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I assume that the paper you have read is the paper by Kobara and Imai in PKC 2001 (or its journal version), which proposed a padding scheme for the McEliece PKE scheme. In Lemma2, the authors showed CPA security of the padded scheme. The first answer is NO. They are not. Let $C(n,t) = \{z \mid z \in \{0,1\}^n, Hw(z) = t\}$, where $Hw(z)$ denotes $z$'s ...