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2012-08-05
18:17 [Pub][ePrint] Breaking and Repairing GCM Security Proofs, by Tetsu Iwata and Keisuke Ohashi and Kazuhiko Minematsu

  In this paper, we study the security proofs of GCM (Galois/Counter Mode of Operation). We first point out that a lemma, which is related to the upper bound on the probability of a counter collision, is invalid. Both the original privacy and authenticity proofs by the designers are based on the lemma. We further show that the observation can be translated into a distinguishing attack that invalidates the main part of the privacy proof. It turns out that the original security proofs of GCM contain a flaw, and hence the claimed security bounds are not justified. A very natural question is then whether the proofs can be repaired. We give an affirmative answer to the question by presenting new security bounds, both for privacy and authenticity. As a result, although the security bounds are larger than what were previously claimed, GCM maintains its provable security. We also show that, when the nonce length is restricted to 96 bits, GCM has better security bounds than a general case of variable length nonces.



18:17 [Pub][ePrint] Robust Smart Card based Password Authentication Scheme against Smart Card Loss Problem, by Ding Wang and Chun-guang Ma

  As the most prevailing two-factor authentication mechanism, smart card based password authentication has been a subject of intensive research in the past decade and hundreds of this type of schemes have been proposed. However, most of them were found severely flawed, especially prone to the smart card loss problem, shortly after they were first put forward, no matter the security is heuristically analyzed or formally proved. In SEC\'12, Wang pointed out that, the main cause of this issue is attributed to the lack of an appropriate security model to fully identify the practical threats. To address the issue, Wang presented three kinds of security models, namely Type I, II and III, and further proposed four concrete schemes, only two of which, i.e. PSCAV and PSCAb, are claimed to be secure under the harshest model, i.e. Type III security model. However, in this paper, we demonstrate that PSCAV still cannot achieve the claimed security goals and is vulnerable to an offline password guessing attack and other attacks in the Type III security mode, while PSCAb has several practical pitfalls. As our main contribution, a robust scheme is presented to cope with the aforementioned defects and it is proven to be secure in the random oracle model. Moreover, the analysis demonstrates that our scheme meets all the proposed criteria and eliminates several hard security threats that are difficult to be tackled at the same time in previous scholarship.



18:17 [Pub][ePrint] New Preimage Attacks Against Reduced SHA-1, by Simon Knellwolf and Dmitry Khovratovich

  This paper shows preimage attacks against reduced SHA-1 up to 57 steps. The best previous attack has been presented at CRYPTO 2009 and was for 48 steps finding a two-block preimage with incorrect padding at the cost of 2159.3 evaluations of the compression function. For the same variant our attacks find a one-block preimage at 2150.6 and a correctly padded two-block preimage at 2151.1 evaluations of the compression function. The improved results come out of a differential view on the meet-in-the-middle technique originally developed by Aoki and Sasaki. The new framework closely relates meet-in-the-middle attacks to differential cryptanalysis which turns out to be particularly useful for hash functions with linear message expansion and weak diffusion properties.



18:17 [Pub][ePrint] Adaptively Secure Multi-Party Computation with Dishonest Majority, by Sanjam Garg and Amit Sahai

  Adaptively secure multiparty computation is an essential and fundamental notion in cryptography. In this work we focus on the basic question of constructing a multiparty computation protocol secure against a \\emph{malicious}, \\emph{adaptive} adversary in the \\emph{stand-alone} setting without assuming an honest majority, in the plain model. It has been believed that this question can be resolved by composing known protocols from the literature. We show that in fact, this belief is fundamentally mistaken. In particular, we show:

\\begin{itemize}

\\item[-]\\textbf{Round inefficiency is unavoidable when using black-box simulation:} There does not exist any $o(\\frac{n}{\\log{n}})$ round protocol that adaptively securely realizes a (natural) $n$-party functionality with a black-box simulator. Note that most previously known protocols in the adaptive security setting relied on black-box simulators.

\\item[-]\\textbf{A constant round protocol using non-black-box simulation:} We construct a \\emph{constant round} adaptively secure multiparty computation protocol in a setting without \\emph{honest majority} that makes crucial use of non-black box techniques.

\\end{itemize}

Taken together, these results give the first resolution to the question of adaptively secure multiparty computation protocols with a malicious dishonest majority in the plain model, open since the first formal treatment of adaptive security for multiparty computation in 1996.



18:17 [Pub][ePrint] Group Signatures with Almost-for-free Revocation, by Benoit Libert and Thomas Peters and Moti Yung

  Group signatures are a central cryptographic primitive where users

can anonymously and accountably sign messages in the name of a group

they belong to. Several efficient constructions with security proofs

in the standard model ({\\it i.e.}, without the random oracle idealization) appeared in the recent years. However, like standard PKIs, group signatures need an efficient revocation system to be practical.

