A system and method for generating keys that can be used for encrypting and decrypting PIN messages transmitted between mobile devices. In one embodiment, the method comprises defining a function f, where f(x,y) = f(y,x), and for each of first and second mobile devices, evaluating f with one of x and y having the value of the PIN of the respective mobile device and hard coding f into the respective mobile device. Each one of the first and second mobile devices can then compute a shared messaging key by evaluating f with the other of x and y having the value of the PIN of the other mobile device. The shared key can then be used specifically to encrypt and decrypt PIN messages transmitted between those two mobile devices.