
Anatomy of a SIM Card - benguild
https://twitter.com/benguild/status/249659847308632064
======
chm
Does anybody have an idea why there's only one pin left untouched?

~~~
jgeorge
Pin 5 (the unconnected pin) is programming voltage input, used only when
writing identity information to the SIM.

These chips are preprogrammed when manufactured, and the programming voltage
pin is left unconnected when placed in the carrier so it's impossible to re-
program the SIM card once made.

In older times, it was possible with the right hardware to reprogram a SIM
card to (for example) have two SIM cards report the same mobile number, so you
could essentially clone a phone number onto multiple handsets instead of
having to pay for multiple cellular lines. These newer style SIM cards prevent
hacking like this simply by making it impossible to reprogram the SIM by not
being able to power the programming circuit.

Source: <http://pinouts.ru/Memory/SmartCardIso_pinout.shtml>

