This paper investigates compression of data encrypted with block ciphers, such as the Advanced Encryption Standard. It is shown that such data can be feasibly compressed without knowledge of the secret key. Block ciphers operating in various chaining modes are considered and it is shown how compression can be achieved without compromising security of the encryption scheme. Further, it is shown that there exists a fundamental limitation to the practical compressibility of block ciphers when no chaining is used between blocks. Some performance results for practical code constructions used to compress binary sources are presented.

This paper presents a coding scheme for the Gaussian wiretap channel based on low-density parity-check (LDPC) codes. The messages are transmitted over punctured bits to hide data from eavesdroppers. The proposed coding scheme is asymptotically effective in the sense that it yields a bit-error rate (BER) very close to 0.5 for an eavesdropper whose signal-to-noise ratio (SNR) is lower than the threshold SNRE, even if the eavesdropper has the ability to use a bitwise maximum a posteriori (MAP) decoder. Such codes also achieve high reliability for the friendly parties provided they have an SNR above a second threshold SNRB. It is shown how asymptotically optimized LDPC codes are designed with differential evolution where the goal is to achieve high reliability between friendly parties while keeping the security gap SNRB/SNREas small as possible to protect against passive eavesdroppers. The proposed coding scheme is encodable in linear time, applicable at finite block lengths, and can be combined with existing cryptographic schemes to deliver improved data security by taking advantage of the stochastic nature of many communication channels.

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.

This paper investigates compression of encrypted data. It has been previously shown that data encrypted with Vernam's scheme, also known as the one-time pad, can be compressed without knowledge of the secret key, therefore this result can be applied to stream ciphers used in practice. However, it was not known how to compress data encrypted with non-stream ciphers. In this paper, we address the problem of compressing data encrypted with block ciphers, such as the advanced encryption standard (AES) used in conjunction with one of the commonly employed chaining modes. We show that such data can be feasibly compressed without knowledge of the key. We present performance results for practical code constructions used to compress binary sources.

This paper presents a coding scheme for the Gaussian wiretap channel based on low-density parity-check (LDPC) codes. The messages are transmitted over punctured bits to hide data from eavesdroppers. It is shown that this method is asymptotically effective in the sense that it yields a BER very close to 0.5 for an eavesdropper whose SNR is lower than the threshold SNRE, even if the eavesdropper has the ability to use a bitwise MAP decoder. Such codes also achieve high reliability for the friendly parties provided they have an SNR above a second threshold SNRB. It is shown how asymptotically optimized LDPC codes can be designed with differential evolution where the goal is to achieve high reliability between friendly parties and security against a passive eavesdropper while keeping the security gap SNRB/SNREas small as possible. The proposed coding scheme is applicable at finite block lengths and can be combined with existing cryptographic schemes to deliver improved data security by taking advantage of the stochastic nature of many communication channels.

While rate-compatible punctured low-density parity-check (RCP-LDPC) codes offer high flexibility in terms of code rate at a relatively low cost in implementation complexity, they are reported to require more decoding iterations than unpunctured LDPC codes. In this paper1we consider layered belief propagation decoding and propose efficient check node layering that significantly accelerates the decoding convergence of RCP-LDPC codes. We show that the proposed layering outperforms both random layering and conventional BP decoding. The performance improvements become more distinctive at high rates and they come at no additional implementation cost.

We consider the problem of rate-compatible puncturing of low-density parity-check (LDPC) codes over additive white Gaussian noise (AWGN) channels. In previous work, it was shown that "good" puncturing distributions exist for LDPC codes but the code length was large. In this correspondence, we give a procedure for determining the puncturing distributions for LDPC codes with short block lengths (a few thousand bits) and show that careful puncturing can produce good performance. We compare the performance of the rate-compatible punctured LDPC codes with dedicated LDPC codes across a range of rates and see that the rate-compatible codes have a favorable complexity/performance tradeoff.

In this paper we treat rate-compatible punctured low-density parity-check (RCP-LDPC) codes. We show how a mother code can be efficiently punctured to achieve good performance over a wide range of rates and discuss the design of a mother code. While this approach can be applied to virtually any time-varying channel, we apply it to the ultra wide band (UWB) channel, e.g. 802.15.3a, and we show that RCP-LDPC codes perform well as compared to a selection of fixed rate unpunctured codes. They reduce the implementation complexity and enable the use of retransmission protocols based on incremental redundancy to increase the throughput