The LNCS two-volume set 13905 and LNCS 13906 constitutes the refereed proceedings of the 21st International Conference on Applied Cryptography and Network Security, ACNS 2023, held in Tokyo, Japan, during June 19-22, 2023. The 53 full papers included in these proceedings were carefully reviewed and selected from a total of 263 submissions. They are organized in topical sections as follows: Part I: side-channel and fault attacks; symmetric cryptanalysis; web security; elliptic curves and pairings; homomorphic cryptography; machine learning; and lattices and codes. Part II: embedded security; privacy-preserving protocols; isogeny-based cryptography; encryption; advanced primitives; multiparty computation; and Blockchain.