This book constitutes the refereed proceedings of the 12th International Workshop on Security, IWSEC 2017, held in Hiroshima, Japan, in August/September 2017.  The 11 regular papers and 3 short papers presented in this volume were carefully reviewed and selected from 37 submissions. They were organized in topical sections named: post-quantum cryptography; system security; public key cryptosystems; cryptanalysis; and cryptographic protocols.