A Framework for Identity-Based Encryption with Almost Tight Security

We show a framework for constructing identity-based encryption IBE schemes that are almost tightly secure in the multi-challenge and multi-instance setting. In particular, we formalize a new notion called broadcast encoding, analogously to encoding notions by Attrapadung Eurocrypt 2014 and Wee TCC 2014. We then show that it can be converted into such an IBE. By instantiating the framework using several encoding schemes new or known ones, we obtain the following:We obtain almost tightly secure IBE in the multi-challenge, multi-instance setting, both in composite and prime-order groups. The latter resolves the open problem posed by Hofheinz et al. PKC 2015.We obtain the first almost tightly secure IBE with sub-linear size public parameters master public keys. In particular, we can set the size of the public parameters to constant at the cost of longer ciphertexts and private keys. This gives a partial solution to the open problem posed by Chen and Wee Crypto 2013. By applying a variant of the Canetti-Halevi-Katz transformation to our schemes, we obtain several CCA-secure PKE schemes with tight security in the multi-challenge, multi-instance setting. One of our schemes achieves very small ciphertext overhead, consisting of less than 12 group elements. This significantly improves the state-of-the-art construction by Libert et al.i¾?in ePrint Archive which requires 47 group elements. Furthermore, by modifying one of our IBE schemes obtained above, we can make it anonymous. This gives the first anonymous IBE whose security is almost tightly shown in the multi-challenge setting.