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.

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. They cover a broad spectrum of research within the conference's scope, including code-, hash-, isogeny-, and lattice-based cryptography, multivariate cryptography, and quantum cryptanalysis.