@article{107,
abstract = {We introduce the notion of “non-malleable codes” which relaxes the notion of error correction and error detection. Informally, a code is non-malleable if the message contained in a modified codeword is either the original message, or a completely unrelated value. In contrast to error correction and error detection, non-malleability can be achieved for very rich classes of modifications. We construct an efficient code that is non-malleable with respect to modifications that affect each bit of the codeword arbitrarily (i.e., leave it untouched, flip it, or set it to either 0 or 1), but independently of the value of the other bits of the codeword. Using the probabilistic method, we also show a very strong and general statement: there exists a non-malleable code for every “small enough” family F of functions via which codewords can be modified. Although this probabilistic method argument does not directly yield efficient constructions, it gives us efficient non-malleable codes in the random-oracle model for very general classes of tampering functions—e.g., functions where every bit in the tampered codeword can depend arbitrarily on any 99% of the bits in the original codeword. As an application of non-malleable codes, we show that they provide an elegant algorithmic solution to the task of protecting functionalities implemented in hardware (e.g., signature cards) against “tampering attacks.” In such attacks, the secret state of a physical system is tampered, in the hopes that future interaction with the modified system will reveal some secret information. This problem was previously studied in the work of Gennaro et al. in 2004 under the name “algorithmic tamper proof security” (ATP). We show that non-malleable codes can be used to achieve important improvements over the prior work. In particular, we show that any functionality can be made secure against a large class of tampering attacks, simply by encoding the secret state with a non-malleable code while it is stored in memory.},
author = {Dziembowski, Stefan and Pietrzak, Krzysztof Z and Wichs, Daniel},
journal = {Journal of the ACM},
number = {4},
publisher = {ACM},
title = {{Non-malleable codes}},
doi = {10.1145/3178432},
volume = {65},
year = {2018},
}
@inproceedings{559,
abstract = {Proofs of space (PoS) were suggested as more ecological and economical alternative to proofs of work, which are currently used in blockchain designs like Bitcoin. The existing PoS are based on rather sophisticated graph pebbling lower bounds. Much simpler and in several aspects more efficient schemes based on inverting random functions have been suggested, but they don’t give meaningful security guarantees due to existing time-memory trade-offs. In particular, Hellman showed that any permutation over a domain of size N can be inverted in time T by an algorithm that is given S bits of auxiliary information whenever (Formula presented). For functions Hellman gives a weaker attack with S2· T≈ N2 (e.g., S= T≈ N2/3). To prove lower bounds, one considers an adversary who has access to an oracle f: [ N] → [N] and can make T oracle queries. The best known lower bound is S· T∈ Ω(N) and holds for random functions and permutations. We construct functions that provably require more time and/or space to invert. Specifically, for any constant k we construct a function [N] → [N] that cannot be inverted unless Sk· T∈ Ω(Nk) (in particular, S= T≈ (Formula presented). Our construction does not contradict Hellman’s time-memory trade-off, because it cannot be efficiently evaluated in forward direction. However, its entire function table can be computed in time quasilinear in N, which is sufficient for the PoS application. Our simplest construction is built from a random function oracle g: [N] × [N] → [ N] and a random permutation oracle f: [N] → N] and is defined as h(x) = g(x, x′) where f(x) = π(f(x′)) with π being any involution without a fixed point, e.g. flipping all the bits. For this function we prove that any adversary who gets S bits of auxiliary information, makes at most T oracle queries, and inverts h on an ϵ fraction of outputs must satisfy S2· T∈ Ω(ϵ2N2).},
author = {Abusalah, Hamza M and Alwen, Joel F and Cohen, Bram and Khilko, Danylo and Pietrzak, Krzysztof Z and Reyzin, Leonid},
isbn = {978-331970696-2},
location = {Hong Kong, China},
pages = {357 -- 379},
publisher = {Springer},
title = {{Beyond Hellman’s time-memory trade-offs with applications to proofs of space}},
doi = {10.1007/978-3-319-70697-9_13},
volume = {10625},
year = {2017},
}
@inproceedings{605,
abstract = {Position based cryptography (PBC), proposed in the seminal work of Chandran, Goyal, Moriarty, and Ostrovsky (SIAM J. Computing, 2014), aims at constructing cryptographic schemes in which the identity of the user is his geographic position. Chandran et al. construct PBC schemes for secure positioning and position-based key agreement in the bounded-storage model (Maurer, J. Cryptology, 1992). Apart from bounded memory, their security proofs need a strong additional restriction on the power of the adversary: he cannot compute joint functions of his inputs. Removing this assumption is left as an open problem. We show that an answer to this question would resolve a long standing open problem in multiparty communication complexity: finding a function that is hard to compute with low communication complexity in the simultaneous message model, but easy to compute in the fully adaptive model. On a more positive side: we also show some implications in the other direction, i.e.: we prove that lower bounds on the communication complexity of certain multiparty problems imply existence of PBC primitives. Using this result we then show two attractive ways to “bypass” our hardness result: the first uses the random oracle model, the second weakens the locality requirement in the bounded-storage model to online computability. The random oracle construction is arguably one of the simplest proposed so far in this area. Our results indicate that constructing improved provably secure protocols for PBC requires a better understanding of multiparty communication complexity. This is yet another example where negative results in one area (in our case: lower bounds in multiparty communication complexity) can be used to construct secure cryptographic schemes.},
author = {Brody, Joshua and Dziembowski, Stefan and Faust, Sebastian and Pietrzak, Krzysztof Z},
editor = {Kalai, Yael and Reyzin, Leonid},
isbn = {978-331970499-9},
location = {Baltimore, MD, United States},
pages = {56 -- 81},
publisher = {Springer},
title = {{Position based cryptography and multiparty communication complexity}},
doi = {10.1007/978-3-319-70500-2_3},
volume = {10677},
year = {2017},
}
@inproceedings{609,
abstract = {Several cryptographic schemes and applications are based on functions that are both reasonably efficient to compute and moderately hard to invert, including client puzzles for Denial-of-Service protection, password protection via salted hashes, or recent proof-of-work blockchain systems. Despite their wide use, a definition of this concept has not yet been distilled and formalized explicitly. Instead, either the applications are proven directly based on the assumptions underlying the function, or some property of the function is proven, but the security of the application is argued only informally. The goal of this work is to provide a (universal) definition that decouples the efforts of designing new moderately hard functions and of building protocols based on them, serving as an interface between the two. On a technical level, beyond the mentioned definitions, we instantiate the model for four different notions of hardness. We extend the work of Alwen and Serbinenko (STOC 2015) by providing a general tool for proving security for the first notion of memory-hard functions that allows for provably secure applications. The tool allows us to recover all of the graph-theoretic techniques developed for proving security under the older, non-composable, notion of security used by Alwen and Serbinenko. As an application of our definition of moderately hard functions, we prove the security of two different schemes for proofs of effort (PoE). We also formalize and instantiate the concept of a non-interactive proof of effort (niPoE), in which the proof is not bound to a particular communication context but rather any bit-string chosen by the prover.},
author = {Alwen, Joel F and Tackmann, Björn},
editor = {Kalai, Yael and Reyzin, Leonid},
isbn = {978-331970499-9},
location = {Baltimore, MD, United States},
pages = {493 -- 526},
publisher = {Springer},
title = {{Moderately hard functions: Definition, instantiations, and applications}},
doi = {10.1007/978-3-319-70500-2_17},
volume = {10677},
year = {2017},
}
@article{6196,
abstract = {PMAC is a simple and parallel block-cipher mode of operation, which was introduced by Black and Rogaway at Eurocrypt 2002. If instantiated with a (pseudo)random permutation over n-bit strings, PMAC constitutes a provably secure variable input-length (pseudo)random function. For adversaries making q queries, each of length at most l (in n-bit blocks), and of total length σ ≤ ql, the original paper proves an upper bound on the distinguishing advantage of Ο(σ2/2n), while the currently best bound is Ο (qσ/2n).In this work we show that this bound is tight by giving an attack with advantage Ω (q2l/2n). In the PMAC construction one initially XORs a mask to every message block, where the mask for the ith block is computed as τi := γi·L, where L is a (secret) random value, and γi is the i-th codeword of the Gray code. Our attack applies more generally to any sequence of γi’s which contains a large coset of a subgroup of GF(2n). We then investigate if the security of PMAC can be further improved by using τi’s that are k-wise independent, for k > 1 (the original distribution is only 1-wise independent). We observe that the security of PMAC will not increase in general, even if the masks are chosen from a 2-wise independent distribution, and then prove that the security increases to O(q<2/2n), if the τi are 4-wise independent. Due to simple extension attacks, this is the best bound one can hope for, using any distribution on the masks. Whether 3-wise independence is already sufficient to get this level of security is left as an open problem.},
author = {Gazi, Peter and Pietrzak, Krzysztof Z and Rybar, Michal},
issn = {2519-173X},
journal = {IACR Transactions on Symmetric Cryptology},
number = {2},
pages = {145--161},
publisher = {Ruhr University Bochum},
title = {{The exact security of PMAC}},
doi = {10.13154/TOSC.V2016.I2.145-161},
volume = {2016},
year = {2017},
}
@inproceedings{635,
abstract = {Memory-hard functions (MHFs) are hash algorithms whose evaluation cost is dominated by memory cost. As memory, unlike computation, costs about the same across different platforms, MHFs cannot be evaluated at significantly lower cost on dedicated hardware like ASICs. MHFs have found widespread applications including password hashing, key derivation, and proofs-of-work. This paper focuses on scrypt, a simple candidate MHF designed by Percival, and described in RFC 7914. It has been used within a number of cryptocurrencies (e.g., Litecoin and Dogecoin) and has been an inspiration for Argon2d, one of the winners of the recent password-hashing competition. Despite its popularity, no rigorous lower bounds on its memory complexity are known. We prove that scrypt is optimally memory-hard, i.e., its cumulative memory complexity (cmc) in the parallel random oracle model is Ω(n2w), where w and n are the output length and number of invocations of the underlying hash function, respectively. High cmc is a strong security target for MHFs introduced by Alwen and Serbinenko (STOC’15) which implies high memory cost even for adversaries who can amortize the cost over many evaluations and evaluate the underlying hash functions many times in parallel. Our proof is the first showing optimal memory-hardness for any MHF. Our result improves both quantitatively and qualitatively upon the recent work by Alwen et al. (EUROCRYPT’16) who proved a weaker lower bound of Ω(n2w/ log2 n) for a restricted class of adversaries.},
