CryptoDB

Shweta Agrawal

Publications

Year
Venue
Title
2020
EUROCRYPT
Boneh, Waters and Zhandry (CRYPTO 2014) used multilinear maps to provide a solution to the long-standing problem of public-key broadcast encryption (BE) where all parameters in the system are small. In this work, we improve their result by providing a solution that uses only {\it bilinear} maps and Learning With Errors (LWE). Our scheme is fully collusion-resistant against any number of colluders, and can be generalized to an identity-based broadcast system with short parameters. Thus, we reclaim the problem of optimal broadcast encryption from the land of Obfustopia''. Our main technical contribution is a ciphertext policy attribute based encryption (CP-ABE) scheme which achieves special efficiency properties -- its ciphertext size, secret key size, and public key size are all independent of the size of the circuits supported by the scheme. We show that this special CP-ABE scheme implies BE with optimal parameters; but it may also be of independent interest. Our constructions rely on a novel interplay of bilinear maps and LWE, and are proven secure in the generic group model.
2020
EUROCRYPT
Candidates of Indistinguishability Obfuscation (iO) can be categorized as direct'' or bootstrapping based''. Direct constructions rely on high degree multilinear maps [GGH13,GGHRSW13] and provide heuristic guarantees, while bootstrapping based constructions [LV16,Lin17,LT17,AJLMS19,Agr19,JLMS19] rely, in the best case, on bilinear maps as well as new variants of the Learning With Errors (LWE) assumption and pseudorandom generators. Recent times have seen exciting progress in the construction of indistinguishability obfuscation (iO) from bilinear maps (along with other assumptions) [LT17,AJLMS19,JLMS19,Agr19]. As a notable exception, a recent work by Agrawal [Agr19] provided a construction for iO without using any maps. This work identified a new primitive, called Noisy Linear Functional Encryption (NLinFE) that provably suffices for iO and gave a direct construction of NLinFE from new assumptions on lattices. While a preliminary cryptanalysis for the new assumptions was provided in the original work, the author admitted the necessity of performing significantly more cryptanalysis before faith could be placed in the security of the scheme. Moreover, the author did not suggest concrete parameters for the construction. In this work, we fill this gap by undertaking the task of thorough cryptanalytic study of NLinFE. We design two attacks that let the adversary completely break the security of the scheme. Our attacks are completely new and unrelated to attacks that were hitherto used to break other candidates of iO. To achieve this, we develop new cryptanalytic techniques which (we hope) will inform future designs of the primitive of NLinFE. From the knowledge gained by our cryptanalytic study, we suggest modifications to the scheme. We provide a new scheme which overcomes the vulnerabilities identified before. We also provide a thorough analysis of all the security aspects of this scheme and argue why plausible attacks do not work. We additionally provide concrete parameters with which the scheme may be instantiated. We believe the security of NLinFE stands on significantly firmer footing as a result of this work.
2020
PKC
Inner product functional encryption ( ${mathsf {IPFE}}$ ) [ 1 ] is a popular primitive which enables inner product computations on encrypted data. In ${mathsf {IPFE}}$ , the ciphertext is associated with a vector $varvec{x}$ , the secret key is associated with a vector $varvec{y}$ and decryption reveals the inner product $langle varvec{x},varvec{y} angle$ . Previously, it was known how to achieve adaptive indistinguishability ( $mathsf {IND}$ ) based security for ${mathsf {IPFE}}$ from the $mathsf {DDH}$ , $mathsf {DCR}$ and $mathsf {LWE}$ assumptions [ 8 ]. However, in the stronger simulation ( $mathsf {SIM}$ ) based security game, it was only known how to support a restricted adversary that makes all its key requests either before or after seeing the challenge ciphertext, but not both. In more detail, Wee [ 46 ] showed that the $mathsf {DDH}$ -based scheme of Agrawal et al. (Crypto 2016) achieves semi-adaptive simulation-based security, where the adversary must make all its key requests after seeing the challenge ciphertext. On the other hand, O’Neill showed that all $mathsf {IND}$ -secure ${mathsf {IPFE}}$ schemes (which may be based on $mathsf {DDH}$ , $mathsf {DCR}$ and $mathsf {LWE}$ ) satisfy $mathsf {SIM}$ based security in the restricted model where the adversary makes all its key requests before seeing the challenge ciphertext. In this work, we resolve the question of $mathsf {SIM}$ -based security for ${mathsf {IPFE}}$ by showing that variants of the ${mathsf {IPFE}}$ constructions by Agrawal et al. , based on $mathsf {DDH}$ , Paillier and $mathsf {LWE}$ , satisfy the strongest possible adaptive $mathsf {SIM}$ -based security where the adversary can make an unbounded number of key requests both before and after seeing the (single) challenge ciphertext. This establishes optimal security of the ${mathsf {IPFE}}$ schemes, under all hardness assumptions on which it can (presently) be based.
