This book constitutes the proceedings of the 9th International Workshop on Code-Based Cryptography, CBCrypto 2021, which was held during June 21-22, 2021. The workshop was initially planned to take place in Munich, Germany, but changed to an online event due to the COVID-19 pandemic.

The 6 papers presented in this volume were carefully reviewed and selected from 14 submissions. These contributions span all aspects of code-based cryptography, from design to implementation, and including studies of security, new systems, and improved decoding algorithms.

A Rank Metric Code-Based Group Signature Scheme.- The Rank-Based Cryptography Library.- Security Analysis of a Cryptosystem based on Subspace Subcodes.- Information-Set Decoding with Hints.- A Correction to A Code-Based Blind Signature Scheme.- Performance bounds for QC-MDPC codes decoders.