Practical Witness Encryption for Algebraic Languages Or How to Encrypt Under Groth-Sahai Proofs

Witness encryption (WE) is a recent powerful encryption paradigm, which allows to encrypt a message using the description of a hard problem (a word in an NP-language) and someone who knows a solution to this problem (a witness) is able to efficiently decrypt the ciphertext. Recent work thereby focuses on constructing WE for NP complete languages (and thus NP). While this rich expressiveness allows flexibility w.r.t. applications, it makes existing instantiations impractical. Thus, it is interesting to study practical variants of WE schemes for subsets of NP that are still expressive enough for many cryptographic applications. We show that such WE schemes can be generically constructed from smooth projective hash functions (SPHFs). In terms of concrete instantiations of SPHFs (and thus WE), we target languages of statements proven in the popular Groth-Sahai (GS) non-interactive witness-indistinguishable/zero-knowledge proof framework. This allows us to provide a novel way to encrypt. In particular, encryption is with respect to a GS proof and efficient decryption can only be done by the respective prover. The so obtained constructions are entirely practical. To illustrate our techniques, we apply them in context of privacy-preserving exchange of information

Witness Encryption for Succinct Functional Commitments and Applications

Witness encryption (WE), introduced by Garg, Gentry, Sahai, and Waters (STOC 2013) allows one to encrypt a message to a statement $\mathsf{x}$ for some NP language $\mathcal{L}$, such that any user holding a witness for $\mathsf{x} \in \mathcal{L}$ can decrypt the ciphertext. The extreme power of this primitive comes at the cost of its elusiveness: a practical construction from established cryptographic assumptions is currently out of reach. In this work we introduce and construct a new notion of encryption that has a strong flavor of WE and that, crucially, we can build from well-studied assumptions (based on bilinear pairings) for interesting classes of computation. Our new notion, witness encryption for (succinct) functional commitment, takes inspiration from a prior weakening of witness encryption introduced by Benhamouda and Lin (TCC 2020). In a nutshell, theirs is a WE where: the encryption statement consists of a (non compressible) commitment $\mathsf{cm}$, a function $G$ and a value $y$; the decryption witness consists of a (non succinct) NIZK proof about the fact that $\mathsf{cm}$ opens to $v$ such that $y=G(v)$. Benhamouda and Lin showed how to apply this primitive to obtain MPC with non-interactive and reusability properties---dubbed mrNISC---replacing the requirement of WE in existing round-collapsing techniques. Our new WE-like notion is motivated by supporting both commitments of a fixed size and fixed decryption complexity, independent $|v|$---in contrast to the work by Benhamouda and Lin where this complexity is linear. As a byproduct, our efficiency profile substantially improves the offline stage of mrNISC protocols. Our work solves the additional challenges that arise from relying on computationally binding commitments and computational soundness (of functional commitments), as opposed to statistical binding and unconditional soundness (of NIZKs), used in Benhamouda and Lin\u27s work. To tackle them, we not only modify their basic blueprint, but also model and instantiate different types of projective hash functions as building blocks. Furthermore, as one of our main contributions, we show the first pairing-based construction of functional commitments for NC1 circuits with linear verification. Our techniques are of independent interest and may highlight new avenues to design practical variants of witness encryption. As an additional contribution, we show that our new WE-flavored primitive and its efficiency properties are versatile: we discuss its further applications and show how to extend this primitive to better suit these settings

Universally Composable Two-Server PAKE

Two-Server Password Authenticated Key Exchange (2PAKE) protocols apply secret shar-ing techniques to achieve protection against server-compromise attacks. 2PAKE protocols eliminate the need for password hashing and remain secure as long as one of the servers remains honest. This concept has also been explored in connection with two-server password authenticated secret sharing (2PASS) protocols for which game-based and universally composable versions have been proposed. In contrast, universally composable PAKE protocols exist currently only in the single-server scenario and all proposed 2PAKE protocols use game-based security definitions. In this paper we propose the first construction of an universally composable 2PAKE protocol, alongside with its ideal functionality. The protocol is proven UC-secure in the standard model, assuming a common reference string which is a common assumption to many UC-secure PAKE and PASS protocols. The proposed protocol remains secure for arbitrary password distributions. As one of the building blocks we define and construct a new cryptographic primitive, called Trapdoor Distributed Smooth Projective Hash Function (TD-SPHF), which could be of independent interest

