This book constitutes the refereed proceedings of the 12th International Conference on Applied Cryptography and Network Security, ACNS 2014, held in Lausanne, Switzerland, in June 2014. The 33 revised full papers included in this volume were carefully reviewed and selected from 147 submissions. They are organized in topical sections on key exchange; primitive construction; attacks (public-key cryptography); hashing; cryptanalysis and attacks (symmetric cryptography); network security; signatures; system security; and secure computation.