Quantum keystroke logging

Abstract: We propose a \textit{quantum keystroke logging} scheme in the context of GKP-based quantum communication. The central idea is that, if a communication provider controls the preparation of encoded states, it may be able to infer logical operations applied by users prior to transmission. We show that phase estimation can be adapted to this setting despite two obstacles: GKP codewords are not eigenstates of the logical operators, and realistic communication involves one-shot operations rather than repeated applications. Our approach relies on three observations: the geometric phase associated with closed trajectories in phase space manifests as an effective Pauli-$Z$ rotation on an ancilla; the quantum Fourier transform (QFT) need only reproduce the correct probability distribution and can thus be simplified using auxiliary ancilla; and, in oscillator systems, cross-Kerr nonlinearities provide a natural mechanism to reduce the circuit depth of the QFT. Together, these tools enable a malicious provider to extract user inputs without disturbing the transmitted codewords, thereby demonstrating a sufficient condition for quantum keystroke logging.