Rating:
Challenge
We're given a Python service that runs a custom "quantum" block cipher on 8-bit blocks using Qiskit:
NQUBITS = 8 ROUNDS = 12 MAX_QUERIES = 1
Each connection generates secret = urandom(13). Of those, secret[0..11] drive the encryption rounds and secret[12] chooses an output slice. The interface lets us:
Encrypt one (1) chosen byte (MAX_QUERIES = 1).
Get the encrypted flag (no limit, but deterministic).
Each call returns a slice of 256 complex amplitudes per 8-bit block of plaintext.
The encryption circuit uses 16 qubits: 8 data qubits + 8 "key" qubits initialized to secret[0]. Each round does:
python
add_key # CRY(π/15) gates from key qubits to data qubits
permutation # iswap chain over data qubits (fixed)
rxz # RZX(π/e) on 4 disjoint pairs (fixed)
evolve_key # X gates flipping key register from secret[r] → secret[r+1]
Output: Statevector[secret[12]*256 : secret[12]*256+256].
