Encryption -- Cross-Domain Attribute-Based Access Control Encryption -- Grain-128AEADv2: Strengthening the Initialization Against Key Reconstruction -- Partition Oracles from Weak Key Forgeries -- Practical Privacy-Preserving Face Identification based on FunctionHiding Functional Encryption -- The Matrix Reloaded: Multiplication Strategies in FrodoKEM -- Signatures -- BlindOR: An Effcient Lattice-Based Blind Signature Scheme from OR-Proofs -- Effcient Threshold-Optimal ECDSA -- GMMT: A Revocable Group Merkle Multi-Tree Signature Scheme -- Issuer-Hiding Attribute-Based Credentials -- Report and Trace Ring Signatures -- Selectively Linkable Group Signatures - Stronger Security and Preserved Verifiability -- Cryptographic Schemes and Protocols -- FO-like Combiners and Hybrid Post-Quantum Cryptography -- Linear-time oblivious permutations for SPDZ -- On the Higher-bit Version of Approximate Inhomogeneous Short Integer Solution Problem -- Practical Continuously Non-Malleable Randomness Encoders in the Random Oracle Model -- Attacks and Counter-Measures -- Countermeasures against Backdoor Attacks towards Malware Detectors -- Free By Design: On the Feasibility Of Free-Riding Attacks Against Zero-Rated Services -- Function-private Conditional Disclosure of Secrets and Multi-evaluation Threshold Distributed Point Functions -- How Distance-bounding can Detect Internet Traffc Hijacking -- SoK: Secure Memory Allocation -- Toward Learning Robust Detectors from Imbalanced Datasets Leveraging Weighted Adversarial Training -- Towards Quantum Large-Scale Password Guessing on Real-World Distributions -- Attestation and Verification -- Anonymous Transactions with Revocation and Auditing in Hyperledger Fabric -- Attestation Waves: Platform Trust via Remote Power Analysis -- How (not) to Achieve both Coercion Resistance and Cast as Intended Verifiability in Remote eVoting -- Subversion-Resistant Quasi-Adaptive NIZK and Applications to Modular zk-SNARKs -- THC: Practical and Cost-Effective Verification of Delegated Computation -- Tiramisu: Black-Box Simulation Extractable NIZKs in the Updatable CRS Model.

This book constitutes the refereed proceedings of the 20th International Conference on Cryptology and Network Security, CANS 2021, which was held during December 13-15, 2021. The conference was originally planned to take place in Vienna, Austria, and changed to an online event due to the COVID-19 pandemic. The 25 full and 3 short papers presented in these proceedings were carefully reviewed and selected from 85 submissions. They were organized in topical sections as follows: Encryption; signatures; cryptographic schemes and protocols; attacks and counter-measures; and attestation and verification.