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 low-complexity 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.
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