In the thesis we study deniable encryption, as proposed by Canetti et al. (CRYPTO 1997). Standard encryption schemes guarantee good security level unless the adversary is able to force the sender and/or receiver to reveal her secret knowledge. Assuming that the adversary knows true ciphertext, the se- cret inputs usually commits the sender/receiver to the true plaintext. On the contrary, deniable scheme is equipped with algorithms that provide alternative secrets which makes the adversary believe that different plaintext was encrypted. We recall the most important results in the area, in particular, the schemes of Canetti et al. (CRYPTO 1997), the scheme of Klonowski et al. (SOFSEM 2008) based on ElGamal encryption, schemes of O'Neill et al. (CRYPTO 2011), and schemes and impossibility result of Bendlin et al. (ASIACRYPT 2011). In ad- dition to presenting known results in an unified environment, we deeply investi- gate simulatable-encryption based schemes. In particular, we construct a scheme that is bideniable, and both of its induced schemes are receiver-deniable (in the flexible/multi-distributional setting). We also disprove part of the results of Bendlin et al. (ASIACRYPT 2011) by showing that their construction of fully bideniable scheme is wrong. This result is verified using computer simulation....

The objective of this study is to investigate and analyse covert action as an option in national security policy. To achieve this aim, the study focused on aspects such as changes in the current international security environment; new challenges that exist; and a conceptual framework of covert action as an element of intelligence. An analysis of the conduct of covert action by the United States of America (US) during the Cold War era as well as the post-Cold War era was also done with the specific intention of identifying problem areas, reasons for success, as well as legislative control measures instituted to regulate the activity. A similar study referring to the situation in South Africa, with the focus on the pre- and post-1994 eras, is also presented. The nature of covert action and especially negative perceptions within society, have led to the questioning of the use of covert action as a legitimate option within security policy. However, as shown in the analysis of case studies, reality indicates that governments continue to conduct covert action. The importance of the study lies in its clarification of the concept of covert action, not only for policy-makers but also for intelligence functionaries. It indicates the measures that should be in place for covert action to be an effective element of national security options; its advantages and disadvantages; the circumstances in which it should be conducted; and the fact that it is still a viable option in the current security milieu. / Dissertation (MSS (Political Science))--University of Pretoria, 2007. / Political Sciences / unrestricted

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