About the Fujisaki-Okamoto Transformation in the Code-based Algorithms of the NIST Post-Quantum Call

Post-quantum encryption schemes use variants of the Fujisaki-Okamoto transformation in order to construct a highly secure key encapsulation mechanism from a weakly secure public key encryption scheme. In the third round of the NIST post-quantum cryptography standardization call, all the candidates for the key encapsulation mechanism category use some of these transformations. This work studies how the mentioned transformations are applied in the code-based candidates of the NIST third round. These are Classic McEliece (finalist), BIKE (alternative) and HQC (alternative). Studying the differences between the transformations gives a better understanding of these candidates.Project P2QProMeTe (PID2020-112586RB-I00/AEI/10.13039/501100011033)ORACLE Project, with reference PCI2020-120691-2, funded by MCIN/AEI/10.13039/501100011033 and European Union “NextGenerationEU/PRTR”Spanish State Research Agency (AEI) of the Ministry of Science and Innovation (MCIN)EU Horizon 2020 research and innovation programme, project SPIRS (Grant Agreement No. 952622)Peer reviewe