Book excerpt: This book constitutes the refereed proceedings of the 12th International Conference on Practice and Theory in Public-Key Cryptography, PKC 2009, held in Irvine, CA, USA, in March 2009. The 28 revised full papers presented were carefully reviewed and selected from 112 submissions. The papers are organized in topical sections on number theory, applications and protocols, multi-party protocols, identity-based encryption, signatures, encryption, new cryptosystems and optimizations, as well as group signatures and anonymous credentials.