Miniature CCA2 PK Encryption: Tight Security Without Redundancy

We present a minimalist public-key cryptosystem, as compact as ElGamal, but with adaptive chosen-ciphertext security under the gap Diffie-Hellman assumption in the random oracle model. The novelty is a dual-hash device that provides tight redundancy-free implicit validation. Compared to previous constructions, ours features a tight security reduction, both in efficacy and efficiency, to a classic and essentially non-interactive complexity assumption, and without resorting to asymmetric/symmetric-key hybrid constructions. The system is very compact: on elliptic curves with 80-bit security, a 160-bit plaintext becomes a 320-bit ciphertext. It is also very simple and has a number of practical advantages, and we hope to see it adopted widely.

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