  In this paper, we investigate the secure coding issue for a wiretap channel
model with fixed main channel and varying wiretap channel, by assuming that
legitimate parties can obtain the wiretapping channel state information (CSI)
after some time delay. For the symmetric degraded delay CSI case, we present an
explicit weak security scheme by constructing secure polar codes on a one-time
pad chaining structure, and prove its weak security, reliability and capability
of approaching the secrecy capacity of perfect CSI case with delay CSI
assumption. Further for the symmetric no-degraded delay CSI case, we present a
modified multi-block chaining structure in which the original subset of frozen
bit is designed for conveying functional random bits securely. Then we combine
this modified multi-block chaining structure with the weak security scheme to
construct an explicit strong security polar coding scheme, and prove its strong
security, reliability and also the capability of approaching the secrecy
capacity of perfect CSI case with delay CSI assumption. At last, we carry out
stimulations to prove the performance of both secure schemes.
