see: * postfix book

Create new key

root$ cd /usr/lib/ssl/misc
root$ ./CA.pl -newca        # create CA (Certificate Authority)
root$ ./CA.pl -newreq       # create certificate request
root$ ./CA.pl -sign         # sign it with your CA.
root$ openssl rsa < newreq.pem > key.pem # need decrypted key for postfix
root$ chmod 400 *.pem

see also: InstallPostfix, StunnelAdmin