“Your cell phone is always working, even if you’re not using it, if for no other reason than to refresh and receive new text or email messages. It always seeks the nearest cell tower for a connection. And each cell tower then records that connection and memorializes it, stores it, down to the day, hour, minute, and second. So if the government has your cell phone number, they can go back and subpoena those cell-tower records—called cell-site location information, or CSLI—and retrace your steps. Not just the calls you made. Not just the text messages you sent. But every place you walked. Every
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