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How Do You Intercept a Text Message?Turn your cell phone into a spy gadget.

By Christopher Beam
Posted Wednesday, March 7, 2007, at 6:53 PM ET
Text messaging.

Retail giant Wal-Mart fired an employee Monday for eavesdropping on phone calls and intercepting text messages between the company's media-relations staff and a New York Times reporter. How do you intercept a text message?

Turn your own cell phone into a surveillance gadget. There are a few ways to do this. One method, phone cloning, lets you intercept incoming messages and send outgoing ones as if your phone were the original. If both phones are near the same broadcast tower, you can also listen in on calls. To clone a phone, you have to make a copy of its SIM card, which stores the phone's identifying information. This requires a SIM reader that can read the card's unique cryptographic key and transfer it to another phone. (Warning: This is super illegal, but there are still sites that show you how.) The problem with cloning is that it only lets you intercept messages sent to one phone number. Plus, you need physical access to the target phone to make it work—something Wal-Mart's technician probably didn't have.

It's also possible to intercept unencrypted or poorly encrypted messages directly as they're broadcast over cellular channels. (If the network uses sophisticated encryption, you might be out of luck.) To steal messages with your phone, you would need to upload illegal "firmware" onto your phone. This essentially turns your phone into a radio and allows it to pick up all the texts broadcast on a given channel—instead of limiting you to the ones addressed to you. You'd also need to know the network for the target phone—Verizon, Cingular, T-Mobile, etc.—and you'd have to make sure that both your phone and the target are within range of the same base station. This method isn't too expensive since you don't need much more than a computer, a phone, and some firmware that any serious techie could find online for free.

Wal-Mart isn't discussing details of the method its employee used, but a spokesperson did say he was able to intercept messages that included certain keywords. Companies like Global Security Solutions and Homeland Security Strategies develop interceptors for law-enforcement purposes. (Prices reportedly run as high as nearly $1 million.) These fancy devices essentially work the same way as the firmware method outlined above, but they have antennas for longer range and may run more smoothly.

Got a question about today's news? Ask the Explainer.

Explainer thanks Patrick Traynor of Pennsylvania State University.

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Christopher Beam is a Slate political reporter.
Photograph of a person text messaging by James Woodson/Digital Vision.
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