If by 'security reasons' you mean both to protect your family and also be an excellent pet, I would have to recommend a Rottweiler. I have a male Rottie raised from 6months old with a newborn and he is gentle as a lamb with the child.But he is also a protective dog and intelligent.Dogs can sense aggression and Rotties are no different. If a part of the "pack" or family feels threatened he stiffens up and growls, and at 130 pounds that is enough to scare most people away.However, Rottweilers are a friendly, exuberant breed. They need walks every day and lots of space.This is a site with cited info, as most of what I said is personal experience. However, I think you will find the site corroborates my experiences.
www.web-rover.com