Book excerpt: This book constitutes the refereed proceedings of the 4th International Conference on Codes, Cryptology and Information Security, C2SI 2023, held in Rabat, Morocco, during May 29–31, 2023. The 21 full papers included in this book were carefully reviewed and selected from 62 submissions. They were organized in topical sections as follows: Invited Papers, Cryptography, Information Security, Discrete Mathematics, Coding Theory.