The two-volume set LNCS 11891 and 11892 constitutes the proceedings of the 17th International Conference on Theory of Cryptography, TCC 2019, held in Nuremberg, Germany, in December 2019.The 43 full papers presented were carefully reviewed and selected from 147 submissions. The Theory of Cryptography Conference deals with the paradigms, approaches, and techniques used to conceptualize natural cryptographic problems and provide algorithmic solutions to them and much more.