Stingrays and other IMSI-catchers violate the cell tower agreements by producing an illegal signal, pretending to be a cell tower.They control the encryption flag, which will cause a phone to downgrade security either to the least secure algorithm, or disable encryption completely. As far as a MITM goes, they may pass along the phone call data to a legitimate tower, or they may simply send back an error code the user sees as a call failure.
Virtually all modern phones technology is rooted in GSM technology. GSM standard was intentionally designed to be able to run with weak encryption. This allowed the export of GSM to Eastern Europe. Modern cell phones support different protocols, classic GSM, GPRS, HSDPA, 3G, 4G, …
Each of these was developed at their own time and the most basic protocol allowed for optional encryption or was using a “proprietary” encryption protocol that was vulnerable. This means your iPhone can still speak basic, vulnerable GSM – and can be tricked to use it.
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