Book excerpt: This book constitutes the refereed proceedings of the 9th International Conference on Applied Cryptography and Network Security, ACNS 2011, held in Nerja, Spain, in June 2011. The 31 revised full papers included in this volume were carefully reviewed and selected from 172 submissions. They are organized in topical sessions on malware and intrusion detection; attacks, applied crypto; signatures and friends; eclectic assortment; theory; encryption; broadcast encryption; and security services.