  Covert communication allows us to transmit messages in such a way that it is
not possible to detect that the communication is occurring. This provides
protection in situations where knowledge that people are talking to each other
may be incriminating to them. In this work, we study how covert communication
can be used for a different purpose: secret key expansion. First, we show that
any message transmitted in a secure covert protocol is also secret and
therefore unknown to an adversary. We then propose a protocol that uses covert
communication where the amount of key consumed in the protocol is smaller than
the transmitted key, thus leading to secure secret key expansion. We derive
precise conditions showing that secret key expansion from covert communication
is possible when there are sufficiently low levels of noise for a given
security level. We conclude by examining how secret key expansion from covert
communication can be performed in a computational security model.
