This volume constitutes the proceedings of the 12th International Conference on post-quantum cryptography, PQCrypto 2021, held in Daejeon, South Korea in July 2021.The 25 full papers presented in this volume were carefully reviewed and selected from 65 submissions.

Code-Based Cryptography.- Decoding supercodes of Gabidulin codes and applications to cryptanalysis.- LESS-FM: Fine-tuning Signatures from a Code-based Cryptographic Group Action.- Classical and Quantum algorithms for generic Syndrome Decoding problems and applications to the Lee metric.- Multivariate Cryptography.- Improving Thomae-Wolf Algorithm for Solving Underdetermined.- Multivariate Quadratic Polynomial Problem.- New Practical Multivariate Signatures from a Nonlinear Modifier.- On the Effect of Projection on Rank Attacks in Multivariate Cryptography.- Quantum Algorithms.- Quantum Key Search for Ternary LWE.- A fusion algorithm for solving the hidden shift problem in finite abelian groups.- The “quantum annoying” property of password-authenticated key exchange protocols.- Implementation and Side channel attack.- Differential Power Analysis of the Picnic Signature Scheme.- Implementation of Lattice Trapdoors on Modules and Applications.- Verifying Post-Quantum Signatures in 8 KiB of RAM.- Fast NEON-based multiplication for lattice-based NIST Post-Quantum Cryptography finalists.- Isogeny.- CSI-RAShi: Distributed key generation for CSIDH.- SimS: a Simplification of SiGamal.- Memory Optimization Techniques for Computing Discrete Logarithms in Compressed SIKE.- Lattice-Based Cryptography.- Generating cryptographically-strong random lattice bases and recognizing rotations of Z.- Zero-Knowledge Proofs for Committed Symmetric Boolean Functions.- Short Identity-Based Signatures with Tight Security from Lattices.- On Removing Rejection Conditions in Practical Lattice-Based Signatures.- Secure Hybrid Encryption In the Standard Model from Hard Learning Problems.- Cryptanalysis.- Attack on Beyond-Birthday-Bound MACs in Quantum Setting.- An algebraic approach to the Rank Support Learning problem.- Quantum Indistinguishability for Public Key Encryption.- A Practical Adaptive Key Recovery Attack on the LGM (GSW-like) Cryptosystem.

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