Key distribution. Episode 1: Quantum menace

We study the possibility of applying related key attacks to cryptographic devices with hybrid key systems, when a session key is generated from a long-term «classical» key and a key generated via quantum key distribution (QKD). For the simplest system that XORes quantum key and long-term key we study the impact of compromising the «quantum» part. We show that for many QKD systems the probability of a quantum key recovery by an attacker is close to 1. This leads to the increase of success probability when applying related key attack. Therefore we propose to use key derivation functions for key update.