author = {Alwen, Joel F and Chen, Binchi and Pietrzak, Krzysztof Z and Reyzin, Leonid and Tessaro, Stefano},
editor = {Coron, Jean-Sébastien and Buus Nielsen, Jesper},
isbn = {978-331956616-0},
location = {Paris, France},
pages = {33 -- 62},
publisher = {Springer},
title = {{Scrypt is maximally memory hard}},
doi = {10.1007/978-3-319-56617-7_2},
volume = {10212},
year = {2017},
}
@inproceedings{637,
abstract = {For many cryptographic primitives, it is relatively easy to achieve selective security (where the adversary commits a-priori to some of the choices to be made later in the attack) but appears difficult to achieve the more natural notion of adaptive security (where the adversary can make all choices on the go as the attack progresses). A series of several recent works shows how to cleverly achieve adaptive security in several such scenarios including generalized selective decryption (Panjwani, TCC ’07 and Fuchsbauer et al., CRYPTO ’15), constrained PRFs (Fuchsbauer et al., ASIACRYPT ’14), and Yao garbled circuits (Jafargholi and Wichs, TCC ’16b). Although the above works expressed vague intuition that they share a common technique, the connection was never made precise. In this work we present a new framework that connects all of these works and allows us to present them in a unified and simplified fashion. Moreover, we use the framework to derive a new result for adaptively secure secret sharing over access structures defined via monotone circuits. We envision that further applications will follow in the future. Underlying our framework is the following simple idea. It is well known that selective security, where the adversary commits to n-bits of information about his future choices, automatically implies adaptive security at the cost of amplifying the adversary’s advantage by a factor of up to 2n. However, in some cases the proof of selective security proceeds via a sequence of hybrids, where each pair of adjacent hybrids locally only requires some smaller partial information consisting of m ≪ n bits. The partial information needed might be completely different between different pairs of hybrids, and if we look across all the hybrids we might rely on the entire n-bit commitment. Nevertheless, the above is sufficient to prove adaptive security, at the cost of amplifying the adversary’s advantage by a factor of only 2m ≪ 2n. In all of our examples using the above framework, the different hybrids are captured by some sort of a graph pebbling game and the amount of information that the adversary needs to commit to in each pair of hybrids is bounded by the maximum number of pebbles in play at any point in time. Therefore, coming up with better strategies for proving adaptive security translates to various pebbling strategies for different types of graphs.},
author = {Jafargholi, Zahra and Kamath Hosdurg, Chethan and Klein, Karen and Komargodski, Ilan and Pietrzak, Krzysztof Z and Wichs, Daniel},
editor = {Katz, Jonathan and Shacham, Hovav},
isbn = {978-331963687-0},
location = {Santa Barbara, CA, United States},
pages = {133 -- 163},
publisher = {Springer},
title = {{Be adaptive avoid overcommitting}},
doi = {10.1007/978-3-319-63688-7_5},
volume = {10401},
year = {2017},
}
@inproceedings{640,
abstract = {Data-independent Memory Hard Functions (iMHFS) are finding a growing number of applications in security; especially in the domain of password hashing. An important property of a concrete iMHF is specified by fixing a directed acyclic graph (DAG) Gn on n nodes. The quality of that iMHF is then captured by the following two pebbling complexities of Gn: – The parallel cumulative pebbling complexity Π∥cc(Gn) must be as high as possible (to ensure that the amortized cost of computing the function on dedicated hardware is dominated by the cost of memory). – The sequential space-time pebbling complexity Πst(Gn) should be as close as possible to Π∥cc(Gn) (to ensure that using many cores in parallel and amortizing over many instances does not give much of an advantage). In this paper we construct a family of DAGs with best possible parameters in an asymptotic sense, i.e., where Π∥cc(Gn) = Ω(n2/ log(n)) (which matches a known upper bound) and Πst(Gn) is within a constant factor of Π∥cc(Gn). Our analysis relies on a new connection between the pebbling complexity of a DAG and its depth-robustness (DR) – a well studied combinatorial property. We show that high DR is sufficient for high Π∥cc. Alwen and Blocki (CRYPTO’16) showed that high DR is necessary and so, together, these results fully characterize DAGs with high Π∥cc in terms of DR. Complementing these results, we provide new upper and lower bounds on the Π∥cc of several important candidate iMHFs from the literature. We give the first lower bounds on the memory hardness of the Catena and Balloon Hashing functions in a parallel model of computation and we give the first lower bounds of any kind for (a version) of Argon2i. Finally we describe a new class of pebbling attacks improving on those of Alwen and Blocki (CRYPTO’16). By instantiating these attacks we upperbound the Π∥cc of the Password Hashing Competition winner Argon2i and one of the Balloon Hashing functions by O (n1.71). We also show an upper bound of O(n1.625) for the Catena functions and the two remaining Balloon Hashing functions.},
author = {Alwen, Joel F and Blocki, Jeremiah and Pietrzak, Krzysztof Z},
editor = {Coron, Jean-Sébastien and Buus Nielsen, Jesper},
isbn = {978-331956616-0},
location = {Paris, France},
pages = {3 -- 32},
publisher = {Springer},
title = {{Depth-robust graphs and their cumulative memory complexity}},
doi = {10.1007/978-3-319-56617-7_1},
volume = {10212},
year = {2017},
}
@inproceedings{648,
abstract = {Pseudoentropy has found a lot of important applications to cryptography and complexity theory. In this paper we focus on the foundational problem that has not been investigated so far, namely by how much pseudoentropy (the amount seen by computationally bounded attackers) diﬀers from its information-theoretic counterpart (seen by unbounded observers), given certain limits on attacker’s computational power? We provide the following answer for HILL pseudoentropy, which exhibits a threshold behavior around the size exponential in the entropy amount:– If the attacker size (s) and advantage () satisfy s (formula presented) where k is the claimed amount of pseudoentropy, then the pseudoentropy boils down to the information-theoretic smooth entropy. – If s (formula presented) then pseudoentropy could be arbitrarily bigger than the information-theoretic smooth entropy. Besides answering the posted question, we show an elegant application of our result to the complexity theory, namely that it implies the clas-sical result on the existence of functions hard to approximate (due to Pippenger). In our approach we utilize non-constructive techniques: the duality of linear programming and the probabilistic method.},
author = {Skórski, Maciej},
editor = {Jäger, Gerhard and Steila, Silvia},
isbn = {978-331955910-0},
location = {Bern, Switzerland},
pages = {600 -- 613},
publisher = {Springer},
title = {{On the complexity of breaking pseudoentropy}},
doi = {10.1007/978-3-319-55911-7_43},
volume = {10185},
year = {2017},
}
@inproceedings{650,
abstract = {In this work we present a short and unified proof for the Strong and Weak Regularity Lemma, based on the cryptographic tech-nique called low-complexity approximations. In short, both problems reduce to a task of finding constructively an approximation for a certain target function under a class of distinguishers (test functions), where dis-tinguishers are combinations of simple rectangle-indicators. In our case these approximations can be learned by a simple iterative procedure, which yields a unified and simple proof, achieving for any graph with density d and any approximation parameter the partition size. The novelty in our proof is: (a) a simple approach which yields both strong and weaker variant, and (b) improvements when d = o(1). At an abstract level, our proof can be seen a refinement and simplification of the “analytic” proof given by Lovasz and Szegedy.},
author = {Skórski, Maciej},
editor = {Jäger, Gerhard and Steila, Silvia},
issn = {03029743},
location = {Bern, Switzerland},
pages = {586 -- 599},
publisher = {Springer},
title = {{A cryptographic view of regularity lemmas: Simpler unified proofs and refined bounds}},
doi = {10.1007/978-3-319-55911-7_42},
volume = {10185},
year = {2017},
}
@inproceedings{6526,
abstract = {This paper studies the complexity of estimating Rényi divergences of discrete distributions: p observed from samples and the baseline distribution q known a priori. Extending the results of Acharya et al. (SODA'15) on estimating Rényi entropy, we present improved estimation techniques together with upper and lower bounds on the sample complexity. We show that, contrarily to estimating Rényi entropy where a sublinear (in the alphabet size) number of samples suffices, the sample complexity is heavily dependent on events occurring unlikely in q, and is unbounded in general (no matter what an estimation technique is used). For any divergence of integer order bigger than 1, we provide upper and lower bounds on the number of samples dependent on probabilities of p and q (the lower bounds hold for non-integer orders as well). We conclude that the worst-case sample complexity is polynomial in the alphabet size if and only if the probabilities of q are non-negligible. This gives theoretical insights into heuristics used in the applied literature to handle numerical instability, which occurs for small probabilities of q. Our result shows that they should be handled with care not only because of numerical issues, but also because of a blow up in the sample complexity.},
author = {Skórski, Maciej},
booktitle = {2017 IEEE International Symposium on Information Theory (ISIT)},
isbn = {9781509040964},
location = {Aachen, Germany},
publisher = {IEEE},
title = {{On the complexity of estimating Rènyi divergences}},
doi = {10.1109/isit.2017.8006529},
year = {2017},
}
@inproceedings{6527,
abstract = {A memory-hard function (MHF) ƒn with parameter n can be computed in sequential time and space n. Simultaneously, a high amortized parallel area-time complexity (aAT) is incurred per evaluation. In practice, MHFs are used to limit the rate at which an adversary (using a custom computational device) can evaluate a security sensitive function that still occasionally needs to be evaluated by honest users (using an off-the-shelf general purpose device). The most prevalent examples of such sensitive functions are Key Derivation Functions (KDFs) and password hashing algorithms where rate limits help mitigate off-line dictionary attacks. As the honest users' inputs to these functions are often (low-entropy) passwords special attention is given to a class of side-channel resistant MHFs called iMHFs.
Essentially all iMHFs can be viewed as some mode of operation (making n calls to some round function) given by a directed acyclic graph (DAG) with very low indegree. Recently, a combinatorial property of a DAG has been identified (called "depth-robustness") which results in good provable security for an iMHF based on that DAG. Depth-robust DAGs have also proven useful in other cryptographic applications. Unfortunately, up till now, all known very depth-robust DAGs are impractically complicated and little is known about their exact (i.e. non-asymptotic) depth-robustness both in theory and in practice.
