On Two Modifications of the McEliece PKE and the CFS Signature Scheme

This paper analyzes two schemes from a 2019 work by Liu, Yang, Han and Wang, namely, adapting the McEliece cryptosystem to obtain a new public-key encryption scheme, and designing an efficient CFS-like digital signature. It is shown that the new encryption scheme based on McEliece, even if it has longer public keys, is not more secure than the standard one. The security gap between the original scheme and its modification is presented. Moreover, while the proposed parameters for the digital signature scheme lead to a significant performance improvement, however, they introduce a vulnerability in the protocol. A key-forgery attack using the Support Splitting Algorithm as a subroutine is described, with a study of the computational cost of retrieving the secret key from the public one.