2019
CRYPTO
Constructing Attribute Based Encryption (ABE) [56] for uniform models of computation from standard assumptions, is an important problem, about which very little is known. The only known ABE schemes in this setting that (i) avoid reliance on multilinear maps or indistinguishability obfuscation, (ii) support unbounded length inputs and (iii) permit unbounded key requests to the adversary in the security game, are by Waters from Crypto, 2012 [57] and its variants. Waters provided the first ABE for Deterministic Finite Automata (DFA) satisfying the above properties, from a parametrized or “q-type” assumption over bilinear maps. Generalizing this construction to Nondeterministic Finite Automata (NFA) was left as an explicit open problem in the same work, and has seen no progress to date. Constructions from other assumptions such as more standard pairing based assumptions, or lattice based assumptions has also proved elusive.In this work, we construct the first symmetric key attribute based encryption scheme for nondeterministic finite automata (NFA) from the learning with errors (LWE) assumption. Our scheme supports unbounded length inputs as well as unbounded length machines. In more detail, secret keys in our construction are associated with an NFA M of unbounded length, ciphertexts are associated with a tuple $(\mathbf {x}, m)$ where $\mathbf {x}$ is a public attribute of unbounded length and m is a secret message bit, and decryption recovers m if and only if $M(\mathbf {x})=1$.Further, we leverage our ABE to achieve (restricted notions of) attribute hiding analogous to the circuit setting, obtaining the first predicate encryption and bounded key functional encryption schemes for NFA from LWE. We achieve machine hiding in the single/bounded key setting to obtain the first reusable garbled NFA from standard assumptions. In terms of lower bounds, we show that secret key functional encryption even for DFAs, with security against unbounded key requests implies indistinguishability obfuscation ($\mathsf {iO}$) for circuits; this suggests a barrier in achieving full fledged functional encryption for NFA.
2019
EUROCRYPT
Constructing indistinguishability obfuscation ($\mathsf{iO}$iO) [17] is a central open question in cryptography. We provide new methods to make progress towards this goal. Our contributions may be summarized as follows:1.Bootstrapping. In a recent work, Lin and Tessaro [71] (LT) show that $\mathsf{iO}$iO may be constructed using (i) Functional Encryption ($\mathsf {FE}$FE) for polynomials of degree L, (ii) Pseudorandom Generators ($\mathsf{PRG}$PRG) with blockwise localityL and polynomial expansion, and (iii) Learning With Errors ($\mathsf{LWE}$LWE). Since there exist constructions of $\mathsf {FE}$FE for quadratic polynomials from standard assumptions on bilinear maps [16, 68], the ideal scenario would be to set $L=2$L=2, yielding $\mathsf{iO}$iO from widely believed assumptionsUnfortunately, it was shown soon after [18, 73] that $\mathsf{PRG}$PRG with block locality 2 and the expansion factor required by the LT construction, concretely $\varOmega (n \cdot 2^{b(3+\epsilon )})$Ω(n·2b(3+ϵ)), where n is the input length and b is the block length, do not exist. In the worst case, these lower bounds rule out 2-block local $\mathsf{PRG}$PRG with stretch $\varOmega (n \cdot 2^{b(2+\epsilon )})$Ω(n·2b(2+ϵ)). While [18, 73] provided strong negative evidence for constructing $\mathsf{iO}$iO based on bilinear maps, they could not rule out the possibility completely; a tantalizing gap has remained. Given the current state of lower bounds, the existence of 2 block local $\mathsf{PRG}$PRG with expansion factor $\varOmega (n \cdot 2^{b(1+\epsilon )})$Ω(n·2b(1+ϵ)) remains open, although this stretch does not suffice for the LT bootstrapping, and is hence unclear to be relevant for $\mathsf{iO}$iO.In this work, we improve the state of affairs as follows.