In this work we build and analyze (both formally and empirically) several exceedingly simple and efficient to navigate practical DAGs for use in iMHFs and other applications. For each DAG we:
*Prove that their depth-robustness is asymptotically maximal.
*Prove bounds of at least 3 orders of magnitude better on their exact depth-robustness compared to known bounds for other practical iMHF.
*Implement and empirically evaluate their depth-robustness and aAT against a variety of state-of-the art (and several new) depth-reduction and low aAT attacks.
We find that, against all attacks, the new DAGs perform significantly better in practice than Argon2i, the most widely deployed iMHF in practice.
Along the way we also improve the best known empirical attacks on the aAT of Argon2i by implementing and testing several heuristic versions of a (hitherto purely theoretical) depth-reduction attack. Finally, we demonstrate practicality of our constructions by modifying the Argon2i code base to use one of the new high aAT DAGs. Experimental benchmarks on a standard off-the-shelf CPU show that the new modifications do not adversely affect the impressive throughput of Argon2i (despite seemingly enjoying significantly higher aAT).
},
author = {Alwen, Joel F and Blocki, Jeremiah and Harsha, Ben},
booktitle = {Proceedings of the 2017 ACM SIGSAC Conference on Computer and Communications Security},
isbn = {9781450349468},
location = {Dallas, TX, USA},
pages = {1001--1017},
publisher = {ACM Press},
title = {{Practical graphs for optimal side-channel resistant memory-hard functions}},
doi = {10.1145/3133956.3134031},
year = {2017},
}
@inproceedings{697,
abstract = {De, Trevisan and Tulsiani [CRYPTO 2010] show that every distribution over n-bit strings which has constant statistical distance to uniform (e.g., the output of a pseudorandom generator mapping n-1 to n bit strings), can be distinguished from the uniform distribution with advantage epsilon by a circuit of size O( 2^n epsilon^2). We generalize this result, showing that a distribution which has less than k bits of min-entropy, can be distinguished from any distribution with k bits of delta-smooth min-entropy with advantage epsilon by a circuit of size O(2^k epsilon^2/delta^2). As a special case, this implies that any distribution with support at most 2^k (e.g., the output of a pseudoentropy generator mapping k to n bit strings) can be distinguished from any given distribution with min-entropy k+1 with advantage epsilon by a circuit of size O(2^k epsilon^2). Our result thus shows that pseudoentropy distributions face basically the same non-uniform attacks as pseudorandom distributions. },
author = {Pietrzak, Krzysztof Z and Skórski, Maciej},
issn = {18688969},
location = {Warsaw, Poland},
publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik},
title = {{Non uniform attacks against pseudoentropy}},
doi = {10.4230/LIPIcs.ICALP.2017.39},
volume = {80},
year = {2017},
}
@inproceedings{710,
abstract = {We revisit the problem of estimating entropy of discrete distributions from independent samples, studied recently by Acharya, Orlitsky, Suresh and Tyagi (SODA 2015), improving their upper and lower bounds on the necessary sample size n. For estimating Renyi entropy of order alpha, up to constant accuracy and error probability, we show the following * Upper bounds n = O(1) 2^{(1-1/alpha)H_alpha} for integer alpha>1, as the worst case over distributions with Renyi entropy equal to H_alpha. * Lower bounds n = Omega(1) K^{1-1/alpha} for any real alpha>1, with the constant being an inverse polynomial of the accuracy, as the worst case over all distributions on K elements. Our upper bounds essentially replace the alphabet size by a factor exponential in the entropy, which offers improvements especially in low or medium entropy regimes (interesting for example in anomaly detection). As for the lower bounds, our proof explicitly shows how the complexity depends on both alphabet and accuracy, partially solving the open problem posted in previous works. The argument for upper bounds derives a clean identity for the variance of falling-power sum of a multinomial distribution. Our approach for lower bounds utilizes convex optimization to find a distribution with possibly worse estimation performance, and may be of independent interest as a tool to work with Le Cam’s two point method. },
author = {Obremski, Maciej and Skórski, Maciej},
issn = {18688969},
location = {Berkeley, USA},
publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik},
title = {{Renyi entropy estimation revisited}},
doi = {10.4230/LIPIcs.APPROX-RANDOM.2017.20},
volume = {81},
year = {2017},
}
@phdthesis{838,
abstract = {In this thesis we discuss the exact security of message authentications codes HMAC , NMAC , and PMAC . NMAC is a mode of operation which turns a fixed input-length keyed hash function f into a variable input-length function. A practical single-key variant of NMAC called HMAC is a very popular and widely deployed message authentication code (MAC). PMAC is a block-cipher based mode of operation, which also happens to be the most famous fully parallel MAC. NMAC was introduced by Bellare, Canetti and Krawczyk Crypto’96, who proved it to be a secure pseudorandom function (PRF), and thus also a MAC, under two assumptions. Unfortunately, for many instantiations of HMAC one of them has been found to be wrong. To restore the provable guarantees for NMAC , Bellare [Crypto’06] showed its security without this assumption. PMAC was introduced by Black and Rogaway at Eurocrypt 2002. If instantiated with a pseudorandom permutation over n -bit strings, PMAC constitutes a provably secure variable input-length PRF. For adversaries making q queries, each of length at most ` (in n -bit blocks), and of total length σ ≤ q` , the original paper proves an upper bound on the distinguishing advantage of O ( σ 2 / 2 n ), while the currently best bound is O ( qσ/ 2 n ). In this work we show that this bound is tight by giving an attack with advantage Ω( q 2 `/ 2 n ). In the PMAC construction one initially XORs a mask to every message block, where the mask for the i th block is computed as τ i := γ i · L , where L is a (secret) random value, and γ i is the i -th codeword of the Gray code. Our attack applies more generally to any sequence of γ i ’s which contains a large coset of a subgroup of GF (2 n ). As for NMAC , our first contribution is a simpler and uniform proof: If f is an ε -secure PRF (against q queries) and a δ - non-adaptively secure PRF (against q queries), then NMAC f is an ( ε + `qδ )-secure PRF against q queries of length at most ` blocks each. We also show that this ε + `qδ bound is basically tight by constructing an f for which an attack with advantage `qδ exists. Moreover, we analyze the PRF-security of a modification of NMAC called NI by An and Bellare that avoids the constant rekeying on multi-block messages in NMAC and allows for an information-theoretic analysis. We carry out such an analysis, obtaining a tight `q 2 / 2 c bound for this step, improving over the trivial bound of ` 2 q 2 / 2 c . Finally, we investigate, if the security of PMAC can be further improved by using τ i ’s that are k -wise independent, for k > 1 (the original has k = 1). We observe that the security of PMAC will not increase in general if k = 2, and then prove that the security increases to O ( q 2 / 2 n ), if the k = 4. Due to simple extension attacks, this is the best bound one can hope for, using any distribution on the masks. Whether k = 3 is already sufficient to get this level of security is left as an open problem. Keywords: Message authentication codes, Pseudorandom functions, HMAC, PMAC. },
author = {Rybar, Michal},
pages = {86},
publisher = {IST Austria},
title = {{(The exact security of) Message authentication codes}},
doi = {10.15479/AT:ISTA:th_828},
year = {2017},
}
@article{1187,
abstract = {We construct efficient authentication protocols and message authentication codes (MACs) whose security can be reduced to the learning parity with noise (LPN) problem. Despite a large body of work—starting with the (Formula presented.) protocol of Hopper and Blum in 2001—until now it was not even known how to construct an efficient authentication protocol from LPN which is secure against man-in-the-middle attacks. A MAC implies such a (two-round) protocol.},
author = {Kiltz, Eike and Pietrzak, Krzysztof Z and Venturi, Daniele and Cash, David and Jain, Abhishek},
journal = {Journal of Cryptology},
number = {4},
pages = {1238 -- 1275},
publisher = {Springer},
title = {{Efficient authentication from hard learning problems}},
doi = {10.1007/s00145-016-9247-3},
volume = {30},
year = {2017},
}
@inproceedings{1174,
abstract = {Security of cryptographic applications is typically defined by security games. The adversary, within certain resources, cannot win with probability much better than 0 (for unpredictability applications, like one-way functions) or much better than 1/2 (indistinguishability applications for instance encryption schemes). In so called squared-friendly applications the winning probability of the adversary, for different values of the application secret randomness, is not only close to 0 or 1/2 on average, but also concentrated in the sense that its second central moment is small. The class of squared-friendly applications, which contains all unpredictability applications and many indistinguishability applications, is particularly important for key derivation. Barak et al. observed that for square-friendly applications one can beat the "RT-bound", extracting secure keys with significantly smaller entropy loss. In turn Dodis and Yu showed that in squared-friendly applications one can directly use a "weak" key, which has only high entropy, as a secure key. In this paper we give sharp lower bounds on square security assuming security for "weak" keys. We show that any application which is either (a) secure with weak keys or (b) allows for entropy savings for keys derived by universal hashing, must be square-friendly. Quantitatively, our lower bounds match the positive results of Dodis and Yu and Barak et al. (TCC\'13, CRYPTO\'11) Hence, they can be understood as a general characterization of squared-friendly applications. While the positive results on squared-friendly applications where derived by one clever application of the Cauchy-Schwarz Inequality, for tight lower bounds we need more machinery. In our approach we use convex optimization techniques and some theory of circular matrices.},
author = {Skórski, Maciej},
issn = {18688969},
location = {Hannover, Germany},
publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik},
title = {{Lower bounds on key derivation for square-friendly applications}},
doi = {10.4230/LIPIcs.STACS.2017.57},
volume = {66},
year = {2017},
}
@inproceedings{1175,
abstract = {We study space complexity and time-space trade-offs with a focus not on peak memory usage but on overall memory consumption throughout the computation. Such a cumulative space measure was introduced for the computational model of parallel black pebbling by [Alwen and Serbinenko ’15] as a tool for obtaining results in cryptography. We consider instead the non- deterministic black-white pebble game and prove optimal cumulative space lower bounds and trade-offs, where in order to minimize pebbling time the space has to remain large during a significant fraction of the pebbling. We also initiate the study of cumulative space in proof complexity, an area where other space complexity measures have been extensively studied during the last 10–15 years. Using and extending the connection between proof complexity and pebble games in [Ben-Sasson and Nordström ’08, ’11] we obtain several strong cumulative space results for (even parallel versions of) the resolution proof system, and outline some possible future directions of study of this, in our opinion, natural and interesting space measure.},
