Bad Qubits — Learn Quantum by Doing

Multiple choice

In device-independent QKD (DI-QKD), what is the basis for the security guarantee, given that the internal workings of the devices are untrusted ('black boxes')?

An observed CHSH value S > 2 cannot be faked by any classical or pre-programmed box, so a genuine violation certifies real entanglement from the input-output statistics alone
The adversary is assumed to lack a large fault-tolerant quantum computer, so cannot run Shor's algorithm
Verification requires returning the devices to a trusted bank that holds a secret table of preparation bases
The devices are assumed to emit the four BB84 states and measure in the claimed basis, exactly as in standard BB84