This mini-course provides a focused introduction to code-based cryptography with an emphasis on quasi-cyclic code families and their cryptanalytic challenges. Participants will explore the structure and properties of quasi-cyclic low-density parity-check (QC-LDPC) and quasi-cyclic moderate-density parity-check (QC-MDPC) codes. The course includes a foundational overview of the McEliece and Niederreiter cryptosystems underlying code-based cryptography. We then turn to cryptanalytic considerations, culminating in a detailed examination of the decoding failure attack presented by Guo, Johansson, and Stankovski in eprint 2016/858, which demonstrates how error patterns leaked via decryption failures can compromise security in QC-MDPC-based schemes.