author = {Alwen, Joel F and De Rezende, Susanna and Nordstrom, Jakob and Vinyals, Marc},
editor = {Papadimitriou, Christos},
issn = {18688969},
location = {Berkeley, CA, United States},
pages = {38:1--38--21},
publisher = {Schloss Dagstuhl - Leibniz-Zentrum für Informatik},
title = {{Cumulative space in black-white pebbling and resolution}},
doi = {10.4230/LIPIcs.ITCS.2017.38},
volume = {67},
year = {2017},
}
@inproceedings{1176,
abstract = {The algorithm Argon2i-B of Biryukov, Dinu and Khovratovich is currently being considered by the IRTF (Internet Research Task Force) as a new de-facto standard for password hashing. An older version (Argon2i-A) of the same algorithm was chosen as the winner of the recent Password Hashing Competition. An important competitor to Argon2i-B is the recently introduced Balloon Hashing (BH) algorithm of Corrigan-Gibs, Boneh and Schechter. A key security desiderata for any such algorithm is that evaluating it (even using a custom device) requires a large amount of memory amortized across multiple instances. Alwen and Blocki (CRYPTO 2016) introduced a class of theoretical attacks against Argon2i-A and BH. While these attacks yield large asymptotic reductions in the amount of memory, it was not, a priori, clear if (1) they can be extended to the newer Argon2i-B, (2) the attacks are effective on any algorithm for practical parameter ranges (e.g., 1GB of memory) and (3) if they can be effectively instantiated against any algorithm under realistic hardware constrains. In this work we answer all three of these questions in the affirmative for all three algorithms. This is also the first work to analyze the security of Argon2i-B. In more detail, we extend the theoretical attacks of Alwen and Blocki (CRYPTO 2016) to the recent Argon2i-B proposal demonstrating severe asymptotic deficiencies in its security. Next we introduce several novel heuristics for improving the attack's concrete memory efficiency even when on-chip memory bandwidth is bounded. We then simulate our attacks on randomly sampled Argon2i-A, Argon2i-B and BH instances and measure the resulting memory consumption for various practical parameter ranges and for a variety of upperbounds on the amount of parallelism available to the attacker. Finally we describe, implement, and test a new heuristic for applying the Alwen-Blocki attack to functions employing a technique developed by Corrigan-Gibs et al. for improving concrete security of memory-hard functions. We analyze the collected data and show the effects various parameters have on the memory consumption of the attack. In particular, we can draw several interesting conclusions about the level of security provided by these functions. · For the Alwen-Blocki attack to fail against practical memory parameters, Argon2i-B must be instantiated with more than 10 passes on memory - beyond the "paranoid" parameter setting in the current IRTF proposal. · The technique of Corrigan-Gibs for improving security can also be overcome by the Alwen-Blocki attack under realistic hardware constraints. · On a positive note, both the asymptotic and concrete security of Argon2i-B seem to improve on that of Argon2i-A.},
author = {Alwen, Joel F and Blocki, Jeremiah},
isbn = {978-150905761-0},
location = {Paris, France},
publisher = {IEEE},
title = {{Towards practical attacks on Argon2i and balloon hashing}},
doi = {10.1109/EuroSP.2017.47},
year = {2017},
}
@article{1592,
abstract = {A modular approach to constructing cryptographic protocols leads to simple designs but often inefficient instantiations. On the other hand, ad hoc constructions may yield efficient protocols at the cost of losing conceptual simplicity. We suggest a new design paradigm, structure-preserving cryptography, that provides a way to construct modular protocols with reasonable efficiency while retaining conceptual simplicity. A cryptographic scheme over a bilinear group is called structure-preserving if its public inputs and outputs consist of elements from the bilinear groups and their consistency can be verified by evaluating pairing-product equations. As structure-preserving schemes smoothly interoperate with each other, they are useful as building blocks in modular design of cryptographic applications. This paper introduces structure-preserving commitment and signature schemes over bilinear groups with several desirable properties. The commitment schemes include homomorphic, trapdoor and length-reducing commitments to group elements, and the structure-preserving signature schemes are the first ones that yield constant-size signatures on multiple group elements. A structure-preserving signature scheme is called automorphic if the public keys lie in the message space, which cannot be achieved by compressing inputs via a cryptographic hash function, as this would destroy the mathematical structure we are trying to preserve. Automorphic signatures can be used for building certification chains underlying privacy-preserving protocols. Among a vast number of applications of structure-preserving protocols, we present an efficient round-optimal blind-signature scheme and a group signature scheme with an efficient and concurrently secure protocol for enrolling new members.},
author = {Abe, Masayuki and Fuchsbauer, Georg and Groth, Jens and Haralambiev, Kristiyan and Ohkubo, Miyako},
journal = {Journal of Cryptology},
number = {2},
pages = {363 -- 421},
publisher = {Springer},
title = {{Structure preserving signatures and commitments to group elements}},
doi = {10.1007/s00145-014-9196-7},
volume = {29},
year = {2016},
}
@inproceedings{1653,
abstract = {A somewhere statistically binding (SSB) hash, introduced by Hubáček and Wichs (ITCS ’15), can be used to hash a long string x to a short digest y = H hk (x) using a public hashing-key hk. Furthermore, there is a way to set up the hash key hk to make it statistically binding on some arbitrary hidden position i, meaning that: (1) the digest y completely determines the i’th bit (or symbol) of x so that all pre-images of y have the same value in the i’th position, (2) it is computationally infeasible to distinguish the position i on which hk is statistically binding from any other position i’. Lastly, the hash should have a local opening property analogous to Merkle-Tree hashing, meaning that given x and y = H hk (x) it should be possible to create a short proof π that certifies the value of the i’th bit (or symbol) of x without having to provide the entire input x. A similar primitive called a positional accumulator, introduced by Koppula, Lewko and Waters (STOC ’15) further supports dynamic updates of the hashed value. These tools, which are interesting in their own right, also serve as one of the main technical components in several recent works building advanced applications from indistinguishability obfuscation (iO).
The prior constructions of SSB hashing and positional accumulators required fully homomorphic encryption (FHE) and iO respectively. In this work, we give new constructions of these tools based on well studied number-theoretic assumptions such as DDH, Phi-Hiding and DCR, as well as a general construction from lossy/injective functions.},
author = {Okamoto, Tatsuaki and Pietrzak, Krzysztof Z and Waters, Brent and Wichs, Daniel},
location = {Auckland, New Zealand},
pages = {121 -- 145},
publisher = {Springer},
title = {{New realizations of somewhere statistically binding hashing and positional accumulators}},
doi = {10.1007/978-3-662-48797-6_6},
volume = {9452},
year = {2016},
}
@inproceedings{1225,
abstract = {At Crypto 2015 Fuchsbauer, Hanser and Slamanig (FHS) presented the first standard-model construction of efficient roundoptimal blind signatures that does not require complexity leveraging. It is conceptually simple and builds on the primitive of structure-preserving signatures on equivalence classes (SPS-EQ). FHS prove the unforgeability of their scheme assuming EUF-CMA security of the SPS-EQ scheme and hardness of a version of the DH inversion problem. Blindness under adversarially chosen keys is proven under an interactive variant of the DDH assumption. We propose a variant of their scheme whose blindness can be proven under a non-interactive assumption, namely a variant of the bilinear DDH assumption. We moreover prove its unforgeability assuming only unforgeability of the underlying SPS-EQ but no additional assumptions as needed for the FHS scheme.},
author = {Fuchsbauer, Georg and Hanser, Christian and Kamath Hosdurg, Chethan and Slamanig, Daniel},
location = {Amalfi, Italy},
pages = {391 -- 408},
publisher = {Springer},
title = {{Practical round-optimal blind signatures in the standard model from weaker assumptions}},
doi = {10.1007/978-3-319-44618-9_21},
volume = {9841},
year = {2016},
}
@inproceedings{1229,
abstract = {Witness encryption (WE) was introduced by Garg et al. [GGSW13]. A WE scheme is defined for some NP language L and lets a sender encrypt messages relative to instances x. A ciphertext for x can be decrypted using w witnessing x ∈ L, but hides the message if x ∈ L. Garg et al. construct WE from multilinear maps and give another construction [GGH+13b] using indistinguishability obfuscation (iO) for circuits. Due to the reliance on such heavy tools, WE can cur- rently hardly be implemented on powerful hardware and will unlikely be realizable on constrained devices like smart cards any time soon. We construct a WE scheme where encryption is done by simply computing a Naor-Yung ciphertext (two CPA encryptions and a NIZK proof). To achieve this, our scheme has a setup phase, which outputs public parameters containing an obfuscated circuit (only required for decryption), two encryption keys and a common reference string (used for encryption). This setup need only be run once, and the parame- ters can be used for arbitrary many encryptions. Our scheme can also be turned into a functional WE scheme, where a message is encrypted w.r.t. a statement and a function f, and decryption with a witness w yields f (m, w). Our construction is inspired by the functional encryption scheme by Garg et al. and we prove (selective) security assuming iO and statistically simulation-sound NIZK. We give a construction of the latter in bilinear groups and combining it with ElGamal encryption, our ciphertexts are of size 1.3 kB at a 128-bit security level and can be computed on a smart card.},
author = {Abusalah, Hamza M and Fuchsbauer, Georg and Pietrzak, Krzysztof Z},
location = {Guildford, UK},
pages = {285 -- 303},
publisher = {Springer},
title = {{Offline witness encryption}},
doi = {10.1007/978-3-319-39555-5_16},
volume = {9696},
year = {2016},
}
@inproceedings{1231,
