Schematics of the two intercept-resend attack strategies. Quantum channels are represented by continuous arrows, while dashed arrows represent lossless classical channels. Letters P and M denote preparation and measurement devices, respectively. (a) Eve breaks the quantum channel between Alice and Bob and inserts both of her devices in between. (b) Eve places her preparation device at a location close to, possibly inside, Bob’s lab and connects her measurement and preparation devices by a lossless classical channel.