Book excerpt: This book constitutes the refereed proceedings of the 6th International Workshop on Practice and Theory in Public Key Cryptosystems, PKC 2003, held in Miami, Florida, USA in January 2003. The 26 revised full papers presented were carefully reviewed and selected from 105 submissions. The papers are organized in topical sections on Diffie-Hellman based schemes, threshold cryptography, reduction proofs, broadcast and tracing, digital signatures, specialized multiparty cryptography, cryptanalysis, elliptic curves: implementation attacks, implementation and hardware issues, new public key schemes, and elliptic curves: general issues.