abstract = {We study the time-and memory-complexities of the problem of computing labels of (multiple) randomly selected challenge-nodes in a directed acyclic graph. The w-bit label of a node is the hash of the labels of its parents, and the hash function is modeled as a random oracle. Specific instances of this problem underlie both proofs of space [Dziembowski et al. CRYPTO’15] as well as popular memory-hard functions like scrypt. As our main tool, we introduce the new notion of a probabilistic parallel entangled pebbling game, a new type of combinatorial pebbling game on a graph, which is closely related to the labeling game on the same graph. As a first application of our framework, we prove that for scrypt, when the underlying hash function is invoked n times, the cumulative memory complexity (CMC) (a notion recently introduced by Alwen and Serbinenko (STOC’15) to capture amortized memory-hardness for parallel adversaries) is at least Ω(w · (n/ log(n))2). This bound holds for adversaries that can store many natural functions of the labels (e.g., linear combinations), but still not arbitrary functions thereof. We then introduce and study a combinatorial quantity, and show how a sufficiently small upper bound on it (which we conjecture) extends our CMC bound for scrypt to hold against arbitrary adversaries. We also show that such an upper bound solves the main open problem for proofs-of-space protocols: namely, establishing that the time complexity of computing the label of a random node in a graph on n nodes (given an initial kw-bit state) reduces tightly to the time complexity for black pebbling on the same graph (given an initial k-node pebbling).},
author = {Alwen, Joel F and Chen, Binyi and Kamath Hosdurg, Chethan and Kolmogorov, Vladimir and Pietrzak, Krzysztof Z and Tessaro, Stefano},
location = {Vienna, Austria},
pages = {358 -- 387},
publisher = {Springer},
title = {{On the complexity of scrypt and proofs of space in the parallel random oracle model}},
doi = {10.1007/978-3-662-49896-5_13},
volume = {9666},
year = {2016},
}
@inproceedings{1233,
abstract = {About three decades ago it was realized that implementing private channels between parties which can be adaptively corrupted requires an encryption scheme that is secure against selective opening attacks. Whether standard (IND-CPA) security implies security against selective opening attacks has been a major open question since. The only known reduction from selective opening to IND-CPA security loses an exponential factor. A polynomial reduction is only known for the very special case where the distribution considered in the selective opening security experiment is a product distribution, i.e., the messages are sampled independently from each other. In this paper we give a reduction whose loss is quantified via the dependence graph (where message dependencies correspond to edges) of the underlying message distribution. In particular, for some concrete distributions including Markov distributions, our reduction is polynomial.},
author = {Fuchsbauer, Georg and Heuer, Felix and Kiltz, Eike and Pietrzak, Krzysztof Z},
location = {Tel Aviv, Israel},
pages = {282 -- 305},
publisher = {Springer},
title = {{Standard security does imply security against selective opening for markov distributions}},
doi = {10.1007/978-3-662-49096-9_12},
volume = {9562},
year = {2016},
}
@inproceedings{1235,
abstract = {A constrained pseudorandom function (CPRF) F: K×X → Y for a family T of subsets of χ is a function where for any key k ∈ K and set S ∈ T one can efficiently compute a short constrained key kS, which allows to evaluate F(k, ·) on all inputs x ∈ S, while the outputs on all inputs x /∈ S look random even given kS. Abusalah et al. recently constructed the first constrained PRF for inputs of arbitrary length whose sets S are decided by Turing machines. They use their CPRF to build broadcast encryption and the first ID-based non-interactive key exchange for an unbounded number of users. Their constrained keys are obfuscated circuits and are therefore large. In this work we drastically reduce the key size and define a constrained key for a Turing machine M as a short signature on M. For this, we introduce a new signature primitive with constrained signing keys that let one only sign certain messages, while forging a signature on others is hard even when knowing the coins for key generation.},
author = {Abusalah, Hamza M and Fuchsbauer, Georg},
location = {Guildford, UK},
pages = {445 -- 463},
publisher = {Springer},
title = {{Constrained PRFs for unbounded inputs with short keys}},
doi = {10.1007/978-3-319-39555-5_24},
volume = {9696},
year = {2016},
}
@inproceedings{1236,
abstract = {A constrained pseudorandom function F: K × X → Y for a family T ⊆ 2X of subsets of X is a function where for any key k ∈ K and set S ∈ T one can efficiently compute a constrained key kS which allows to evaluate F (k, ·) on all inputs x ∈ S, while even given this key, the outputs on all inputs x ∉ S look random. At Asiacrypt’13 Boneh and Waters gave a construction which supports the most general set family so far. Its keys kc are defined for sets decided by boolean circuits C and enable evaluation of the PRF on any x ∈ X where C(x) = 1. In their construction the PRF input length and the size of the circuits C for which constrained keys can be computed must be fixed beforehand during key generation. We construct a constrained PRF that has an unbounded input length and whose constrained keys can be defined for any set recognized by a Turing machine. The only a priori bound we make is on the description size of the machines. We prove our construction secure assuming publiccoin differing-input obfuscation. As applications of our constrained PRF we build a broadcast encryption scheme where the number of potential receivers need not be fixed at setup (in particular, the length of the keys is independent of the number of parties) and the first identity-based non-interactive key exchange protocol with no bound on the number of parties that can agree on a shared key.},
author = {Abusalah, Hamza M and Fuchsbauer, Georg and Pietrzak, Krzysztof Z},
location = {San Francisco, CA, USA},
pages = {413 -- 428},
publisher = {Springer},
title = {{Constrained PRFs for unbounded inputs}},
doi = {10.1007/978-3-319-29485-8_24},
volume = {9610},
year = {2016},
}
@inproceedings{1365,
abstract = {A memory-hard function (MHF) f is equipped with a space cost σ and time cost τ parameter such that repeatedly computing fσ,τ on an application specific integrated circuit (ASIC) is not economically advantageous relative to a general purpose computer. Technically we would like that any (generalized) circuit for evaluating an iMHF fσ,τ has area × time (AT) complexity at Θ(σ2 ∗ τ). A data-independent MHF (iMHF) has the added property that it can be computed with almost optimal memory and time complexity by an algorithm which accesses memory in a pattern independent of the input value. Such functions can be specified by fixing a directed acyclic graph (DAG) G on n = Θ(σ ∗ τ) nodes representing its computation graph. In this work we develop new tools for analyzing iMHFs. First we define and motivate a new complexity measure capturing the amount of energy (i.e. electricity) required to compute a function. We argue that, in practice, this measure is at least as important as the more traditional AT-complexity. Next we describe an algorithm A for repeatedly evaluating an iMHF based on an arbitrary DAG G. We upperbound both its energy and AT complexities per instance evaluated in terms of a certain combinatorial property of G. Next we instantiate our attack for several general classes of DAGs which include those underlying many of the most important iMHF candidates in the literature. In particular, we obtain the following results which hold for all choices of parameters σ and τ (and thread-count) such that n = σ ∗ τ. -The Catena-Dragonfly function of [FLW13] has AT and energy complexities O(n1.67). -The Catena-Butterfly function of [FLW13] has complexities is O(n1.67). -The Double-Buffer and the Linear functions of [CGBS16] both have complexities in O(n1.67). -The Argon2i function of [BDK15] (winner of the Password Hashing Competition [PHC]) has complexities O(n7/4 log(n)). -The Single-Buffer function of [CGBS16] has complexities O(n7/4 log(n)). -Any iMHF can be computed by an algorithm with complexities O(n2/ log1 −ε(n)) for all ε > 0. In particular when τ = 1 this shows that the goal of constructing an iMHF with AT-complexity Θ(σ2 ∗ τ ) is unachievable. Along the way we prove a lemma upper-bounding the depth-robustness of any DAG which may prove to be of independent interest.},
author = {Alwen, Joel F and Blocki, Jeremiah},
location = {Santa Barbara, CA, USA},
pages = {241 -- 271},
publisher = {Springer},
title = {{Efficiently computing data-independent memory-hard functions}},
doi = {10.1007/978-3-662-53008-5_9},
volume = {9815},
year = {2016},
}
@inproceedings{1366,
abstract = {We study the problem of devising provably secure PRNGs with input based on the sponge paradigm. Such constructions are very appealing, as efficient software/hardware implementations of SHA-3 can easily be translated into a PRNG in a nearly black-box way. The only existing sponge-based construction, proposed by Bertoni et al. (CHES 2010), fails to achieve the security notion of robustness recently considered by Dodis et al. (CCS 2013), for two reasons: (1) The construction is deterministic, and thus there are high-entropy input distributions on which the construction fails to extract random bits, and (2) The construction is not forward secure, and presented solutions aiming at restoring forward security have not been rigorously analyzed. We propose a seeded variant of Bertoni et al.’s PRNG with input which we prove secure in the sense of robustness, delivering in particular concrete security bounds. On the way, we make what we believe to be an important conceptual contribution, developing a variant of the security framework of Dodis et al. tailored at the ideal permutation model that captures PRNG security in settings where the weakly random inputs are provided from a large class of possible adversarial samplers which are also allowed to query the random permutation. As a further application of our techniques, we also present an efficient sponge-based key-derivation function (which can be instantiated from SHA-3 in a black-box fashion), which we also prove secure when fed with samples from permutation-dependent distributions.},
author = {Gazi, Peter and Tessaro, Stefano},
location = {Vienna, Austria},
pages = {87 -- 116},
publisher = {Springer},
title = {{Provably robust sponge-based PRNGs and KDFs}},
doi = {10.1007/978-3-662-49890-3_4},
volume = {9665},
year = {2016},
}
@article{1479,
abstract = {Most entropy notions H(.) like Shannon or min-entropy satisfy a chain rule stating that for random variables X,Z, and A we have H(X|Z,A)≥H(X|Z)−|A|. That is, by conditioning on A the entropy of X can decrease by at most the bitlength |A| of A. Such chain rules are known to hold for some computational entropy notions like Yao’s and unpredictability-entropy. For HILL entropy, the computational analogue of min-entropy, the chain rule is of special interest and has found many applications, including leakage-resilient cryptography, deterministic encryption, and memory delegation. These applications rely on restricted special cases of the chain rule. Whether the chain rule for conditional HILL entropy holds in general was an open problem for which we give a strong negative answer: we construct joint distributions (X,Z,A), where A is a distribution over a single bit, such that the HILL entropy H HILL (X|Z) is large but H HILL (X|Z,A) is basically zero.
Our counterexample just makes the minimal assumption that NP⊈P/poly. Under the stronger assumption that injective one-way function exist, we can make all the distributions efficiently samplable.
