Nowadays, with wireless sensor networks (WSNs) being widely applied to diverse applications, heterogeneous sensor networks (HSNs), which can simultaneously support multiple sensing tasks in a common sensor field, are being considered as the general form of WSN system deployment. In HSNs, each application generates data packets with a different size, thereby resulting in fairness issues in terms of the network performance. In this paper, we present the design and performance evaluation of a differentiated channel access scheme (abbreviated to DiffCA) to resolve the fairness problem in HSNs. DiffCA achieves fair performance among the application groups by providing each node with an additional backoff counter, whose value varies according to the size of the packets. A mathematical model based on the discrete time Markov chain is presented and is analyzed to measure the performance of DiffCA. The numerical results show that the performance degradation of disadvantaged application groups can be effectively compensated for by DiffCA. Simulation results are given to verify the accuracy of the numerical model.

Entropy is a basic and important concept in information theory. It is also often used as a measure of the unpredictability of a cryptographic key in cryptography research areas. Ubiquitous computing (Ubi-comp) has emerged rapidly as an exciting new paradigm. In this special issue, we mainly selected and discussed papers related with ore theories based on the graph theory to solve computational problems on cryptography and security, practical technologies; applications and services for Ubi-comp including secure encryption techniques, identity and authentication; credential cloning attacks and countermeasures; switching generator with resistance against the algebraic and side channel attacks; entropy-based network anomaly detection; applied cryptography using chaos function, information hiding and watermark, secret sharing, message authentication, detection and modeling of cyber attacks with Petri Nets, and quantum flows for secret key distribution, etc.Keywords: applied cryptography; enhanced security; ubiquitous computing Entropy is a basic and important concept in information theory introduced by Claude E. Shannon. It is also often used as a measure of the unpredictability of a cryptographic key in cryptography research areas. Ubi-comp has emerged rapidly as an exciting new paradigm. Together with these trends, applied cryptography and security have become rising big issues for providing secure and trusted computing in the next generation of information and communications. A detailed discussion of these issues includes applied cryptography and security concerns that cover amongst others, confidentiality, integrity, and availability including various application areas. In particular, these topics will comprehensively focus on the important aspects of entropy-based applied cryptography and enhanced security for Ubi-comp. Topics in Ubi-comp include entropy-based applied cryptographic aspects, entropy-based hash functions, mathematical and algorithmic foundations of applied cryptography, advanced design and analysis of cryptographic algorithms, authentication and access control, privacy protection and trust computing, entropy-based network security issues, information hiding and digital forensics, security issues in cloud computing and mobile social networks. This special issue aims to provide advanced theories and applications. Furthermore, researchers contribute with original research and review articles that present state-of-the-art research outcomes, practical results in entropy-based applied cryptographic models, and enhanced security system for Ubi-comp.During our working period, we received a total of 33 submissions from at least 10 countries where the corresponding authors were majorly counted by the deadline for paper submission. All these submissions were found with significant contributions in main interested topics of our special issue. However, only 14 high quality papers were accepted after two or three-round strict and rigorous review processes. These accepted papers mainly look at our issu...