As a rule technology improves. Processors become faster, storage space becomes more plentiful, and components become smaller. We’ve seen computers go from slow machines with very little storage that were as big as a room to tiny little powerhouses with gigabytes of storage that fit in your pocket. Cellular technology is no different. Cellular inceptors, for example, can now be concealed in a printer:
Stealth Cell Tower is an antagonistic GSM base station in the form of an innocuous office printer. It brings the covert design practice of disguising cellular infrastructure as other things – like trees and lamp-posts – indoors, while mimicking technology used by police and intelligence agencies to surveil mobile phone users.
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Stealth Cell Tower is a Hewlett Packard Laserjet 1320 printer modified to contain and power components required implement a GSM 900 Base Station.
These components comprise:
- BladeRF x40
- Raspberry Pi 3
- 2x short GSM omnidirectional antennae with magnetic base
- 2x SMA cable
- Cigarette-lighter-to-USB-charger circuit (converting 12-24v to 5v)
- 1x USB Micro cable (cut and soldered to output of USB charger)
- 1x USB A cable (cut and soldered to printer mainboard)
The HP Laserjet 1320 was chosen not only for its surprisingly unmentionable appearance but also because it had (after much trial and error) the minimal unused interior volumes required to host the components. No cables, other than the one standard power-cord, are externally visible. More so, care has been taken to ensure the printer functions normally when connected via USB cable to the standard socket in the rear.
It’s an impressive project that illustrates a significant problem. Cellular interceptors work because the protocols used by the Global System for Mobile Communications (GSM) standard are insecure. At one time this probably wasn’t taken seriously because it was believed that very few actors had the resources necessary to build equipment that could exploit the weaknesses in GSM. Today a hobbyist can buy such equipment for a very low price and conceal it in a printer, which means inserting an interceptor into an office environment is trivial.
Fortunate, Long-Term Evolution (LTE) is a more secure protocol. Unfortunate, most cell phones don’t use LTE for phones calls and text messages. Until everything is switched over to LTE the threat posed by current cellular interceptors should not be taken lightly.