Book excerpt: Here are the refereed proceedings of the 9th International Conference on Theory and Practice in Public-Key Cryptography, PKC 2006, held in New York City in April 2006. The 34 revised full papers presented are organized in topical sections on cryptanalysis and protocol weaknesses, distributed crypto-computing, encryption methods, cryptographic hash and applications, number theory algorithms, pairing-based cryptography, cryptosystems design and analysis, signature and identification, authentication and key establishment, multi-party computation, and PKI techniques.