This book constitutes the refereed conference proceedings of the 20th Australasian Conference on Information Security and Privacy, ACISP 2015, held in Brisbane, QLD, Australia, in June/July 2015. The 28 revised full papers presented in this volume were carefully revised and selected from 112 submissions. The papers are organized in topical sections on symmetric cryptanalysis; public key cryptography; identity-based encryption; digital signatures; security protocols; privacy protocols; symmetric constructions; homomorphic encryption and obfuscation.