On Shortening Ciphertexts: New Constructions for Compact Public Key and Stateful Encryption Schemes

We present new constructions of (conventional) public key and stateful public key encryption schemes which produce ciphertexts of compact size while providing both efficiency and strong security. Our public key encryption scheme incurs only one group element ciphertext expansion (defined as the size of the ciphertext minus the size of the plaintext message) but compared with the previous scheme in the literature, its encryption algorithm is more efficient. Our stateful encryption scheme resolves the problem of ciphertext expansion of the existing schemes in the literature and hence can be served as a favorable alternative. Both of our schemes do not depend on the external lengthpreserving cipher constructed from the expensive strong pseudo random permutation. We provide security analysis of our schemes against chosen ciphertext attack under the well-known computational assumptions, in the random oracle model. We envision that our schemes can serve as efficient public key primitives suitable for implementing on resource constrained devices.