A wiretap channel I is one of the channel models that was proved to achieve unconditional security. However, it has been an open problem in realizing such a channel model in a practical network environment. The paper is committed to solve the open problem by introducing a novel approach for building wiretap channel I in which the eavesdropper sees a binary symmetric channel (BSC) with error probability p while the main channel is error free. By taking advantage of the feedback and low density parity check (LDPC) codes, our scheme adds randomness to the feedback signals from the destination for keeping an eavesdropper ignorant; on the other hand, redundancy is added and encoded by the LDPC codes such that a legitimate receiver can correctly receive and decode the signals. With the proposed approach, unconditionally-secure communication can be achieved through interactive communications, in which the legitimate partner can realize the secret information transmission without a pre-shared secret key even if the eavesdropper has better channel from the beginning.
A wiretap channel I is one of the channel models that was proved to achieve unconditional security. However, it has been an open problem in realizing such a channel model in a practical network environment. The paper is committed to solve the open problem by introducing a novel approach for building wiretap channel I in which the eavesdropper sees a binary symmetric channel (BSC) with error probability p while the main channel is error free. By taking advantage of the feedback and low density parity check (LDPC) codes, our scheme adds randomness to the feedback signals from the destination for keeping an eavesdropper ignorant; on the other hand, redundancy is added and encoded by the LDPC codes such that a legitimate receiver can correctly receive and decode the signals. With the proposed approach, unconditionally-secure communication can be achieved through interactive communications, in which the legitimate partner can realize the secret information transmission without a pre-shared secret key even if the eavesdropper has better channel from the beginning.
This paper investigates an unconditional communication model based on wiretap channel I. We will firstly review our previous study on build wiretap channel I in which the eavesdropper sees a binary symmetric channel (BSC) with error probability p while the main channel is error free. To make p as close to 0.5 as possible, the paper introduces a novel security code which helps turning the original system to achieve unconditional secure communications. Extensive simulation is conducted and the results verify our approach.
Due to the broadcast nature, wireless links are open to malicious intrusions from outsiders, which makes the security issues a critical concern in the wireless communicationsover them. Physical-layer security techniques, which are based on the Shannon’s unconditional secrecy model, are effective in addressing the security issue while meeting the required performance level. According to the Wyner’s wiretap channel model, to achieve unconditionally security communication, the first step is to build up a wiretap channel with better channel quality between the legitimate communication peers than that of the eavesdropper; and the second step is to employ a robust security code to ensure that the legitimate users experience negligible errors while the eavesdropper is subject to 0.5 error probability. \nMotivated by this idea, in this thesis, we build wiretap channels for the single antenna systems without resorting to the spatial degree in commonly observed the multiple-input multiple-output (MIMO) systems. Firstly, to build effective wiretap channels, we design a novel scheme, called multi-round two-way communications (MRTWC). By taking feedback mechanisms into the design of Low Density Parity Check (LDPC) codes, our scheme adds randomness to the feedback signals from the destination to keep the eavesdropper ignorant while adding redundancy with the LDPC codes so that the legitimate receiver can correctly receive and decode the signals. Then, the channel BERs are specifically quantified according to the crossover probability in the case of Binary Symmetric Channel (BSC), or the Signal to Noise Ratio (SNR) in the case of AWGN and Rayleigh channels. Thus, the novel scheme can be utilized to address the security and reliability. Meanwhile, we develop a cross-layer approach to building the wiretap channel, which is suitable for high dynamic scenarios. By taking advantage of multiple parameters freedom in the discrete fractional Fourier transform (DFRFT) for single antenna systems, the proposed scheme introduces a distortion parameter instead of a general signal parameter for wireless networks based on DFRFT. The transmitter randomly flip-flops the uses of the distortion parameter and the general signal parameter to confuse the eavesdropper. An upper-layer cipher sequence will be employed to control the flip-flops. This cryptographic sequence in the higher layer is combined with the physical layer security scheme with random parameter fipping in DFRFT to guarantee security advantages over the main communication channel. \nAs the efforts on the second step, this thesis introduces a novel approach to generate security codes, which can be used for encoding with low complexity by taking advantage of a matrix general inverse algorithm. The novel constructions of the security codes are based on binary and non-binary resilient functions. With the proposed security codes, we prove that our novel security codes can ensure 0.5 error probability seen by the wiretapper while close to zero by the intended receiver if the error probability of the wiretapper’s channel is over a derived threshold. Therefore, the unconditionally secure communication of legitimate partners can be guaranteed. It has been proved mathematically that the non-binary security codes could achieve closer to the security capacity bound than any other reported short-length security codes under BSC. \nFinally, we develop the framework of associating the wiretap channel building approach with the security codes. The advantages between legitimate partners are extended via developing the security codes on top of our cross-layer DFRFT and feedback MRTWC security communication model. In this way, the proposed system could ensure almost zero information obtained by the eavesdroppers while still keeping rather lower error transmissions for legitimate users. Extensive experiments are carried out to verify the proposed security schemes and demonstrate the feasibility and implement ability. An USRP testbed is also constructed, under which the physical layer security mechanisms are implemented and tested. Our study shows that our proposed security schemes can be implemented in practical communications settings.
