In recent years, research has been conducted aimed at finding alternative asymmetric systems other than traditional systems such as RSA (Rivest-Shamir-Adleman algorithm) and ECC (Elliptic-curve cryptography). One of the most promising is code-based cryptosystems since their security is based on well-known NP-hard problems. Especially, the most interesting cryptosystem is system proposed by Misoczki et al. based on QC-MDPC codes which use the modified BitFlip algorithm as the decoding algorithm. This work presents a comparison of different variants of MDPC decoding algorithms and their impact on the cryptosystem. We present a complete analysis of modification of this algorithm and new results of the likelihood of correct word decoding for security systems which ensure security level 2128and 2256.