(a)Weakening requirements on Boolean PRGs: In this work, we show that the narrow window of expansion factors left open by lower bounds do suffice for $\mathsf{iO}$iO. We show a new method to construct $\mathsf {FE}$FE for $\mathsf {NC}_1$NC1 from (i) $\mathsf {FE}$FE for degree L polynomials, (ii) $\mathsf{PRG}$PRGs of block locality L and expansion factor $\tilde{\varOmega }(n \cdot 2^{b(1+\epsilon )})$Ω~(n·2b(1+ϵ)), and (iii) $\mathsf{LWE}$LWE (or $\mathsf{RLWE}$RLWE).(b)Broadening class of sufficient randomness generators: Our bootstrapping theorem may be instantiated with a broader class of pseudorandom generators than hitherto considered for $\mathsf{iO}$iO, and may circumvent lower bounds known for the arithmetic degree of $\mathsf{iO}$iO-sufficient $\mathsf{PRG}$PRGs [18, 73]; in particular, these may admit instantiations with arithmetic degree 2, yielding $\mathsf{iO}$iO with the additional assumptions of $\mathsf{SXDH}$SXDH on Bilinear maps and $\mathsf{LWE}$LWE. In more detail, we may use the following two classes of $\mathsf{PRG}$PRG:i.Non-Boolean PRGs: We may use pseudorandom generators whose inputs and outputs need not be Boolean but may be integers restricted to a small (polynomial) range. Additionally, the outputs are not required to be pseudorandom but must only satisfy a milder indistinguishability property (We note that our notion of non Boolean PRGs is qualitatively similar to the notion of $\varDelta$Δ RGs defined in the concurrent work of Ananth, Jain and Sahai [9]. We emphasize that the methods of [9] and the present work are very different, but both works independently discover the same notion of weak PRG as sufficient for building $\mathsf{iO}$iO.).ii.Correlated Noise Generators: We introduce an even weaker class of pseudorandom generators, which we call correlated noise generators ($\mathsf{CNG}$CNG) which may not only be non-Boolean but are required to satisfy an even milder (seeming) indistinguishability property than $\varDelta$Δ RG.(c)Assumptions and Efficiency. Our bootstrapping theorems can be based on the hardness of the Learning With Errors problem or its ring variant ($\mathsf{LWE}/ \mathsf{RLWE}$LWE/RLWE) and can compile $\mathsf {FE}$FE for degree L polynomials directly to $\mathsf {FE}$FE for $\mathsf {NC}_1$NC1. Previous work compiles $\mathsf {FE}$FE for degree L polynomials to $\mathsf {FE}$FE for $\mathsf {NC}_0$NC0 to $\mathsf {FE}$FE for $\mathsf {NC}_1$NC1 to $\mathsf{iO}$iO [12, 45, 68, 72].Our method for bootstrapping to $\mathsf {NC}_1$NC1 does not go via randomized encodings as in previous works, which makes it simpler and more efficient than in previous works.2.Instantiating Primitives. In this work, we provide the first direct candidate of $\mathsf {FE}$FE for constant degree polynomials from new assumptions on lattices. Our construction is new and does not go via multilinear maps or graded encoding schemes as all previous constructions. Together with the bootstrapping step above, this yields a completely new candidate for$\mathsf{iO}$iO (as well as $\mathsf {FE}$FE for $\mathsf {NC}_1$NC1), which makes no use of multilinear or even bilinear maps. Our construction is based on the ring learning with errors assumption ($\mathsf{RLWE}$RLWE) as well as new untested assumptions on NTRU rings.We provide a detailed security analysis and discuss why previously known attacks in the context of multilinear maps, especially zeroizing and annihilation attacks, do not appear to apply to our setting. We caution that our construction must yet be subject to rigorous cryptanalysis by the community before confidence can be gained in its security. However, we believe that the significant departure from known multilinear map based constructions opens up a new and potentially fruitful direction to explore in the quest for $\mathsf{iO}$iO.Our construction is based entirely on lattices, due to which one may hope for post quantum security. Note that this feature is not enjoyed by instantiations that make any use of bilinear maps even if secure instances of weak PRGs, as identified by the present work, the follow-up by Lin and Matt [69] and the independent work by Ananth, Jain and Sahai [9] are found.
