  A lower bound on the secrecy capacity of the wiretap channel with state
information available causally at both the encoder and decoder is established.
The lower bound is shown to be strictly larger than that for the noncausal case
by Liu and Chen. Achievability is proved using block Markov coding, Shannon
strategy, and key generation from common state information. The state sequence
available at the end of each block is used to generate a key, which is used to
enhance the transmission rate of the confidential message in the following
block. An upper bound on the secrecy capacity when the state is available
noncausally at the encoder and decoder is established and is shown to coincide
with the lower bound for several classes of wiretap channels with state.
