Book excerpt: This book constitutes the refereed proceedings of the 12th International Conference on the Theory and Application of Cryptology and Information Security, held in Shanghai, China, December 2006. The 30 revised full papers cover attacks on hash functions, stream ciphers, biometrics and ECC computation, id-based schemes, public-key schemes, RSA and factorization, construction of hash function, protocols, block ciphers, and signatures.