International Association for Cryptologic Research

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2015-06-21
18:17 [Pub][ePrint]

Random numbers are essential for cryptography. In most real-world systems, these values come from a cryptographic pseudorandom number generator (PRNG), which in turn is seeded by an entropy source. The security of the entire cryptographic system then relies on the accuracy of the claimed amount of entropy provided by the source. If the entropy source provides less unpredictability than is expected, the security of the cryptographic mechanisms is undermined. For this reason, correctly estimating the amount of entropy available from a source is critical.

In this paper, we develop a set of tools for estimating entropy, based on mechanisms that attempt to predict the next sample in a sequence based on all previous samples.

These mechanisms are called predictors. We develop a framework for using predictors to estimate entropy, and test them experimentally against both simulated and real noise sources. For comparison, we subject the entropy estimates defined in the August 2012 draft of NIST Special Publication 800-90B to the same tests, and compare their performance.

09:17 [Pub][ePrint]

It takes time for theoretical advances to get used in practical schemes. Anonymous credential schemes are no exception. For instance, existing schemes suited for real-world use lack formal, composable definitions, partly because they do not support straight-line extraction and rely on random oracles for their security arguments.

To address this gap, we propose unlinkable redactable signatures (URS), a new building block for privacy-enhancing protocols, which we use to construct the first efficient UC-secure anonymous credential system that supports multiple issuers, selective disclosure of attributes, and pseudonyms. Our scheme is one of the first such systems for which both the size of a credential and its presentation proof are independent of the number of attributes issued in a credential. Moreover, our new credential scheme does not rely on random oracles.

As an important intermediary step, we address the problem of building a functionality for a complex credential system that can cover many different features. Namely, we design a core building block for a single issuer that supports credential issuance and presentation with respect to pseudonyms and then show how to construct a full-fledged credential system with multiple issuers in a modular way. We expect this flexible definitional approach to be of independent interest.

09:17 [Pub][ePrint]

Universal Computational Extractors (UCEs), introduced by Bellare, Hoang and Keelveedhi (CRYPTO 2013), are a framework of assumptions on hash functions that allow to instantiate random oracles in a large variety of settings. Brzuska, Farshim and Mittelbach (CRYPTO 2014) showed that a large class of UCE assumptions with \\emph{computationally} unpredictable sources cannot be achieved, if indistinguishability obfuscation exists. In the process of circumventing obfuscation-based attacks, new UCE notions emerged, most notably UCEs with respect to \\emph{statistically} unpredictable sources that suffice for a large class of applications. However, the only standard model constructions of UCEs are for a small subclass considering only $q$-query sources which are \\emph{strongly statistically} unpredictable (Brzuska, Mittelbach; Asiacrypt 2014).

The contributions of this paper are threefold:

1) We show a surprising equivalence for the notions of strong unpredictability and (plain) unpredictability thereby lifting the construction from Brzuska and Mittelbach to achieve $q$-query UCEs for statistically unpredictable sources. This yields standard model instantiations for various ($q$-query) primitives including, deterministic public-key encryption, message-locked encryption, multi-bit point obfuscation, CCA-secure encryption, and more. For some of these, our construction yields the first standard model candidate.

2) We study the blow-up that occurs in indistinguishability obfuscation proof techniques due to puncturing and state the \\emph{Superfluous Padding Assumption} for indistinguishability obfuscation which allows us to lift the $q$-query restriction of our construction. We validate the assumption by showing that it holds for virtual black-box obfuscation.

3) Brzuska and Mittelbach require a strong form of point obfuscation secure in the presence of auxiliary input for their construction of UCEs. We show that this assumption is indeed necessary for the construction of injective UCEs.

09:17 [Pub][ePrint]

QUIC is a secure transport

protocol developed by Google and implemented in Chrome in 2013, currently

representing one of the most promising solutions to decreasing latency

while intending to provide security properties similar with TLS.

In this work we shed some light on QUIC\'s strengths and weaknesses

in terms of its provable security and performance guarantees in the presence of attackers.

We first introduce a security model for analyzing performance-driven protocols like QUIC

and prove that QUIC satisfies our definition under reasonable assumptions on the protocol\'s building blocks.

However, we find that QUIC does not satisfy the traditional notion of forward secrecy that is provided by some modes of TLS,

e.g., TLS-DHE.

Our analyses also reveal that with simple bit-flipping and replay attacks on some

public parameters exchanged during the handshake, an

adversary could easily prevent QUIC from achieving minimal latency

advantages either by having it fall back to TCP or by causing

the client and server to have an inconsistent view of their

handshake leading to a failure to complete the connection.

We have implemented these attacks and demonstrated that they

are practical.

Our results suggest that QUIC\'s security weaknesses are introduced by the very mechanisms used to reduce latency,

which highlights the seemingly inherent trade off between minimizing latency and providing `good\' security guarantees.

