Secret key generation from static channels with untrusted relays

The generation of symmetric cryptographic keys based on reciprocal channel properties has evolved as a promising approach in recent years, especially for devices with only little computational power. Different schemes individually addressing constraints from special setups have been proposed, but most schemes generally suffer from bad performance in case of (nearly) static scenarios. Addressing this dilemma, we present a novel approach for physical layer based key generation with two static users supported by a third party. In contrast to other solutions, this approach does not rely on special hardware like multiple antennas and does not let the third party know anything about the secret key, as the helper is potentially not fully trusted. The scheme does not evade to other diversity domains, but still exploits temporal and spatial diversity. We present thorough performance and security analysis, also taking a passive attacker into account.