In contrast to the pessimistic view on perfect secrecy, studies on information-theoretic secrecy have shown that perfect secrecy may be possible in practice. A typical example is the Wyner’s wire-tap channel concept. However, existing approaches mostly depend on unrealistic noise or error-rate assumptions. We propose a more practical way in this paper which uses advanced space-time transmissions for enhanced information-theoretical secrecy. Based on the fundamental limits of MIMO blind channel identification, space-time transmissions with proper randomization can effectively prevent the adversary from channel estimation while allowing reliable transmission between the authorized parties. The high error rate suffered by the adversary enhances secret channel capacity. This paper points out a new and practical approach for secret-key agreement which achieves perfect secrecy based on the physical-layer signal processing techniques.
In contrast to the pessimistic view on perfect secrecy, studies on information-theoretic secrecy have shown that perfect secrecy may be possible in practice. A typical example is the Wyner’s wire-tap channel concept. However, existing approaches mostly depend on unrealistic noise or error-rate assumptions. We propose a more practical way in this paper which uses advanced space-time transmissions for enhanced information-theoretical secrecy. Based on the fundamental limits of MIMO blind channel identification, space-time transmissions with proper randomization can effectively prevent the adversary from channel estimation while allowing reliable transmission between the authorized parties. The high error rate suffered by the adversary enhances secret channel capacity. This paper points out a new and practical approach for secret-key agreement which achieves perfect secrecy based on the physical-layer signal processing techniques.
An important building block of the cross-layer network security design is the physical-layer security which addresses problems such as whether the physical-layer can have build-in security and whether physical-layer techniques can assist the upper-layer encryption-based security techniques. This paper provides a positive answer by realizing information-theoretic secrecy with space-time transmissions. While most existing results on information-theoretic secrecy rely on some impractical assumptions, we propose a more practical approach that uses the redundancy of space-time transmissions to create intentional ambiguity to the adversary. Secrecy can be guaranteed even if the adversary has extremely high receiving signal-to-noise ratio. This approach is presented as a new way for secret-key agreement. Simulations in terms of receiving error rate and secret channel capacity are presented.
Information-theoretic secrecy gives the strongest information security that is important for military wireless networks. In this paper, we show that randomized MIMO transmissions can achieve such secrecy. A method is proposed to randomize the MIMO transmission coefficients. The received signal of the adversary has some special statistical distributions, based on which the indeterminacy of the adversary's blind deconvolution is proved. Receiving error rates and secret channel capacity are analyzed and simulated. This paper points out a practical way to realize the well-known wire-tap channel for perfect secrecy, which is one of the interesting challenges in information theory. The proposed method is also useful for key management in military wireless networks.
Secure communication ensures the integrity and confidentiality of communication between connected devices. An information-theoretic approach to secure communication gives the strongest confidentiality guarantee by assuming that the attacker has unlimited computing power. The earliest formal model and definition of information-theoretic secure communication is by Shannon, who employed a secret key shared between communicating parties to provide confidentiality. An alternative elegant information-theoretic approach to secure communication views the natural characteristics of the environment (i.e., channel’s noise) as a resource to build security functionalities. This approach was first proposed by Wyner, and the corresponding secure communication model is called the wiretap channel model. These two approaches introduce two primary resources for providing information-theoretic secure communication: the shared secret key and physical properties of the communication medium. In this thesis, we study how to employ the above two resources for secure message transmission. We study this by using channel’s noise in the wiretap channel model. In this model, a sender is connected to the receiver and the adversary through two noisy channels. We propose a new wiretap encoding scheme with strong secrecy that provides perfect secrecy and reliability, asymptotically. The construction treats the noise in the adversary’s channel as a source of randomness that is extracted and used to hide the message from the adversary. We realize the wiretap channel model using cooperative jamming to evaluate the performance of wiretap codes in practice. We consider a model called keyed wiretap channel that unifies Wyner’s model with Shannon’s model of perfect secrecy for information systems, and propose a keyed encoding schemes with strong secrecy and other properties that are attractive in practice. We also study two-party information-theoretic secret key agreement when the two parties have access to samples of a common source of randomness and use a single message transmission to arrive at a shared random key. We propose a secret key agreement protocol in this setting, prove its security, and show its superior performance compared to other known protocols with the same properties. Finally, we propose an information-theoretic secret key agreement over a virtual wiretap channel created by cooperative jamming.
