## CryptoDB

### Yael Tauman Kalai

#### Publications

Year
Venue
Title
2019
CRYPTO
We construct non-interactive non-malleable commitments without setup in the plain model, under well-studied assumptions.First, we construct non-interactive non-malleable commitments w.r.t. commitment for $\epsilon \log \log n$ tags for a small constant $\epsilon > 0$, under the following assumptions:1.Sub-exponential hardness of factoring or discrete log.2.Quantum sub-exponential hardness of learning with errors (LWE). Second, as our key technical contribution, we introduce a new tag amplification technique. We show how to convert any non-interactive non-malleable commitment w.r.t. commitment for $\epsilon \log \log n$ tags (for any constant $\epsilon >0$) into a non-interactive non-malleable commitment w.r.t. replacement for $2^n$ tags. This part only assumes the existence of sub-exponentially secure non-interactive witness indistinguishable (NIWI) proofs, which can be based on sub-exponential security of the decisional linear assumption.Interestingly, for the tag amplification technique, we crucially rely on the leakage lemma due to Gentry and Wichs (STOC 2011). For the construction of non-malleable commitments for $\epsilon \log \log n$ tags, we rely on quantum supremacy. This use of quantum supremacy in classical cryptography is novel, and we believe it will have future applications. We provide one such application to two-message witness indistinguishable (WI) arguments from (quantum) polynomial hardness assumptions.
2018
EUROCRYPT
2018
CRYPTO
We devise a new partitioned simulation technique for MPC where the simulator uses different strategies for simulating the view of aborting adversaries and non-aborting adversaries. The protagonist of this technique is a new notion of promise zero knowledge (ZK) where the ZK property only holds against non-aborting verifiers. We show how to realize promise ZK in three rounds in the simultaneous-message model assuming polynomially hard DDH (or QR or N$^{th}$-Residuosity).We demonstrate the following applications of our new technique:We construct the first round-optimal (i.e., four round) MPC protocol for general functions based on polynomially hard DDH (or QR or N$^{th}$-Residuosity).We further show how to overcome the four-round barrier for MPC by constructing a three-round protocol for “list coin-tossing” – a slight relaxation of coin-tossing that suffices for most conceivable applications – based on polynomially hard DDH (or QR or N$^{th}$-Residuosity). This result generalizes to randomized input-less functionalities. Previously, four round MPC protocols required sub-exponential-time hardness assumptions and no multi-party three-round protocols were known for any relaxed security notions with polynomial-time simulation against malicious adversaries.In order to base security on polynomial-time standard assumptions, we also rely upon a leveled rewinding security technique that can be viewed as a polynomial-time alternative to leveled complexity leveraging for achieving “non-malleability” across different primitives.
2017
CRYPTO
2017
CRYPTO
2017
ASIACRYPT
2016
TCC
2016
TCC
2016
TCC
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
TCC
2015
CRYPTO
2014
CRYPTO
2014
CRYPTO
2014
EUROCRYPT
2014
TCC
2014
TCC
2014
EPRINT
2014
CRYPTO
2013
TCC
2013
CRYPTO
2013
CRYPTO
2012
TCC
2012
CRYPTO
2011
TCC
2011
CRYPTO
2011
CRYPTO
2011
ASIACRYPT
2010
TCC
2010
TCC
2010
EPRINT
We show tight connections between several cryptographic primitives, namely encryption with weakly random keys, encryption with key-dependent messages (KDM), and obfuscation of point functions with multi-bit output(which we call multi-bit point functions, or MBPFs, for short). These primitives, which have been studied mostly separately in recent works, bear some apparent similarities, both in the flavor of their security requirements and in the flavor of their constructions and assumptions. Still, rigorous connections have not been drawn. Our results can be interpreted as indicating that MBPF obfuscators imply a very strong form of encryption that simultaneously achieves security for weakly-random keys and key-dependent messages as special cases. Similarly, each one of the other primitives implies a certain restricted form of MBPF obfuscation. Our results carry both constructions and impossibility results from one primitive to others. In particular: The recent impossibility result for KDM security of Haitner and Holenstein (TCC 09) carries over to MBPF obfuscators. The Canetti-Dakdouk construction of MBPF obfuscators based on a strong variant of the DDH assumption (EC 08) gives an encryption scheme which is secure w.r.t. any weak key distribution of super-logarithmic min-entropy (and in particular, also has very strong leakage resilient properties). All the recent constructions of encryption schemes that are secure w.r.t. weak keys imply a weak form of MBPF obfuscators. 
2010
CRYPTO
2010
EPRINT
In this work, we present a generic framework for constructing efficient signature scheme, ring signature schemes, and identity based encryption schemes, all in the standard model (without relying on random oracles). We start by abstracting the recent work of Hohenberger and Waters (Crypto 2009), and specifically their prefix method''. We show a transformation taking a signature scheme with a very weak security guarantee (a notion that we call a-priori-message unforgeability under static chosen message attack) and producing a fully secure signature scheme (i.e., existentially unforgeable under adaptive chosen message attack). Our transformation uses the notion of chameleon hash functions, defined by Krawczyk and Rabin (NDSS 2000) and the prefix method''. Constructing such weakly secure schemes seems to be significantly easier than constructing fully secure ones, and we present {\em simple} constructions based on the RSA assumption, the {\em short integer solution} (SIS) assumption, and the {\em computational Diffie-Hellman} (CDH) assumption over bilinear groups. Next, we observe that this general transformation also applies to the regime of ring signatures. Using this observation, we construct new (provably secure) ring signature schemes: one is based on the {\em short integer solution} (SIS) assumption, and the other is based on the CDH assumption over bilinear groups. As a building block for these constructions, we define a primitive that we call {\em ring trapdoor functions}. We show that ring trapdoor functions imply ring signatures under a weak definition, which enables us to apply our transformation to achieve full security. Finally, we show a connection between ring signatures and identity based encryption (IBE) schemes. Using this connection, and using our new constructions of ring signature schemes, we obtain two IBE schemes: The first is based on the {\em learning with error} (LWE) assumption, and is similar to the recently introduced IBE schemes of Peikert, Agrawal-Boyen and Cash-Hofheinz-Kiltz (2009); The second is based on the $d$-linear assumption over bilinear groups.
