In this paper, we address properties of the private quantum channel (PQC), namely a security weakness occurring when at least two copies of the ciphertext are available. We discuss possible definitions of the known-plaintext, known-ciphertext and chosen-plaintext attack. We examine the ability of a PQC to randomize correlations and, finally, we show that specific PQCs encrypting a given set of plaintexts can achieve the approximate encryption of a larger set of plaintexts.
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