This book constitutes the proceedings of the 20th International Conference on Smart Card Research and Advanced Applications, CARDIS 2021, which took place in November 2021. The conference took place in Lübeck, Germany, and changed to a hybrid format due to the COVID-19 pandemic. The 16 full papers presented in this volume were carefully reviewed and selected from 32 submissions. They were organized in topical sections named Side-Channel Attacks, Fault Attacks, Public Key and Secure Implementations.