Quantum keystroke logging

Superdense coding has long been regarded as a secure quantum communication protocol. It is natural to assume that employing logical quantum states with error-correcting capability would not compromise this security. However, in the context of GKP-based quantum communication, we propose a vulnerability that we term quantum keystroke logging. Specifically, consider a large organization that prepares and transmits logical Bell states without ever assembling the full encoded Bell state. Even under this restriction, the organization can still extract the input information without detection, thereby realizing a form of quantum keystroke logging.