09:17 [Pub][ePrint]

PUF-based key generators have been widely considered as a root-of-trust in digital systems. They typically require an error-correcting mechanism (e.g. based on the code-offset method) for dealing with bit errors between the enrollment and reconstruction of keys. When the used PUF does not have full entropy, entropy leakage between the helper data and the device-unique key material can occur. If the entropy level of the PUF becomes too low, the PUF-derived key can be attacked through the publicly available helper data. In this work we provide several solutions for preventing this entropy leakage for PUFs suffering from bias. The methods proposed in this work pose no limit on the amount of bias that can be tolerated, which solves an important open problem for PUF-based key generation. Additionally, the solutions are all evaluated based on reliability, efficiency, leakage and reusability showing that depending on requirements for the key generator different solutions are preferable.

2015-06-20
01:50 [News]

Videos from FSE 2013 are now online.

2015-06-19
20:10 [Event][New]

Submission: 3 February 2016
Notification: 3 February 2016
From March 3 to March 5
Location: Academic City, UAE

2015-06-18
21:17 [Pub][ePrint]

Let P be chosen uniformly from the set P := Perm(S), the set of all permutations over a set S of size N. In Crypto 2015, Minaud and Seurin proved that for any unbounded time adversary A, making at most q queries, the distinguishing advantage between P^r (after sampling P, compose it for r times) and P, denoted Delta(P^r ; P), is at most (2r + 1)q/N. In this paper we provide an alternative simple proof of this result for an upper bound 2q(r+1)^2/N by using well known coefficient H-technique.

2015-06-17
18:17 [Pub][ePrint]

In this paper, we present improved preimage attacks on the reduced-round \\texttt{GOST} hash function family, which serves as the new Russian hash standard, with the aid of techniques such as the rebound attack, the Meet-in-the-Middle preimage attack and the multicollisions. Firstly, the preimage attack on 5-round \\texttt{GOST-256} is proposed which is the first preimage attack for \\texttt{GOST-256} at the hash function level. Then we extend the (previous) attacks on 5-round \\texttt{GOST-256} and 6-round \\texttt{GOST-512} to 6.5 and 7.5 rounds respectively by exploiting the involution property of the \\texttt{GOST} transposition operation.

Secondly, inspired by the preimage attack on \\texttt{GOST-256}, we also study the impacts of four representative truncation patterns on the resistance of the Meet-in-the-Middle preimage attack against \\texttt{AES}-like compression functions, and propose two stronger truncation patterns which make it more difficult to launch this type of attack. Based on our investigations, we are able to slightly improve the previous pseudo preimage attacks on reduced-round \\texttt{Gr{\\o}stl-256}.

18:17 [Pub][ePrint]

There have been several attempts recently at using homomorphic encryption to increase the efficiency of Oblivious RAM protocols. One of the most successful has been Onion ORAM, which achieves O(1) communication overhead with polylogarithmic server com- putation. However, it has a number of drawbacks. It requires a very large block size of B = Ω(log^5 N), with large constants. Although it needs only polylogarithmic computation complexity, that computation consists mostly of expensive homomorphic mul- tiplications. Finally, it achieves O(1) communication complexity but only amortized over a number of accesses. In this work we aim to address these problems, reducing the required block size to Ω(log^3 N), removing almost all of the homomorphic multiplica- tions and achieving O(1) worst-case communication complexity. We achieve this by replacing their homomorphic eviction routine with a much less expensive permute-and-merge one which elim- inates homomorphic multiplications while maintaining the same level of security. In turn, this removes the need for layered encryp- tion that Onion ORAM relies on and reduces both the minimum block size and worst-case bandwidth.

18:17 [Pub][ePrint]

The protection of cryptographic implementations against higher-order attacks has risen to an important topic in the side-channel community after the advent of enhanced measurement equipment that enables the capture of millions of power traces in reasonably short time. However, the preprocessing of multi-million traces for such an attack is still challenging, in particular when in the case of (multivariate) higher-order attacks all traces need to be parsed at least two times. Even worse, partitioning the captured traces into smaller groups to parallelize computations is hardly possible with current techniques.

In this work we introduce procedures that allow iterative computation of correlation in a side-channel analysis attack at any arbitrary order in both univariate and multivariate settings. The advantages of our proposed solutions are manifold: i) they provide stable results, i.e., by increasing the number of used traces high accuracy of the estimations is still maintained, ii) each trace needs to be processed only once and at any time the result of the attack can be obtained (without requiring to reparse the whole trace pull when adding more traces), and iii) the computations can be efficiently parallelized, e.g., by splitting the trace pull into smaller subsets and processing each by a single thread on a multi-threading or cloud-computing platform. In short, our constructions allow efficiently performing higher-order side-channel analysis attacks (e.g., on hundreds of million traces) which is of crucial importance when practical evaluation of the masking schemes need to be performed.