Eurocrypt is a conference devoted to all aspects of cryptologic research, both theoretical and practical, sponsored by the International Association for Cryptologic Research (IACR). Eurocrypt 90 took place in Åarhus, Denmark, in May 1990. From the 85 papers submitted, 42 were selected for presentation at the conference and for inclusion in this volume. In addition to the formal contributions, short abstracts of a number of informal talks are included in these proceedings. The proceedings are organized into sessions on protocols, number-theoretic algorithms, boolean functions, binary sequences, implementations, combinatorial schemes, cryptanalysis, new cryptosystems, signatures and authentication, and impromptu talks.