What's black and white and red all over?

Born of brine yet dried to dust, a pinch of me is often enough. I help to bring out every taste, with pepper close, i’m often placed.

I'm a portal, small and deep, A guarded secret I keep. Though round or square, my form may be, What must be found, to look through me?

I am your key. There are many like me, but this one is yours. I'm not usually metal, nor do I fit into a lock. You can use me to get in, but usually just once.

Hang me right, I'll see you through, I'm also nailed for protection. As in form, I am shaped like you, Can you beat this misdirection?