Multi-recipient public-key encryption with shortened ciphertext

Abstract

In the trivial n-recipient public-key encryption scheme, a ciphertext is a concatenation of independently encrypted messages for n recipients. In this paper, we say that an n-recipient scheme has a “shortened ciphertext” property if the length of the ciphertext is almost a half (or less) of the trivial scheme and the security is still almost the same as the underlying single-recipient scheme. We first present (multi-plaintext, multi-recipient) schemes with the “shortened ciphertext” property for ElGamal scheme and Cramer-Shoup scheme. We next show (single-plaintext, multi-recipient) hybrid encryption schemes with the “shortened ciphertext” property.