silly phone app idea

Anyone tired of losing your apartment key or forgetting your password to unlock a door> I was running to my room yesterday under the freezing weather. When i got to my door, what i pulled out of my pocket was my cellphone. My key was lost somewhere on the way when i run. At that moment, my only wish was to have my phone turn into my key. It's would be so much easier to keep. The phone is significant larger compare to the key. You can argue that making the key bigger can solve the problem but who want to carry a giant key around them anyway ? It's pointless to do that. I am thinking of a basic system where we have lock that receive signal from our cellphone. We can have our app store a binary combination and use as a key to decode the lock in order to open the door in a acceptable distance.