Efficient Adaptively-Secure IB-KEMs and VRFs via Near-Collision Resistance 📺
We construct more efficient cryptosystems with provable security against adaptive attacks, based on simple and natural hardness assumptions in the standard model. Concretely, we describe: – An adaptively-secure variant of the efficient, selectively-secure LWE- based identity-based encryption (IBE) scheme of Agrawal, Boneh, and Boyen (EUROCRYPT 2010). In comparison to the previously most efficient such scheme by Yamada (CRYPTO 2017) we achieve smaller lattice parameters and shorter public keys of size O(log \lambda), where \lambda is the security parameter. – Adaptively-secure variants of two efficient selectively-secure pairing- based IBEs of Boneh and Boyen (EUROCRYPT 2004). One is based on the DBDH assumption, has the same ciphertext size as the cor- responding BB04 scheme, and achieves full adaptive security with public parameters of size only O(log \lambda). The other is based on a q- type assumption and has public key size O(\lambda), but a ciphertext is only a single group element and the security reduction is quadrat- ically tighter than the corresponding scheme by Jager and Kurek (ASIACRYPT 2018). – A very efficient adaptively-secure verifiable random function where proofs, public keys, and secret keys have size O(log \lambda). As a technical contribution we introduce blockwise partitioning, which leverages the assumption that a cryptographic hash function is weak near-collision resistant to prove full adaptive security of cryptosystems.
Short Digital Signatures and ID-KEMs via Truncation Collision Resistance
Truncation collision resistance is a simple non-interactive complexity assumption that seems very plausible for standard cryptographic hash functions like SHA-3. We describe how this assumption can be leveraged to obtain standard-model constructions of public-key cryptosystems that previously seemed to require a programmable random oracle. This includes the first constructions of identity-based key encapsulation mechanisms (ID-KEMs) and digital signatures over bilinear groups with full adaptive security and without random oracles, where a ciphertext or signature consists of only a single element of a prime-order group. We also describe a generic construction of ID-KEMs with full adaptive security from a scheme with very weak security (“selective and non-adaptive chosen-ID security”), and a similar generic construction for digital signatures.
Simple and More Efficient PRFs with Tight Security from LWE and Matrix-DDH
We construct efficient and tightly secure pseudorandom functions (PRFs) with only logarithmic security loss and short secret keys. This yields very simple and efficient variants of well-known constructions, including those of Naor-Reingold (FOCS 1997) and Lewko-Waters (ACM CCS 2009). Most importantly, in combination with the construction of Banerjee, Peikert and Rosen (EUROCRYPT 2012) we obtain the currently most efficient LWE-based PRF from a weak LWE-assumption with a much smaller modulus than the original construction. In comparison to the only previous construction with this property, which is due to Döttling and Schröder (CRYPTO 2015), we use a modulus of similar size, but only a single instance of the underlying PRF, instead of parallel instances, where is the security parameter. Like Döttling and Schröder, our security proof is only almost back-box, due to the fact that the number of queries made by the adversary and its advantage must be known a-priori.Technically, we introduce all-prefix universal hash functions (APUHFs), which are hash functions that are (almost-)universal, even if any prefix of the output is considered. We give simple and very efficient constructions of APUHFs, and show how they can be combined with the augmented cascade of Boneh et al. (ACM CCS 2010) to obtain our results. Along the way, we develop a new and more direct way to prove security of PRFs based on the augmented cascade.