This paper considers the design of practical schemes that enable two terminals, namely a source and a destination, to agree on a secret key in the presence of an eavesdropper (wire-tapper). The channel model considered is a fast Rayleigh fading wiretap channel with the constraints of quadrature phase-shift-keyed source symbols and hard-decision destination quantization. It is assumed that there exists a public feedback channel between the source and destination to facilitate the key-agreement process, however, any communication over the public channel is perfectly observed by the wiretapper. A key-agreement scheme employing irregular low-density parity-check (LDPC) codes is proposed, and its secrecy performance is measured in terms of the key rate between the source and destination and the leakage rate about the secret key at the wiretapper. A code search algorithm is also developed to design irregular LDPC codes to achieve good secrecy performance under various channel conditions.
The model of wiretap channel (WTC) is important as it constitutes the essence of physical layer security. Recently, it has been shown that a noiseless feedback channel from the legitimate receiver to the transmitter helps to increase the secrecy capacity of the WTC. However, the present feedback strategy focuses only on generating the key from the feedback signal and using this key to protect part of the transmitted message. This secret key-based feedback strategy has been proved to be optimal only for some degraded cases. In this paper, we propose a new feedback strategy for the WTC, where the feedback signal is not only used to generate the key but also used to generate cooperative information helping the legitimate parties to improve their encoding and decoding performance. We show that the proposed new strategy performs better than the already existing one, and the results are further explained via a binary example.
Recent accomplishments in physical-layer security research have shown that channel coding for secrecy can be effectively combined with security at other layers, such as cryptography at the application layer, in order to provide a significant security enhancement to communication systems. The goal of this previous work was to inhibit the passive eavesdropper in the wiretap channel model by encoding the ciphertext using nonsystematic low-density parity-check (LDPC) codes prior to transmission and by exploiting the advantage of feedback for legitimate parties. The net result was propagation of a single packet erasure to the detriment of the entire message. The security enhancement was characterized assuming statistically independent packet erasure channels (PECs) for the legitimate receiver and the eavesdropper. In this paper, we go beyond these results by addressing correlated erasure events across the two channels in a wiretap feedback framework. The intuitive notion that high correlation across channels reduces secrecy is shown through the complete characterization of the correlated channel scenario. Furthermore, it is shown that security improvements are still achievable in the face of positive correlation by means of judicious physical-layer design, even when the eavesdropper has a better channel than the legitimate receiver.
This paper studies the problem of secure communication over a wiretap channel p(y,z|x) with a secure feedback link of rate Rf, where X is the channel input, and Y and Z are channel outputs observed by the legitimate receiver and the eavesdropper, respectively. It is shown that the secrecy capacity, the maximum data rate of reliable communication while the intended message is not revealed to the eavesdropper, is upper bounded as Cs(Rf) les maxmin/p(x) {I(X;Y), I(X;Y |Z) + Rf}. The proof of the bound crucially depends on a recursive argument which is used to obtain the single-letter characterization. This upper bound is shown to be tight for the class of physically degraded wiretap channels. A capacity-achieving coding scheme is presented for this case, in which the receiver securely feeds back fresh randomness with rate Rf, generated independent of the received channel output symbols. The transmitter then uses this shared randomness as a secret key on top of Wyner's coding scheme for wiretap channels without feedback. Hence, when a feedback link is available, the receiver should allocate all resources to convey a new key rather than sending back the channel output.