The two-volume set LNCS 10174 and 10175 constitutes the refereed proceedings of the 20th IACR International Conference on the Practice and Theory in Public-Key Cryptography, PKC 2017, held in Amsterdam, The Netherlands, in March 2017. The 34 revised papers presented were carefully reviewed and selected from 160 submissions. They are organized in topical sections such as Cryptanalysis, Protocols, Encryption Schemes, Leakage-Resilient and Non-Malleable Codes, Number Theory and Diffie-Hellman, Encryption with Access Control, Special Signatures, Fully Homomorphic Encryption, Real-World Schemes, Multiparty Computation and Primitives.