2010
EPRINT
Following Gennaro, Gentry, and Parno (Cryptology ePrint Archive 2009/547), we use fully homomorphic encryption to design improved schemes for delegating computation. In such schemes, a delegator outsources the computation of a function $F$ on many, dynamically chosen inputs $x_i$ to a worker in such a way that it is infeasible for the worker to make the delegator accept a result other than $F(x_i)$. The "online stage" of the Gennaro et al. scheme is very efficient: the parties exchange two messages, the delegator runs in time $poly(log T)$, and the worker runs in time $poly(T)$, where $T$ is the time complexity of $F$. However, the "offline stage" (which depends on the function $F$ but not the inputs to be delegated) is inefficient: the delegator runs in time $poly(T)$ and generates a public key of length $poly(T)$ that needs to be accessed by the worker during the online stage. Our first construction eliminates the large public key from the Gennaro et al. scheme. The delegator still invests $poly(T)$ time in the offline stage, but does not need to communicate or publish anything. Our second construction reduces the work of the delegator in the offline stage to $poly(log T)$ at the price of a 4-message (offline) interaction with a $poly(T)$-time worker (which need not be the same as the workers used in the online stage). Finally, we describe a "pipelined" implementation of the second construction that avoids the need to re-run the offline construction after errors are detected (assuming errors are not too frequent).
2010
EPRINT
In recent years, there has been a major effort to design cryptographic schemes that remain secure even if part of the secret key is leaked. This is due to a recent proliferation of side channel attacks which, through various physical means, can recover part of the secret key. We explore the possibility of achieving security even with continual leakage, i.e., even if some information is leaked each time the key is used. We show how to securely update a secret key while information is leaked: We construct schemes that remain secure even if an attacker, {\em at each time period}, can probe the entire memory (containing a secret key) and leak'' up to a $(1-o(1))$ fraction of the secret key. The attacker may also probe the memory during the updates, and leak $O(\log k)$ bits, where $k$ is the security parameter (relying on subexponential hardness allows $k^\epsilon$ bits of leakage during each update process). All of the above is achieved without restricting the model as is done in previous works (e.g. by assuming that only computation leaks information'' [Micali-Reyzin, TCC04]). Specifically, under the decisional linear assumption on bilinear groups (which allows for a leakage rate of $(1/2-o(1))$) or the symmetric external Diffie-Hellman assumption (which allows for a leakage rate of $(1-o(1))$), we achieve the above for public key encryption, identity-based encryption, and signature schemes. Prior to this work, it was not known how to construct public-key encryption schemes even in the more restricted model of [MR]. The main contributions of this work are (1) showing how to securely update a secret key while information is leaked (in the more general model) and (2) giving a public key encryption (and IBE) schemes that are resilient to continual leakage.
2009
CRYPTO
2008
CRYPTO
2007
EPRINT
We present a general framework for constructing two-message oblivious transfer protocols using a modification of Cramer and Shoup's notion of smooth projective hashing (2002). This framework is an abstraction of the two-message oblivious transfer protocols of Naor and Pinkas (2001) and Aiello et al. (2001), whose security is based on the Decisional Diffie Hellman Assumption. In particular, we give two new oblivious transfer protocols. The security of one is based on the Quadratic Residuosity Assumption, and the security of the other is based on the $N$'th Residuosity Assumption. Our security guarantees are not simulation based, but are similar to the guarantees of the aforementioned two constructions. Compared to other applications of smooth projective hashing, in our context we must deal also with maliciously chosen parameters, which raises new technical difficulties. We also improve on prior constructions of factoring-based smooth universal hashing, in that our constructions *do not require that the underlying RSA-composite is a product of safe primes*. In fact, we observe that the safe-prime requirement is unnecessary for many prior constructions. In particular, we observe that the factoring-based CCA secure encryption schemes due to Cramer-Shoup, Gennaro-Lindell and Camenisch-Shoup remain secure even if the underlying RSA-composite is not a product of safe primes. (This holds for the schemes based on the Quadratic Residuosity Assumption as well as the ones based on the $N$'th Residuosity Assumption.)
2005
EUROCRYPT
2005
EPRINT
In the setting of secure multiparty computation, a set of mutually distrustful parties wish to securely compute some joint function of their inputs. In the stand-alone case, it has been shown that {\em every} efficient function can be securely computed. However, in the setting of concurrent composition, broad impossibility results have been proven for the case of no honest majority and no trusted setup phase. These results hold both for the case of general composition (where a secure protocol is run many times concurrently with arbitrary other protocols) and self composition (where a single secure protocol is run many times concurrently). In this paper, we investigate the feasibility of obtaining security in the concurrent setting, assuming that each party has a local clock and that these clocks proceed at approximately the same rate. We show that under this mild timing assumption, it is possible to securely compute {\em any} multiparty functionality under concurrent \emph{self} composition. We also show that it is possible to securely compute {\em any} multiparty functionality under concurrent {\em general} composition, as long as the secure protocol is run only with protocols whose messages are delayed by a specified amount of time. On the negative side, we show that it is impossible to achieve security under concurrent general composition with no restrictions whatsoever on the network (like the aforementioned delays), even in the timing model.

TCC 2017
TCC 2013
Crypto 2012
Crypto 2010
TCC 2007