checking of signatures

[Ankush Grover]

Hey,

 I have downloaded the postfix from the postfix.org site.There is a
file called postfix-2.2.0.tar.gz.sig i know that it contains the
signature for the file against which i have to check
postfix-2.2.0.tar.gz. If the signature matches means there is no
problem in the downloaded file.

But I don't know how to check the signatures.

Morever sometimes rpm files also have signature checking please tell
me the procedure for checking those downloaded files also means if I
have downloaded a software there is also a signature file with that
software how do i check that downloaded software against the signature
file .

Thanks in advance

Regards

Ankush