This book constitutes the refereed proceedings of the 21st International Conference on Cryptology in India, INDOCRYPT 2020, held in Bangalore, India, in December 2020.The 39 full papers presented together with 3 invited abstracts in this book were carefully reviewed and selected from 84 submissions. Apart from its traditional focus on areas in applied and theoretical cryptology, this year INDOCRYPT solicited papers in the area of Formal Methods for Cryptographic Systems as well and much more.