Zaytun: Lattice Based PKE and KEM with Shorter Ciphertext Size

In this paper, we propose a lattice-based encryption scheme with a short ciphertext size. Our scheme is somewhat hybrid of the NTRU type encryptions and RLWE based encryptions. In particular, the ciphertext of the scheme is a ring element as NTRU type encryptions, yet it can be compressible as RLWE based encryption schemes. Furthermore, we present a key-encapsulation mechanism that is more efficient than a direct construction from our encryption scheme. The IND-CPA security of the schemes is based on the RLWE assumption and the NTRU assumption. Our parameterizations show that the schemes enjoy almost the same public key size as the NIST PQC finalist lattice-based candidates, yet the ciphertext size is only about 37% of them.