2019
TCC
Waters [Crypto, 2012] provided the first attribute based encryption scheme ABE for Deterministic Finite Automata (DFA) from a parametrized or “q-type” assumption over bilinear maps. Obtaining a construction from static assumptions has been elusive, despite much progress in the area of ABE.In this work, we construct the first attribute based encryption scheme for DFA from static assumptions on pairings, namely, the $\mathsf{DLIN}$ assumption. Our scheme supports unbounded length inputs, unbounded length machines and unbounded key requests. In more detail, secret keys in our construction are associated with a DFA M of unbounded length, ciphertexts are associated with a tuple $(\mathbf {x}, \mathsf {\mu })$ where $\mathbf {x}$ is a public attribute of unbounded length and $\mathsf {\mu }$ is a secret message bit, and decryption recovers $\mathsf {\mu }$ if and only if $M(\mathbf {x})=1$.Our techniques are at least as interesting as our final result. We present a simple compiler that combines constructions of unbounded ABE schemes for monotone span programs (MSP) in a black box way to construct ABE for DFA. In more detail, we find a way to embed DFA computation into monotone span programs, which lets us compose existing constructions (modified suitably) of unbounded key-policy ABE (${\mathsf {kpABE}}$) and unbounded ciphertext-policy ABE (${\mathsf {cpABE}}$) for MSP in a simple and modular way to obtain key-policy ABE for DFA. Our construction uses its building blocks in a symmetric way – by swapping the use of the underlying ${\mathsf {kpABE}}$ and ${\mathsf {cpABE}}$, we also obtain a construction of ciphertext-policy ABE for DFA.Our work extends techniques developed recently by Agrawal, Maitra and Yamada [Crypto 2019], which show how to construct ABE that support unbounded machines and unbounded inputs by combining ABE schemes that are bounded in one co-ordinate. At the heart of our work is the observation that unbounded, multi-use ABE for MSP already achieve most of what we need to build ABE for DFA.
2018
TCC
We construct Indistinguishability Obfuscation ($\mathsf {iO}$) and Functional Encryption ($\mathsf {FE}$) schemes in the Turing machine model from the minimal assumption of compact $\mathsf {FE}$ for circuits ($\mathsf {CktFE}$). Our constructions overcome the barrier of sub-exponential loss incurred by all prior work. Our contributions are:1.We construct $\mathsf {iO}$ in the Turing machine model from the same assumptions as required in the circuit model, namely, sub-exponentially secure $\mathsf {FE}$ for circuits. The previous best constructions [6, 41] require sub-exponentially secure $\mathsf {iO}$ for circuits, which in turn requires sub-exponentially secure $\mathsf {FE}$ for circuits [5, 15].2.We provide a new construction of single input $\mathsf {FE}$ for Turing machines with unbounded length inputs and optimal parameters from polynomially secure, compact $\mathsf {FE}$ for circuits. The previously best known construction by Ananth and Sahai [7] relies on $\mathsf {iO}$ for circuits, or equivalently, sub-exponentially secure $\mathsf {FE}$ for circuits.3.We provide a new construction of multi-input $\mathsf {FE}$ for Turing machines. Our construction supports a fixed number of encryptors (say k), who may each encrypt a string $\mathbf {x}_i$ of unbounded length. We rely on sub-exponentially secure $\mathsf {FE}$ for circuits, while the only previous construction [10] relies on a strong knowledge type assumption, namely, public coin differing inputs obfuscation. Our techniques are new and from first principles, and avoid usage of sophisticated $\mathsf {iO}$ specific machinery such as positional accumulators and splittable signatures that were used by all relevant prior work [6, 7, 41].
2017
CRYPTO
2017
TCC
2016
CRYPTO
2015
PKC
2015
EUROCRYPT
2014
EPRINT
2013
CRYPTO
2013
ASIACRYPT
2013
ASIACRYPT
2012
CRYPTO
2012
PKC
2011
ASIACRYPT
2010
EPRINT
Network coding is a method for achieving channel capacity in networks. The key idea is to allow network routers to linearly mix packets as they traverse the network so that recipients receive linear combinations of packets. Network coded systems are vulnerable to pollution attacks where a single malicious node floods the network with bad packets and prevents the receiver from decoding correctly. Cryptographic defenses to these problems are based on homomorphic signatures and MACs. These proposals, however, cannot handle mixing of packets from multiple sources, which is needed to achieve the full benefits of network coding. In this paper we address integrity of multi-source mixing. We propose a security model for this setting and provide a generic construction.
2010
PKC
2010
CRYPTO
2010
EUROCRYPT

Asiacrypt 2020
Asiacrypt 2019
TCC 2018
PKC 2018
Crypto 2018
Asiacrypt 2017
Crypto 2017
Eurocrypt 2016
PKC 2015