This two-volume set of LNCS 12146 and 12147 constitutes the refereed proceedings of the 18th International Conference on Applied Cryptography and Network Security, ACNS 2020, held in Rome, Italy, in October 2020. The conference was held virtually due to the COVID-19 pandemic. The 46 revised full papers presented were carefully reviewed and selected from 214 submissions. The papers were organized in topical sections named: cryptographic protocols cryptographic primitives, attacks on cryptographic primitives, encryption and signature, blockchain and cryptocurrency, secure multi-party computation, post-quantum cryptography.