New techniques,languages and paradigms have facilitated the creation of distributed applications in several areas. Perhaps the most promising paradigm is the one that incorporates the mobile agent concept. A mobile agent in a large scale network can be viewed as a software program that travels through a heterogeneous network, crossing various security domains and executing autonomously in its destination. Mobile agent technology extends the traditional network communication model by including mobile processes, which can autonomously migrate to new remote servers. This basic idea results in numerous benefits including flexible, dynamic customisation of the behavior of clients and servers and robust interaction over unreliable networks. In spite of its advantages, widespread adoption of the mobile agent paradigm is being delayed due to various security concerns. Currently available mechanisms for reducing the security risks of this technology do not e±ciently cover all the existing threats. Due to the characteristics of the mobile agent paradigm and the threats to which it is exposed, security mechanisms must be designed to protect both agent hosting servers and agents. Protection to agent-hosting servers' security is a reasonably well researched issue, and many viable mechanisms have been developed to address it. Protecting agents is technically more challenging and solutions to do so are far less developed. The primary added complication is that, as an agent traverses multiple servers that are trusted to different degrees, the agent's owner has no control over the behaviors of the agent-hosting servers. Consequently the hosting servers can subvert the computation of the passing agent. Since it is infeasible to enforce the remote servers to enact the security policy that may prevent the server from corrupting agent's data, cryptographic mechanisms defined by the agent's owner may be one of the feasible solutions to protect agent's data.Hence the focus of this thesis is the development and deployment of cryptographic mechanisms for securing mobile agents in an open environment. Firstly, requirements for securing mobile agents' data are presented. For a sound mobile agent application, the data in an agent that is collected from each visiting server must be provided integrity. In some applications where servers intend to keep anonymous and will reveal their identities only under certain cir- cumstances, privacy is also necessitated. Aimed at these properties, four new schemes are designed to achieve different security levels: two schemes direct at preserving integrity for the agent's data, the other two focus on attaining data privacy. There are four new security techniques designed to support these new schemes. The first one is joint keys to discourage two servers from colluding to forge a victim server's signature. The second one is recoverable key commitment to enable detection of any illegal operation of hosting servers on an agent's data. The third one is conditionally anonymous digital signature schemes, utilising anonymous public-key certificates, to allow any server to digitally sign a document without leaking its identity. The fourth one is servers' pseudonyms that are analogues of identities, to enable servers to be recognised as legitimate servers while their identities remain unknown to anyone. Pseudonyms can be deanonymised with the assistance of authorities. Apart from these new techniques, other mechanisms such as hash chaining relationship and mandatory verification process are adopted in the new schemes. To enable the inter-operability of these mechanisms, a security architecture is therefore developed to integrate compatible techniques to provide a generic solution for securing an agent's data. The architecture can be used independently of the particular mobile agent application under consideration. It can be used for guiding and supporting developers in the analysis of security issues during the design and implementation of services and applications based on mobile agents technology.

In heterogeneous dense networks where spectrum is shared, users' privacy remains one of the major challenges. On a multi-antenna relay-assisted multi-carrier interference channel, each user shares the spectral and spatial resources with all other users. When the receivers are not only interested in their own signals but also in eavesdropping other users' signals, the cross talk on the spectral and spatial channels becomes information leakage. In this paper, we propose a novel secrecy rate enhancing relay strategy that utilizes both spectral and spatial resources, termed as information leakage neutralization. To this end, the relay matrix is chosen such that the effective channel from the transmitter to the colluding eavesdropper is equal to the negative of the effective channel over the relay to the colluding eavesdropper and thus the information leakage to zero. Interestingly, the optimal relay matrix in general is not block-diagonal which encourages users' encoding over the frequency channels. We proposed two information leakage neutralization strategies, namely efficient information leakage neutralization (EFFIN) and local-optimized information leakage neutralization (LOPTIN). EFFIN provides a simple and efficient design of relay processing matrix and precoding matrices at the transmitters in the scenario of limited power and computational resources. LOPTIN, despite its higher complexity, provides a better sum secrecy rate performance by optimizing the relay processing matrix and the precoding matrices jointly. The proposed methods are shown to improve the sum secrecy rates over several state-of-the-art baseline methods.

In heterogeneous dense networks where spectrum is shared, users' privacy remains one of the major challenges. On a multi-antenna relay-assisted multi-carrier interference channel, each user shares the spectral and spatial resources with all other users. When the receivers are not only interested in their own signals but also in eavesdropping other users' signals, the cross talk on the spectral and spatial channels becomes information leakage. In this paper, we propose a novel secrecy rate enhancing relay strategy that utilizes both spectral and spatial resources, termed as information leakage neutralization. To this end, the relay matrix is chosen such that the effective channel from the transmitter to the colluding eavesdropper is equal to the negative of the effective channel over the relay to the colluding eavesdropper and thus the information leakage to zero. Interestingly, the optimal relay matrix in general is not block-diagonal which encourages users' encoding over the frequency channels. We proposed two information leakage neutralization strategies, namely efficient information leakage neutralization (EFFIN) and local-optimized information leakage neutralization (LOPTIN). EFFIN provides a simple and efficient design of relay processing matrix and precoding matrices at the transmitters in the scenario of limited power and computational resources. LOPTIN, despite its higher complexity, provides a better sum secrecy rate performance by optimizing the relay processing matrix and the precoding matrices jointly. The proposed methods are shown to improve the sum secrecy rates over several state-of-the-art baseline methods.