This idea came up when I saw a guy in the train today, carrying a cardboard box with a number written on it.

I stared at the box, and it took a short while before I realised the number was a phone number. I wondered why that was, and then it occurred to me that maybe the guy carrying the box was the recipient of the box. Maybe the phone number on the box was used to call him up to say his parcel was waiting for him…

And then I realised that the number could also have been used to authenticate him when he showed up to collect his parcel. Suppose a caretaker is responsible to hand over parcels to the rightful recipient. All the caretaker knows is the mobile phone of the recipient. The caretaker also has a phone, whose number is secret. If a recipient shows up to claim a parcel, the caretaker simply asks him to show his mobile phone. Next, the caretaker dials the phone number written on the parcel. If the call is displayed on the mobile phone held in the hands of the recipient (with the number of the caretaker clearly shown in the display), this proofs the rightful person showed up to collect the parcel.

Of course this is very similar to the use SMS to send one-time passwords or transaction codes to people signing in to websites, or doing electronic banking. But this protocol doesn’t cost you anything (if the caller does not pick up his phone). Moreover, this protocol allows you to ask someone else to collect the parcel… Simply forward all your incoming calls to his mobile…