This book constitutes the refereed proceedings of the 20th International Conference on Cryptology in India, INDOCRYPT 2019, held in Hyderabad, India, in December 2019.The 28 revised full papers presented in this book were carefully reviewed and selected from 110 submissions (of which 20 were either rejected without being reviewed or withdrawn before the deadline). The focus of the conference includes works on signatures and filter permutators; symmetric key ciphers and hash functions; blockchain, secure computation and blind coupon mechanism; oblivious transfer, obfuscation and privacy amplification; Boolean functions, elliptic curves and lattices; algorithms, attacks and distribution; and efficiency, side-channel resistance and PUFs.