Finally, we show that some more sophisticated cryptographic objects like lossy functions can be used to sample a distribution constituting a counterexample to the chain rule making only a single invocation to the underlying object.},
author = {Krenn, Stephan and Pietrzak, Krzysztof Z and Wadia, Akshay and Wichs, Daniel},
journal = {Computational Complexity},
number = {3},
pages = {567 -- 605},
publisher = {Springer},
title = {{A counterexample to the chain rule for conditional HILL entropy}},
doi = {10.1007/s00037-015-0120-9},
volume = {25},
year = {2016},
}
@article{1177,
abstract = {Boldyreva, Palacio and Warinschi introduced a multiple forking game as an extension of general forking. The notion of (multiple) forking is a useful abstraction from the actual simulation of cryptographic scheme to the adversary in a security reduction, and is achieved through the intermediary of a so-called wrapper algorithm. Multiple forking has turned out to be a useful tool in the security argument of several cryptographic protocols. However, a reduction employing multiple forking incurs a significant degradation of (Formula presented.) , where (Formula presented.) denotes the upper bound on the underlying random oracle calls and (Formula presented.) , the number of forkings. In this work we take a closer look at the reasons for the degradation with a tighter security bound in mind. We nail down the exact set of conditions for success in the multiple forking game. A careful analysis of the cryptographic schemes and corresponding security reduction employing multiple forking leads to the formulation of ‘dependence’ and ‘independence’ conditions pertaining to the output of the wrapper in different rounds. Based on the (in)dependence conditions we propose a general framework of multiple forking and a General Multiple Forking Lemma. Leveraging (in)dependence to the full allows us to improve the degradation factor in the multiple forking game by a factor of (Formula presented.). By implication, the cost of a single forking involving two random oracles (augmented forking) matches that involving a single random oracle (elementary forking). Finally, we study the effect of these observations on the concrete security of existing schemes employing multiple forking. We conclude that by careful design of the protocol (and the wrapper in the security reduction) it is possible to harness our observations to the full extent.},
author = {Kamath Hosdurg, Chethan and Chatterjee, Sanjit},
journal = {Algorithmica},
number = {4},
pages = {1321 -- 1362},
publisher = {Springer},
title = {{A closer look at multiple-forking: Leveraging (in)dependence for a tighter bound}},
doi = {10.1007/s00453-015-9997-6},
volume = {74},
year = {2016},
}
@inproceedings{1179,
abstract = {Computational notions of entropy have recently found many applications, including leakage-resilient cryptography, deterministic encryption or memory delegation. The two main types of results which make computational notions so useful are (1) Chain rules, which quantify by how much the computational entropy of a variable decreases if conditioned on some other variable (2) Transformations, which quantify to which extend one type of entropy implies another.
Such chain rules and transformations typically lose a significant amount in quality of the entropy, and are the reason why applying these results one gets rather weak quantitative security bounds. In this paper we for the first time prove lower bounds in this context, showing that existing results for transformations are, unfortunately, basically optimal for non-adaptive black-box reductions (and it’s hard to imagine how non black-box reductions or adaptivity could be useful here.)
A variable X has k bits of HILL entropy of quality (ϵ,s)
if there exists a variable Y with k bits min-entropy which cannot be distinguished from X with advantage ϵ
by distinguishing circuits of size s. A weaker notion is Metric entropy, where we switch quantifiers, and only require that for every distinguisher of size s, such a Y exists.
We first describe our result concerning transformations. By definition, HILL implies Metric without any loss in quality. Metric entropy often comes up in applications, but must be transformed to HILL for meaningful security guarantees. The best known result states that if a variable X has k bits of Metric entropy of quality (ϵ,s)
, then it has k bits of HILL with quality (2ϵ,s⋅ϵ2). We show that this loss of a factor Ω(ϵ−2)
in circuit size is necessary. In fact, we show the stronger result that this loss is already necessary when transforming so called deterministic real valued Metric entropy to randomised boolean Metric (both these variants of Metric entropy are implied by HILL without loss in quality).
The chain rule for HILL entropy states that if X has k bits of HILL entropy of quality (ϵ,s)
, then for any variable Z of length m, X conditioned on Z has k−m bits of HILL entropy with quality (ϵ,s⋅ϵ2/2m). We show that a loss of Ω(2m/ϵ) in circuit size necessary here. Note that this still leaves a gap of ϵ between the known bound and our lower bound.},
author = {Pietrzak, Krzysztof Z and Maciej, Skorski},
location = {Beijing, China},
pages = {183 -- 203},
publisher = {Springer},
title = {{Pseudoentropy: Lower-bounds for chain rules and transformations}},
doi = {10.1007/978-3-662-53641-4_8},
volume = {9985},
year = {2016},
}
@inproceedings{1644,
abstract = {Increasing the computational complexity of evaluating a hash function, both for the honest users as well as for an adversary, is a useful technique employed for example in password-based cryptographic schemes to impede brute-force attacks, and also in so-called proofs of work (used in protocols like Bitcoin) to show that a certain amount of computation was performed by a legitimate user. A natural approach to adjust the complexity of a hash function is to iterate it c times, for some parameter c, in the hope that any query to the scheme requires c evaluations of the underlying hash function. However, results by Dodis et al. (Crypto 2012) imply that plain iteration falls short of achieving this goal, and designing schemes which provably have such a desirable property remained an open problem. This paper formalizes explicitly what it means for a given scheme to amplify the query complexity of a hash function. In the random oracle model, the goal of a secure query-complexity amplifier (QCA) scheme is captured as transforming, in the sense of indifferentiability, a random oracle allowing R queries (for the adversary) into one provably allowing only r < R queries. Turned around, this means that making r queries to the scheme requires at least R queries to the actual random oracle. Second, a new scheme, called collision-free iteration, is proposed and proven to achieve c-fold QCA for both the honest parties and the adversary, for any fixed parameter c.},
author = {Demay, Grégory and Gazi, Peter and Maurer, Ueli and Tackmann, Björn},
location = {Lugano, Switzerland},
pages = {159 -- 180},
publisher = {Springer},
title = {{Query-complexity amplification for random oracles}},
doi = {10.1007/978-3-319-17470-9_10},
volume = {9063},
year = {2015},
}
@inproceedings{1645,
abstract = {Secret-key constructions are often proved secure in a model where one or more underlying components are replaced by an idealized oracle accessible to the attacker. This model gives rise to information-theoretic security analyses, and several advances have been made in this area over the last few years. This paper provides a systematic overview of what is achievable in this model, and how existing works fit into this view.},
author = {Gazi, Peter and Tessaro, Stefano},
booktitle = {2015 IEEE Information Theory Workshop},
location = {Jerusalem, Israel},
publisher = {IEEE},
title = {{Secret-key cryptography from ideal primitives: A systematic verview}},
doi = {10.1109/ITW.2015.7133163},
year = {2015},
}
@inproceedings{1646,
abstract = {A pseudorandom function (PRF) is a keyed function F : K × X → Y where, for a random key k ∈ K, the function F(k, ·) is indistinguishable from a uniformly random function, given black-box access. A key-homomorphic PRF has the additional feature that for any keys k, k' and any input x, we have F(k+k', x) = F(k, x)⊕F(k', x) for some group operations +,⊕ on K and Y, respectively. A constrained PRF for a family of setsS ⊆ P(X) has the property that, given any key k and set S ∈ S, one can efficiently compute a “constrained” key kS that enables evaluation of F(k, x) on all inputs x ∈ S, while the values F(k, x) for x /∈ S remain pseudorandom even given kS. In this paper we construct PRFs that are simultaneously constrained and key homomorphic, where the homomorphic property holds even for constrained keys. We first show that the multilinear map-based bit-fixing and circuit-constrained PRFs of Boneh and Waters (Asiacrypt 2013) can be modified to also be keyhomomorphic. We then show that the LWE-based key-homomorphic PRFs of Banerjee and Peikert (Crypto 2014) are essentially already prefix-constrained PRFs, using a (non-obvious) definition of constrained keys and associated group operation. Moreover, the constrained keys themselves are pseudorandom, and the constraining and evaluation functions can all be computed in low depth. As an application of key-homomorphic constrained PRFs,we construct a proxy re-encryption schemewith fine-grained access control. This scheme allows storing encrypted data on an untrusted server, where each file can be encrypted relative to some attributes, so that only parties whose constrained keys match the attributes can decrypt. Moreover, the server can re-key (arbitrary subsets of) the ciphertexts without learning anything about the plaintexts, thus permitting efficient and finegrained revocation.},
author = {Banerjee, Abishek and Fuchsbauer, Georg and Peikert, Chris and Pietrzak, Krzysztof Z and Stevens, Sophie},
location = {Warsaw, Poland},
pages = {31 -- 60},
publisher = {Springer},
title = {{Key-homomorphic constrained pseudorandom functions}},
doi = {10.1007/978-3-662-46497-7_2},
volume = {9015},
year = {2015},
}
@inproceedings{1647,
abstract = {Round-optimal blind signatures are notoriously hard to construct in the standard model, especially in the malicious-signer model, where blindness must hold under adversarially chosen keys. This is substantiated by several impossibility results. The only construction that can be termed theoretically efficient, by Garg and Gupta (Eurocrypt’14), requires complexity leveraging, inducing an exponential security loss. We present a construction of practically efficient round-optimal blind signatures in the standard model. It is conceptually simple and builds on the recent structure-preserving signatures on equivalence classes (SPSEQ) from Asiacrypt’14. While the traditional notion of blindness follows from standard assumptions, we prove blindness under adversarially chosen keys under an interactive variant of DDH. However, we neither require non-uniform assumptions nor complexity leveraging. We then show how to extend our construction to partially blind signatures and to blind signatures on message vectors, which yield a construction of one-show anonymous credentials à la “anonymous credentials light” (CCS’13) in the standard model. Furthermore, we give the first SPS-EQ construction under noninteractive assumptions and show how SPS-EQ schemes imply conventional structure-preserving signatures, which allows us to apply optimality results for the latter to SPS-EQ.},
author = {Fuchsbauer, Georg and Hanser, Christian and Slamanig, Daniel},
location = {Santa Barbara, CA, United States},
pages = {233 -- 253},
publisher = {Springer},
title = {{Practical round-optimal blind signatures in the standard model}},
doi = {10.1007/978-3-662-48000-7_12},
volume = {9216},
year = {2015},
}
@inproceedings{1648,
abstract = {Generalized Selective Decryption (GSD), introduced by Panjwani [TCC’07], is a game for a symmetric encryption scheme Enc that captures the difficulty of proving adaptive security of certain protocols, most notably the Logical Key Hierarchy (LKH) multicast encryption protocol. In the GSD game there are n keys k1,..., kn, which the adversary may adaptively corrupt (learn); moreover, it can ask for encryptions Encki (kj) of keys under other keys. The adversary’s task is to distinguish keys (which it cannot trivially compute) from random. Proving the hardness of GSD assuming only IND-CPA security of Enc is surprisingly hard. Using “complexity leveraging” loses a factor exponential in n, which makes the proof practically meaningless. We can think of the GSD game as building a graph on n vertices, where we add an edge i → j when the adversary asks for an encryption of kj under ki. If restricted to graphs of depth ℓ, Panjwani gave a reduction that loses only a factor exponential in ℓ (not n). To date, this is the only non-trivial result known for GSD. In this paper we give almost-polynomial reductions for large classes of graphs. Most importantly, we prove the security of the GSD game restricted to trees losing only a quasi-polynomial factor n3 log n+5. Trees are an important special case capturing real-world protocols like the LKH protocol. Our new bound improves upon Panjwani’s on some LKH variants proposed in the literature where the underlying tree is not balanced. Our proof builds on ideas from the “nested hybrids” technique recently introduced by Fuchsbauer et al. [Asiacrypt’14] for proving the adaptive security of constrained PRFs.},
author = {Fuchsbauer, Georg and Jafargholi, Zahra and Pietrzak, Krzysztof Z},
location = {Santa Barbara, CA, USA},
pages = {601 -- 620},
publisher = {Springer},
title = {{A quasipolynomial reduction for generalized selective decryption on trees}},
doi = {10.1007/978-3-662-47989-6_29},
volume = {9215},
year = {2015},
}
@inproceedings{1649,
abstract = {We extend a commitment scheme based on the learning with errors over rings (RLWE) problem, and present efficient companion zeroknowledge proofs of knowledge. Our scheme maps elements from the ring (or equivalently, n elements from },
author = {Benhamouda, Fabrice and Krenn, Stephan and Lyubashevsky, Vadim and Pietrzak, Krzysztof Z},
location = {Vienna, Austria},
pages = {305 -- 325},
publisher = {Springer},
title = {{Efficient zero-knowledge proofs for commitments from learning with errors over rings}},
doi = {10.1007/978-3-319-24174-6_16},
volume = {9326},
year = {2015},
}
@inproceedings{1650,
abstract = {We consider the task of deriving a key with high HILL entropy (i.e., being computationally indistinguishable from a key with high min-entropy) from an unpredictable source.