Encryption of confidential data with a secret key has become a widespread technique for securing wireless transmissions. However, existing key distribution methods that either deliver the secret key with a key distribution center or exchange the secret key using public-key cryptosystems are unable to establish perfect secret keys necessary for symmetric encryption techniques. This research considers secret key establishment, under the broad research area of information theoretic security, using the reciprocal wireless channel as common randomness for the extraction of perfect secret keys in multiple-input multiple-output (MIMO)communication systems. The presentation discusses the fundamental characteristics of the time-variant MIMO wireless channel and establishes a realistic channel simulation model useful for assessing key establishment algorithms. Computational examples show the accuracy and applicability of the model. The discussion then turns to an investigation of the influence of the spatial and temporal correlation of the channel coefficients on the bound of the key size generated from the common channel, and it is found that a sampling approach exists that can generate a key using the minimum sampling time. A practical key generation protocol is then developed based on an enhancement of a published channel coefficient quantization method that incorporates flexible quantization levels, public transmission of the correlation eigenvector matrix and low-density parity-check (LDPC) coding to improve key agreement. This investigation leads to the development of improved channel quantization techniques that dynamically shift the quantization boundaries at one node based on the information provided by the other node. Analysis based on a closed-form bound for the key error rate (KER) and simulations based on the channel model and measurement data show that the enhanced algorithms are able to dramatically reduce key mismatch and asymptotically approach the KER bound. Finally, other secret key generation algorithms based on channel-encryption rather than quantization are discussed, leading to a new concept for secret key generation using the common wireless channel.
Physical layer security (PLS) is a new paradigm aiming at securing communications between legitimate parties at the physical layer. Conventionally, achieving confidentiality in communication networks relies on cryptographic techniques such as public-key cryptography, secret-key distribution and symmetric encryption. Such techniques are deemed secure based on the assumption of limited computational abilities of a wiretapper. Given the relentless progress in computational capacities and the dynamic topology and proliferation of modern wireless networks, the relevance of the previous techniques in securing communications is more and more questionable and less and less reliable. In contrast to this paradigm, PLS does not assume a specific computational power at any eavesdropper, its premise to guarantee provable security via employing channel coding techniques at the physical layer exploiting the inherent randomness in most communication systems. In this dissertation, we investigate a particular aspect of PLS, which is secret-key agreement, also known as secret-sharing. In this setup, two legitimate parties try to distill a secret-key via the observation of correlated signals through a noisy wireless channel, in the presence of an eavesdropper who must be kept ignorant of the secret-key. Additionally, a noiseless public channel is made available to the legitimate parties to exchange public messages that are also accessible to the eavesdropper. Recall that key agreement is an important aspect toward realizing secure communications in the sense that the key can be used in a one-time pad scheme to send the confidential message. In the first part, our focus is on secret-sharing over Rayleigh fading quasi-static channels. We study the fundamental relationship relating the probability of error and a given target secret-key rate in the high power regime. This is characterized through the diversity multiplexing tradeoff (DMT) concept, that we define for our model and then characterize it. We show that the impact of the secrecy constraint is to reduce the effective number of transmit antennas by the number of antennas at the eavesdropper. Toward this characterization, we provide several schemes achieving the DMT and we highlight disparities between coding for the wiretap channel and coding for secret-key agreement. In the second part of the present work, we consider a fast-fading setting in which the wireless channels change during each channel use. We consider a correlated environment where transmit, legitimate receiver and eavesdropper antennas are correlated. We characterize the optimal strategy achieving the highest secret-key rate. We also identify the impact of correlation matrices and illustrate our analysis with various numerical results. Finally, we study the system from an energy-efficiency point of view and evaluate relevant metrics as the minimum energy required for sharing a secret-key bit and the wideband slope.
Despite the tremendous progress made toward establishing PLS as a new paradigm to guarantee security of communication systems at the physical layer, there is a common belief among researchers and industrials that there are many practical challenges that prevent PLS from flourishing at the industrial scale. Most secure message transmission constructions available to date are tied to strong assumptions on CSI, consider simple channel models and undermine eavesdropping capabilities; thus compromising their practical interest to a big extent. Perhaps arguably, the most likely reasonable way to leverage PLS potential in securing modern wireless communication systems is via secret-key agreement. In the latter setting, the legitimate parties try to agree on a key exploiting availability of a public channel with high capacity which is also accessible to the eavesdropper. Once a key is shared by the legitimate parties, they may use it in a one-time pad encryption, for instance. In this article, we investigate two performance limits of secret-key agreement communications; namely, the secret-key diversity-multiplexing trade-off and the effect of transmit correlation on the secret-key capacity. We show via examples how secret-key agreement offers more flexibility than secure message transmissions. Finally, we explore a few challenges of secret-key agreement concept and propose a few guidelines to overturn them.