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? If it helps, I 
can send you a signature, which is not working.

Thanks for the help!


Daniel

-- 
LeVA