Previous to this work, the only known way to transform unpredictability into a key that was ϵ indistinguishable from having min-entropy was via pseudorandomness, for example by Goldreich-Levin (GL) hardcore bits. This approach has the inherent limitation that from a source with k bits of unpredictability entropy one can derive a key of length (and thus HILL entropy) at most k−2log(1/ϵ) bits. In many settings, e.g. when dealing with biometric data, such a 2log(1/ϵ) bit entropy loss in not an option. Our main technical contribution is a theorem that states that in the high entropy regime, unpredictability implies HILL entropy. Concretely, any variable K with |K|−d bits of unpredictability entropy has the same amount of so called metric entropy (against real-valued, deterministic distinguishers), which is known to imply the same amount of HILL entropy. The loss in circuit size in this argument is exponential in the entropy gap d, and thus this result only applies for small d (i.e., where the size of distinguishers considered is exponential in d).
To overcome the above restriction, we investigate if it’s possible to first “condense” unpredictability entropy and make the entropy gap small. We show that any source with k bits of unpredictability can be condensed into a source of length k with k−3 bits of unpredictability entropy. Our condenser simply “abuses" the GL construction and derives a k bit key from a source with k bits of unpredicatibily. The original GL theorem implies nothing when extracting that many bits, but we show that in this regime, GL still behaves like a “condenser" for unpredictability. This result comes with two caveats (1) the loss in circuit size is exponential in k and (2) we require that the source we start with has no HILL entropy (equivalently, one can efficiently check if a guess is correct). We leave it as an intriguing open problem to overcome these restrictions or to prove they’re inherent.},
author = {Skórski, Maciej and Golovnev, Alexander and Pietrzak, Krzysztof Z},
location = {Kyoto, Japan},
pages = {1046 -- 1057},
publisher = {Springer},
title = {{Condensed unpredictability }},
doi = {10.1007/978-3-662-47672-7_85},
volume = {9134},
year = {2015},
}
@inproceedings{1651,
abstract = {Cryptographic e-cash allows off-line electronic transactions between a bank, users and merchants in a secure and anonymous fashion. A plethora of e-cash constructions has been proposed in the literature; however, these traditional e-cash schemes only allow coins to be transferred once between users and merchants. Ideally, we would like users to be able to transfer coins between each other multiple times before deposit, as happens with physical cash. “Transferable” e-cash schemes are the solution to this problem. Unfortunately, the currently proposed schemes are either completely impractical or do not achieve the desirable anonymity properties without compromises, such as assuming the existence of a trusted “judge” who can trace all coins and users in the system. This paper presents the first efficient and fully anonymous transferable e-cash scheme without any trusted third parties. We start by revising the security and anonymity properties of transferable e-cash to capture issues that were previously overlooked. For our construction we use the recently proposed malleable signatures by Chase et al. to allow the secure and anonymous transfer of coins, combined with a new efficient double-spending detection mechanism. Finally, we discuss an instantiation of our construction.},
author = {Baldimtsi, Foteini and Chase, Melissa and Fuchsbauer, Georg and Kohlweiss, Markulf},
location = {Gaithersburg, MD, USA},
pages = {101 -- 124},
publisher = {Springer},
title = {{Anonymous transferable e-cash}},
doi = {10.1007/978-3-662-46447-2_5},
volume = {9020},
year = {2015},
}
@inproceedings{1652,
abstract = {We develop new theoretical tools for proving lower-bounds on the (amortized) complexity of certain functions in models of parallel computation. We apply the tools to construct a class of functions with high amortized memory complexity in the parallel Random Oracle Model (pROM); a variant of the standard ROM allowing for batches of simultaneous queries. In particular we obtain a new, more robust, type of Memory-Hard Functions (MHF); a security primitive which has recently been gaining acceptance in practice as an effective means of countering brute-force attacks on security relevant functions. Along the way we also demonstrate an important shortcoming of previous definitions of MHFs and give a new definition addressing the problem. The tools we develop represent an adaptation of the powerful pebbling paradigm (initially introduced by Hewitt and Paterson [HP70] and Cook [Coo73]) to a simple and intuitive parallel setting. We define a simple pebbling game Gp over graphs which aims to abstract parallel computation in an intuitive way. As a conceptual contribution we define a measure of pebbling complexity for graphs called cumulative complexity (CC) and show how it overcomes a crucial shortcoming (in the parallel setting) exhibited by more traditional complexity measures used in the past. As a main technical contribution we give an explicit construction of a constant in-degree family of graphs whose CC in Gp approaches maximality to within a polylogarithmic factor for any graph of equal size (analogous to the graphs of Tarjan et. al. [PTC76, LT82] for sequential pebbling games). Finally, for a given graph G and related function fG, we derive a lower-bound on the amortized memory complexity of fG in the pROM in terms of the CC of G in the game Gp.},
author = {Alwen, Joel F and Serbinenko, Vladimir},
booktitle = {Proceedings of the 47th annual ACM symposium on Theory of computing},
location = {Portland, OR, United States},
pages = {595 -- 603},
publisher = {ACM},
title = {{High parallel complexity graphs and memory-hard functions}},
doi = {10.1145/2746539.2746622},
year = {2015},
}
@inproceedings{1654,
abstract = {HMAC and its variant NMAC are the most popular approaches to deriving a MAC (and more generally, a PRF) from a cryptographic hash function. Despite nearly two decades of research, their exact security still remains far from understood in many different contexts. Indeed, recent works have re-surfaced interest for {\em generic} attacks, i.e., attacks that treat the compression function of the underlying hash function as a black box.
Generic security can be proved in a model where the underlying compression function is modeled as a random function -- yet, to date, the question of proving tight, non-trivial bounds on the generic security of HMAC/NMAC even as a PRF remains a challenging open question.
In this paper, we ask the question of whether a small modification to HMAC and NMAC can allow us to exactly characterize the security of the resulting constructions, while only incurring little penalty with respect to efficiency. To this end, we present simple variants of NMAC and HMAC, for which we prove tight bounds on the generic PRF security, expressed in terms of numbers of construction and compression function queries necessary to break the construction. All of our constructions are obtained via a (near) {\em black-box} modification of NMAC and HMAC, which can be interpreted as an initial step of key-dependent message pre-processing.
While our focus is on PRF security, a further attractive feature of our new constructions is that they clearly defeat all recent generic attacks against properties such as state recovery and universal forgery. These exploit properties of the so-called ``functional graph'' which are not directly accessible in our new constructions. },
author = {Gazi, Peter and Pietrzak, Krzysztof Z and Tessaro, Stefano},
location = {Auckland, New Zealand},
pages = {85 -- 109},
publisher = {Springer},
title = {{Generic security of NMAC and HMAC with input whitening}},
doi = {10.1007/978-3-662-48800-3_4},
volume = {9453},
year = {2015},
}
@inproceedings{1668,
abstract = {We revisit the security (as a pseudorandom permutation) of cascading-based constructions for block-cipher key-length extension. Previous works typically considered the extreme case where the adversary is given the entire codebook of the construction, the only complexity measure being the number qe of queries to the underlying ideal block cipher, representing adversary’s secret-key-independent computation. Here, we initiate a systematic study of the more natural case of an adversary restricted to adaptively learning a number qc of plaintext/ciphertext pairs that is less than the entire codebook. For any such qc, we aim to determine the highest number of block-cipher queries qe the adversary can issue without being able to successfully distinguish the construction (under a secret key) from a random permutation.
More concretely, we show the following results for key-length extension schemes using a block cipher with n-bit blocks and κ-bit keys:
Plain cascades of length ℓ=2r+1 are secure whenever qcqre≪2r(κ+n), qc≪2κ and qe≪22κ. The bound for r=1 also applies to two-key triple encryption (as used within Triple DES).
The r-round XOR-cascade is secure as long as qcqre≪2r(κ+n), matching an attack by Gaži (CRYPTO 2013).
