  Denial-of-Service (DoS) threats pose a major challenge to the idea of
physical-layer key generation as the underlying wireless channels for key
extraction are usually public. Identifying this vulnerability, we study the
effect of DoS threats on relay-assisted key generation, and show that a
reactive jamming attack on the distribution phase of relay-assisted key
generation can forbid the nodes from extracting secret keys. To circumvent this
problem, we propose a self-sustainable key generation model, wherein a
frequency-hopping based distribution phase is employed to evade the jamming
attack even though the participating nodes do not share prior credentials. A
salient feature of the idea is to carve out a few bits from the key generation
phase and subsequently use them to pick a frequency band at random for the
broadcast phase. Interesting resource-allocation problems are formulated on how
to extract maximum number of secret bits while also being able to evade the
jamming attack with high probability. Tractable low-complexity solutions are
also provided to the resource-allocation problems, along with insights on the
feasibility of their implementation in practice.
