Cryptanalysis of McEliece cryptosystem variants based on quasi‐cyclic low‐density parity check codes

One of the approaches to modify the McEliece cryptosystem to overcome its large key size is replacing binary Goppa codes with a new structured code. However, this modification makes such cryptosystems encounter some new attacks. There are a few modified McEliece cryptosystem variants which are known to be secure. One of them is the cryptosystem introduced by Baldi et al . which uses quasi‐cyclic low‐density parity check (QC‐LDPC) codes. This cryptosystem is still unbroken as no efficient attack has been reported against it since 2008. In this study, an attack has been applied to this cryptosystem which is feasible when the code length is a multiple of a power of 2. Also an important weakness of this kind of cryptosystem has been pointed out, namely utilising a too low‐weight intentional error vector. The authors have established a new security level for this cryptosystem which is applicable to other McEliece‐like cryptosystems using QC‐LDPC codes. This security level for instance is 2 9.18 times lower than previous ones in the case of n = 4 × 4096 when only one ciphertext is available. The gain of the attack in this study can be increased if more than one ciphertext is available.