This book constitutes the refereed proceedings of the 20th International Conference on Cryptology and Network Security, CANS 2021, which was held during December 13-15, 2021. The conference was originally planned to take place in Vienna, Austria, and changed to an online event due to the COVID-19 pandemic. The 25 full and 3 short papers presented in these proceedings were carefully reviewed and selected from 85 submissions. They were organized in topical sections as follows: Encryption; signatures; cryptographic schemes and protocols; attacks and counter-measures; and attestation and verification.