  In wiretap model of secure communication the goal is to provide (asymptotic)
perfect secrecy and reliable communication over a noisy channel that is
eavesdropped by an adversary with unlimited computational power. This goal is
achieved by taking advantage of the channel noise and without requiring a
shared key. The model has attracted attention in recent years because it
captures eavesdropping attack in wireless communication. The wiretap adversary
is a passive eavesdropping adversary at the physical layer of communication. In
this paper we propose a model for adversarial wiretap (AWTP) channel that
models active adversaries at this layer. We consider a $(\rho_r, \rho_w)$
wiretap adversary who can see a fraction $\rho_r$, and modify a fraction
$\rho_w$, of the sent codeword. The code components that are read and/or
modified can be chosen adaptively, and the subsets of read and modified
components in general, can be different. AWTP codes provide secrecy and
reliability for communication over these channels. We give security and
reliability definitions and measures for these codes, and define secrecy
capacity of an AWTP channel that represents the secrecy potential of the
channel. The paper has two main contributions. First, we prove a tight upper
bound on the rate of AWTP codes with perfect secrecy for $(\rho_r,
\rho_w)$-AWTP channels, and use the bound to derive the secrecy capacity of the
channel. We prove a similar bound for $\epsilon$-secure codes also, but in this
case the bound is not tight. Second, we give an explicit construction for a
capacity achieving AWTP code family, and prove its security and efficiency
properties. We show that AWTP model is a natural generalization of Wyner's
wiretap models and somewhat surprisingly, also provides a direct generalization
for a seemingly unrelated cryptographic primitive, Secure Message Transmission
(SMT).
