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Using the Bonsai trees primitive and Gentry’s CPA-secure (chosen-plaintext attack) public-key encryption (PKE) scheme, we propose an efficient chosen-ciphtertext secure PKE scheme over lattice. If the decision variant of the learning with errors (LWE) problem is hard and the one-time signature used in this scheme is strong unforgeable, the proposed PKE scheme is indistinguishable against the adaptive chosen-ciphtertext attack (IND-CCA2). One of the characters for this scheme is that, before any encryption operation, the encryption algorithm uses a new choice rule to fix the public parameter matrixes used in the encryption operation. With the help of this new choice rule, we can achieve the chosen-ciphtertext security with much shorter the public key size in contrast to the lattice-based encryption scheme proposed in STOC’09 by Peikert. Moreover, as a CCA-secure PKE scheme, the message-tociphtertext expanse factor of this scheme which is controlled efficiently is nearly closed to the message-to-ciphtertext expanse factor of Gentry’s scheme which is CPA secure. Due to the quantum intractability of the LWE problem on which the scheme is based, the proposed PKE scheme is secure even in quantum-era.

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