Howto verify packages with apt-get (gpg?)

[Fred Bowman <nospam@xonx.de>]

hi there.

are debian packages signed with pgp or something similar?
how can packages be verified?

for example, if i want to install the openssl package with
#apt-get install openssl

how can i proof, that the package is ok? md5sum is not satisfactory.

i checked the debian webpage, faq and searched mailing list but with no 
result.
thanx for any hint!
--
kind regards,
fred