Dedication

This thesis is dedicated to my family for their endless support through the difficult times. To my mother Lia, my father Giannis and my sister Maraki. iv Acknowledgements First of all I would like to thank my advisor, Yevgeniy Dodis, for giving me the chance to explore the world of research and cryptography specifically. His enthu-siasm has always been inspiring. Moreover, I want to thank him a lot for being always helpful and giving me a complete freedom. Special thanks to a person who helped me and taught me a lot about research and this is Iftach Haitner. I really feel grateful to have met him both as a researcher and a friend. I also want to thank who served as supervisors either in my undergrad, Efstathios Zachos and Aris Pagourtzis, or in my internships, Krszysztof Pietrzak and Vladimir Kolesnikof. Moreover, I would also like to thank George Kollios. I also want to thank colleagues and friends that I met during my studies. First of all Vasilleios Gkatzelis and his wife Maria Christoforaki. Other people I want to thank are Preyas Popat, Petros Mol, Adriana Lopez-Alt, Andreas Goebel, Vasilis Zikas. Very special thanks to Itay Berman for all the support, the difficult but also happy times we went through. I also want to thank my friends from my homecountry for their continuin