp signatures

[LeVA <leva@az.isten.hu>]

Hello!

I have installed KMail a few days ago, and with it I've installed the 
GnuPG program too. But some of the signatures can not be read by gpg.
There are some messages, which has a signature.asc attached, but KMail 
writes this in the messages window:
"The message is signed, but the validity of the signature can't be 
verified.
Reason: No appropriate crypto plug-in was found."

And when I Save the attached signature, and run cat signature.asc | gpg 
--import, I get this messages:
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

But, sometimes I get messages, which has also a signature file attached, 
and it can be verified by KMail, and the signatures can be imported 
with gpg. For example these keys:

http://www.debian.org/security/keys.txt

I can import those keys, and KMail can verify these keys, when I'm 
getting emails from those guys.

What could be the problem with the other signature files?

Thanks for the help!


Daniel

-- 
LeVA