Characterizing the decoding failure rate of iteratively decoded Low- and
Moderate-Density Parity Check (LDPC/MDPC) codes is paramount to build
cryptosystems based on them, able to achieve indistinguishability under
adaptive chosen ciphertext attacks. In this paper, we provide a statistical
worst-case analysis of our proposed iterative decoder obtained through a simple
modification of the classic in-place bit-flipping decoder. This worst case
analysis allows both to derive the worst-case behaviour of an LDPC/MDPC code
picked among the family with the same length, rate and number of parity checks,
and a code-specific bound on the decoding failure rate. The former result
allows us to build a code-based cryptosystem enjoying the δ-correctness
property required by IND-CCA2 constructions, while the latter result allows us
to discard code instances which may have a decoding failure rate significantly
different from the average one (i.e., representing weak keys), should they be
picked during the key generation procedure