This three-volume set, LNCS 12550, 12551, and 12552, constitutes the refereed proceedings of the 18th International Conference on Theory of Cryptography, TCCC 2020, held in Durham, NC, USA, in November 2020. The total of 71 full papers presented in this three-volume set was carefully reviewed and selected from 167 submissions. Amongst others they cover the following topics: study of known paradigms, approaches, and techniques, directed towards their better understanding and utilization; discovery of new paradigms, approaches and techniques that overcome limitations of the existing ones, formulation and treatment of new cryptographic problems; study of notions of security and relations among them; modeling and analysis of cryptographic algorithms; and study of the complexity assumptions used in cryptography. Due to the Corona pandemic this event was held virtually.