How do i auto sign rpm's with rpmbuild?

[Mark]

Hey,

i'm making quite a few rpm's at this moment but i can't seem to find
out how to auto sign them.

I currently run commands like this:
rpmbuild -ba --sign my_rpm.spec

than it wants me to fill in a passphrase. i want to do that automatically.
It is possible (i don't assume that fedora is typing it all manually
for there packages).
Any idea's on how to do this?

Mark