Physical-layer security based on wiretap codes can
be used to complement cryptographic applications at higher
layers of the protocol stack. We assume a passive eavesdropper
that has access to noise-corrupted codewords with erasures
that are statistically independent to those of the legitimate
communication partners. Our goal is to minimize the information
leaked to the eavesdropper. In this paper we present a lowcomplexity coding scheme for channels with feedback, which
employs extensive interleaving of carefully punctured LDPC
codewords. The key idea is to ensure that every transmitted
packet is crucial for successful decoding. This is achieved by
ensuring that stopping-set bit combinations for coded blocks
are distributed among different packets and by enforcing that
retransmission requests be restricted to the friendly parties. A
probabilistic analysis reveals that an eavesdropper who uses a
message-passing decoding algorithm will experience catastrophic
decoding failure with high probability. This encoder thus provides
physical-layer secrecy which is both independent from, and
complementary of, the cryptographic layer. The proposed scheme
works even when an eavesdropper has a better channel than the
legitimate receiver.