Practical Frameworks For hh-Out-Of-nn Oblivious Transfer With Security Against Covert and Malicious Adversaries

We present two practical frameworks for $h$-out-of-$n$ oblivious transfer ($OT^{n}_{h}$). The first one is secure against covert adversaries who are not always willing to cheat at any price. The security is proven under the ideal/real simulation paradigm (call such security fully simulatable security). The second one is secure against malicious adversaries who are always willing to cheat. It provides fully simulatable security and privacy respectively for the sender and the receiver (call such security one-sided simulatable security). The two frameworks can be implemented from the decisional Diffie-Hellman (DDH) assumption, the decisional $N$-th residuosity assumption, the decisional quadratic residuosity assumption and so on. The DDH-based instantiation of our first framework costs the minimum communication rounds and the minimum computational overhead, compared with existing practical protocols for oblivious transfer with fully simulatable security against covert adversaries or malicious adversaries. Though our second framework is not efficient, compared with existing practical protocols with one-sided simulatable security against malicious adversaries. However, it first provides a way to deal with general $OT^{n}_{h}$ on this security level. What is more, its DDH-based instantiation is more efficient than the existing practical protocols for oblivious transfer with fully simulatable security against malicious adversaries

Secure data storage and retrieval in cloud computing

Nowadays cloud computing has been widely recognised as one of the most inuential information technologies because of its unprecedented advantages. In spite of its widely recognised social and economic benefits, in cloud computing customers lose the direct control of their data and completely rely on the cloud to manage their data and computation, which raises significant security and privacy concerns and is one of the major barriers to the adoption of public cloud by many organisations and individuals. Therefore, it is desirable to apply practical security approaches to address the security risks for the wide adoption of cloud computing

Smooth Projective Hashing for Conditionally Extractable Commitments

The original publication is available at www.springerlink.comInternational audienceThe notion of smooth projective hash functions was proposed by Cramer and Shoup and can be seen as special type of zero-knowledge proof system for a language. Though originally used as a means to build efficient chosen-ciphertext secure public-key encryption schemes, some variations of the Cramer-Shoup smooth projective hash functions also found applications in several other contexts, such as password-based authenticated key exchange and oblivious transfer. In this paper, we first address the problem of building smooth projective hash functions for more complex languages. More precisely, we show how to build such functions for languages that can be described in terms of disjunctions and conjunctions of simpler languages for which smooth projective hash functions are known to exist. Next, we illustrate how the use of smooth projective hash functions with more complex languages can be efficiently associated to extractable commitment schemes and avoid the need for zero-knowledge proofs. Finally, we explain how to apply these results to provide more efficient solutions to two well-known cryptographic problems: a public-key certification which guarantees the knowledge of the private key by the user without random oracles or zero-knowledge proofs and adaptive security for password-based authenticated key exchange protocols in the universal composability framework with erasures

Enhancing Privacy Protection:Set Membership, Range Proofs, and the Extended Access Control

Privacy has recently gained an importance beyond the field of cryptography. In that regard, the main goal behind this thesis is to enhance privacy protection. All of the necessary mathematical and cryptographic preliminaries are introduced at the start of this thesis. We then show in Part I how to improve set membership and range proofs, which are cryptographic primitives enabling better privacy protection. Part II shows how to improve the standards for Machine Readable Travel Documents (MRTDs), such as biometric passports. Regarding set membership proofs, we provide an efficient protocol based on the Boneh-Boyen signature scheme. We show that alternative signature schemes can be used and we provide a general protocol description that can be applied for any secure signature scheme. We also show that signature schemes in our design can be replaced by cryptographic accumulators. For range proofs, we provide interactive solutions where the range is divided in a base u and the u-ary digits are handled by one of our set membership proofs. A general construction is also provided for any set membership proof. We additionally explain how to handle arbitrary ranges with either two range proofs or with an improved solution based on sumset representation. These efficient solutions achieve, to date, the lowest asymptotical communication load. Furthermore, this thesis shows that the first efficient non-interactive range proof is insecure. This thesis thus provides the first efficient and secure non-interactive range proof. In the case of MRTDs, two standards exist: one produced by the International Civil Aviation Organization (ICAO) and the other by the European Union, which is called the Extended Access Control (EAC). Although this thesis focuses on the EAC, which is supposed to solve all privacy concerns, it shows that both standards fail to provide complete privacy protection. Lastly, we provide several solutions to improve them