We fully characterize the security of Gaži and Tessaro’s two-call },
author = {Gazi, Peter and Lee, Jooyoung and Seurin, Yannick and Steinberger, John and Tessaro, Stefano},
location = {Istanbul, Turkey},
pages = {319 -- 341},
publisher = {Springer},
title = {{Relaxing full-codebook security: A refined analysis of key-length extension schemes}},
doi = {10.1007/978-3-662-48116-5_16},
volume = {9054},
year = {2015},
}
@inproceedings{1669,
abstract = {Computational notions of entropy (a.k.a. pseudoentropy) have found many applications, including leakage-resilient cryptography, deterministic encryption or memory delegation. The most important tools to argue about pseudoentropy are chain rules, which quantify by how much (in terms of quantity and quality) the pseudoentropy of a given random variable X decreases when conditioned on some other variable Z (think for example of X as a secret key and Z as information leaked by a side-channel). In this paper we give a very simple and modular proof of the chain rule for HILL pseudoentropy, improving best known parameters. Our version allows for increasing the acceptable length of leakage in applications up to a constant factor compared to the best previous bounds. As a contribution of independent interest, we provide a comprehensive study of all known versions of the chain rule, comparing their worst-case strength and limitations.},
author = {Pietrzak, Krzysztof Z and Skórski, Maciej},
location = {Guadalajara, Mexico},
pages = {81 -- 98},
publisher = {Springer},
title = {{The chain rule for HILL pseudoentropy, revisited}},
doi = {10.1007/978-3-319-22174-8_5},
volume = {9230},
year = {2015},
}
@inproceedings{1671,
abstract = {This paper studies the concrete security of PRFs and MACs obtained by keying hash functions based on the sponge paradigm. One such hash function is KECCAK, selected as NIST’s new SHA-3 standard. In contrast to other approaches like HMAC, the exact security of keyed sponges is not well understood. Indeed, recent security analyses delivered concrete security bounds which are far from existing attacks. This paper aims to close this gap. We prove (nearly) exact bounds on the concrete PRF security of keyed sponges using a random permutation. These bounds are tight for the most relevant ranges of parameters, i.e., for messages of length (roughly) l ≤ min{2n/4, 2r} blocks, where n is the state size and r is the desired output length; and for l ≤ q queries (to the construction or the underlying permutation). Moreover, we also improve standard-model bounds. As an intermediate step of independent interest, we prove tight bounds on the PRF security of the truncated CBC-MAC construction, which operates as plain CBC-MAC, but only returns a prefix of the output.},
author = {Gazi, Peter and Pietrzak, Krzysztof Z and Tessaro, Stefano},
location = {Santa Barbara, CA, United States},
pages = {368 -- 387},
publisher = {Springer},
title = {{The exact PRF security of truncation: Tight bounds for keyed sponges and truncated CBC}},
doi = {10.1007/978-3-662-47989-6_18},
volume = {9215},
year = {2015},
}
@inproceedings{1672,
abstract = {Composable notions of incoercibility aim to forbid a coercer from using anything beyond the coerced parties’ inputs and outputs to catch them when they try to deceive him. Existing definitions are restricted to weak coercion types, and/or are not universally composable. Furthermore, they often make too strong assumptions on the knowledge of coerced parties—e.g., they assume they known the identities and/or the strategies of other coerced parties, or those of corrupted parties— which makes them unsuitable for applications of incoercibility such as e-voting, where colluding adversarial parties may attempt to coerce honest voters, e.g., by offering them money for a promised vote, and use their own view to check that the voter keeps his end of the bargain. In this work we put forward the first universally composable notion of incoercible multi-party computation, which satisfies the above intuition and does not assume collusions among coerced parties or knowledge of the corrupted set. We define natural notions of UC incoercibility corresponding to standard coercion-types, i.e., receipt-freeness and resistance to full-active coercion. Importantly, our suggested notion has the unique property that it builds on top of the well studied UC framework by Canetti instead of modifying it. This guarantees backwards compatibility, and allows us to inherit results from the rich UC literature. We then present MPC protocols which realize our notions of UC incoercibility given access to an arguably minimal setup—namely honestly generate tamper-proof hardware performing a very simple cryptographic operation—e.g., a smart card. This is, to our knowledge, the first proposed construction of an MPC protocol (for more than two parties) that is incoercibly secure and universally composable, and therefore the first construction of a universally composable receipt-free e-voting protocol.},
author = {Alwen, Joel F and Ostrovsky, Rafail and Zhou, Hongsheng and Zikas, Vassilis},
location = {Santa Barbara, CA, United States},
pages = {763 -- 780},
publisher = {Springer},
title = {{Incoercible multi-party computation and universally composable receipt-free voting}},
doi = {10.1007/978-3-662-48000-7_37},
volume = {9216},
year = {2015},
}
@inproceedings{1675,
abstract = {Proofs of work (PoW) have been suggested by Dwork and Naor (Crypto’92) as protection to a shared resource. The basic idea is to ask the service requestor to dedicate some non-trivial amount of computational work to every request. The original applications included prevention of spam and protection against denial of service attacks. More recently, PoWs have been used to prevent double spending in the Bitcoin digital currency system. In this work, we put forward an alternative concept for PoWs - so-called proofs of space (PoS), where a service requestor must dedicate a significant amount of disk space as opposed to computation. We construct secure PoS schemes in the random oracle model (with one additional mild assumption required for the proof to go through), using graphs with high “pebbling complexity” and Merkle hash-trees. We discuss some applications, including follow-up work where a decentralized digital currency scheme called Spacecoin is constructed that uses PoS (instead of wasteful PoW like in Bitcoin) to prevent double spending. The main technical contribution of this work is the construction of (directed, loop-free) graphs on N vertices with in-degree O(log logN) such that even if one places Θ(N) pebbles on the nodes of the graph, there’s a constant fraction of nodes that needs Θ(N) steps to be pebbled (where in every step one can put a pebble on a node if all its parents have a pebble).},
author = {Dziembowski, Stefan and Faust, Sebastian and Kolmogorov, Vladimir and Pietrzak, Krzysztof Z},
location = {Santa Barbara, CA, United States},
pages = {585 -- 605},
publisher = {Springer},
title = {{Proofs of space}},
doi = {10.1007/978-3-662-48000-7_29},
volume = {9216},
year = {2015},
}
@inproceedings{1474,
abstract = {Cryptographic access control offers selective access to encrypted data via a combination of key management and functionality-rich cryptographic schemes, such as attribute-based encryption. Using this approach, publicly available meta-data may inadvertently leak information on the access policy that is enforced by cryptography, which renders cryptographic access control unusable in settings where this information is highly sensitive. We begin to address this problem by presenting rigorous definitions for policy privacy in cryptographic access control. For concreteness we set our results in the model of Role-Based Access Control (RBAC), where we identify and formalize several different flavors of privacy, however, our framework should serve as inspiration for other models of access control. Based on our insights we propose a new system which significantly improves on the privacy properties of state-of-the-art constructions. Our design is based on a novel type of privacy-preserving attribute-based encryption, which we introduce and show how to instantiate. We present our results in the context of a cryptographic RBAC system by Ferrara et al. (CSF'13), which uses cryptography to control read access to files, while write access is still delegated to trusted monitors. We give an extension of the construction that permits cryptographic control over write access. Our construction assumes that key management uses out-of-band channels between the policy enforcer and the users but eliminates completely the need for monitoring read/write access to the data.},
author = {Ferrara, Anna and Fuchsbauer, Georg and Liu, Bin and Warinschi, Bogdan},
location = {Verona, Italy},
pages = {46--60},
publisher = {IEEE},
title = {{Policy privacy in cryptographic access control}},
doi = {10.1109/CSF.2015.11},
year = {2015},
}
@inproceedings{2045,
abstract = {We introduce and study a new notion of enhanced chosen-ciphertext security (ECCA) for public-key encryption. Loosely speaking, in the ECCA security experiment, the decryption oracle provided to the adversary is augmented to return not only the output of the decryption algorithm on a queried ciphertext but also of a randomness-recovery algorithm associated to the scheme. Our results mainly concern the case where the randomness-recovery algorithm is efficient. We provide constructions of ECCA-secure encryption from adaptive trapdoor functions as defined by Kiltz et al. (EUROCRYPT 2010), resulting in ECCA encryption from standard number-theoretic assumptions. We then give two applications of ECCA-secure encryption: (1) We use it as a unifying concept in showing equivalence of adaptive trapdoor functions and tag-based adaptive trapdoor functions, resolving an open question of Kiltz et al. (2) We show that ECCA-secure encryption can be used to securely realize an approach to public-key encryption with non-interactive opening (PKENO) originally suggested by Damgård and Thorbek (EUROCRYPT 2007), resulting in new and practical PKENO schemes quite different from those in prior work. Our results demonstrate that ECCA security is of both practical and theoretical interest.},
author = {Dachman Soled, Dana and Fuchsbauer, Georg and Mohassel, Payman and O’Neill, Adam},
booktitle = {Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)},
editor = {Krawczyk, Hugo},
location = {Buenos Aires, Argentina},
pages = {329 -- 344},
publisher = {Springer},
title = {{Enhanced chosen-ciphertext security and applications}},
doi = {10.1007/978-3-642-54631-0_19},
volume = {8383},
year = {2014},
}
@inproceedings{2046,
abstract = {We introduce policy-based signatures (PBS), where a signer can only sign messages conforming to some authority-specified policy. The main requirements are unforgeability and privacy, the latter meaning that signatures not reveal the policy. PBS offers value along two fronts: (1) On the practical side, they allow a corporation to control what messages its employees can sign under the corporate key. (2) On the theoretical side, they unify existing work, capturing other forms of signatures as special cases or allowing them to be easily built. Our work focuses on definitions of PBS, proofs that this challenging primitive is realizable for arbitrary policies, efficient constructions for specific policies, and a few representative applications.},
author = {Bellare, Mihir and Fuchsbauer, Georg},
booktitle = {Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)},
editor = {Krawczyk, Hugo},
location = {Buenos Aires, Argentina},
pages = {520 -- 537},
publisher = {Springer},
title = {{Policy-based signatures}},
doi = {10.1007/978-3-642-54631-0_30},
volume = {8383},
year = {2014},
}