Despite years of research, membership revocation remains a non-trivial problem: many existing solutions do not scale well due to

either high overhead or constraining operational requirements (like

the need for all users to update their keys after each revocation). Only recently, Libert, Peters and Yung (Eurocrypt\'12) suggested a new

scalable revocation method, based on the Naor-Naor-Lotspiech (NNL)

broadcast encryption framework, that interacts nicely with techniques

for building group signatures in the standard model. While promising, their mechanism introduces important storage requirements at group members. Namely, membership certificates, which used to have constant size in existing standard model constructions, now have polylog size in the maximal cardinality of the group (NNL, after all, is a tree-based technique and such dependency is naturally expected).

In this paper we show how to obtain private keys of {\\it constant}

size. To this end, we introduce a new technique to leverage the NNL

subset cover framework in the context of group signatures but,

perhaps surprisingly, without logarithmic relationship between the

size of private keys and the group cardinality. Namely, we provide a

way for users to efficiently prove their membership of one of the

generic subsets in the NNL subset cover framework. This technique

makes our revocable group signatures competitive with ordinary group

signatures ({\\it i.e.}, without revocation) in the standard model. Moreover, unrevoked members (as in PKIs) still do not need to update their keys at each revocation.



15:17 [Pub][ePrint] Biclique Cryptanalysis of TWINE, by Mustafa \\c{C}oban and Ferhat Karako\\c{c} and \\\"{O}zkan Bozta\\c{s}

  TWINE is a lightweight block cipher proposed at ECRYPT Workshop on Lightweight Cryptography 2011, Belgium.

The cipher consists of 36 rounds and has two versions TWINE-80 and TWINE-128 supporting key lengths of 80 and 128 bits, respectively.

The block length of the two versions is 64-bit.

In this paper, we present the first single-key attacks on the both versions of the cipher.

In these attacks, we use the recently developed biclique technique.

The complexities of the attacks on TWINE-80 and TWINE-128 are $2^{79.10}$ and $2^{126.82}$ respectively and the data requirement for the two attacks is $2^{60}$.



15:17 [Pub][ePrint] Programmable encryption and key-dependent messages, by Dominique Unruh

  We present the notion of PROG-KDM security for public-key encryption

schemes. This security notion captures both KDM security and

revealing of secret keys (key corruptions) in a single

definition. This is achieved by requiring the existence of a

simulator that can program ciphertexts when a secret key is

revealed, i.e., the simulator can delay the decision what plaintext

is contained in what ciphertext to the moment where the ciphertext

is opened. The definition is formulated in the random oracle model.

We show that PROG-KDM security can be achieved by showing that a

natural and practical construction in the ideal cipher model is

PROG-KDM secure (hybrid encryption using authenticated CBC

encryption).



15:17 [Pub][ePrint] Scalable Group Signatures with Revocation, by Benoit Libert and Thomas Peters and Moti Yung

  Group signatures are a central cryptographic primitive, simultaneously supporting accountability and anonymity. They allow

users to anonymously sign messages on behalf of a group they are

members of. The recent years saw the appearance of several

constructions with security proofs in the standard model ({\\it

i.e.}, without appealing to the random oracle heuristic). For a

digital signature scheme to be adopted, an efficient revocation

scheme (as in regular PKI) is absolutely necessary.

Despite over a decade of extensive research, membership revocation

remains a non-trivial problem in group signatures: all existing

solutions are not truly scalable due to either high overhead (e.g., large group public key size), or limiting operational requirement (the need for all users to follow the system\'s entire history). In the standard model, the situation is even worse as many existing solutions are not readily adaptable. To fill this gap and tackle this challenge, we describe a new revocation approach based, perhaps

somewhat unexpectedly, on the Naor-Naor-Lotspiech framework which was introduced for a different problem (namely, that of broadcast encryption). Our mechanism yields efficient and scalable revocable group signatures in the standard model. In particular, the size of signatures and the verification cost are independent of the number of

revocations and the maximal cardinality $N$ of the group while other

complexities are at most polylogarithmic in $N$. Moreover, the

schemes are history-independent: unrevoked group members do not have

to update their keys when a revocation occurs.



15:17 [Pub][ePrint] The Stream Cipher Core of the 3GPP Encryption Standard 128-EEA3: Timing Attacks and Countermeasures, by Gautham Sekar

  The core of the 3rd Generation Partnership Project (3GPP) encryption standard 128-EEA3 is a stream cipher called ZUC. It was designed by the Chinese Academy of Sciences and proposed for inclusion in the cellular wireless standards called \"Long Term Evolution\" or \"4G\". The LFSR-based cipher uses a 128-bit key. In this paper, we first show timing attacks on ZUC that can recover, with about 71.43% success rate, (i) one bit of the secret key immediately, and (ii) information involving 6 other key bits. The time, memory and data requirements of the attacks are negligible. While we see potential improvements to the attacks, we

also suggest countermeasures.



15:17 [Pub][ePrint] A Generalised Formula for Calculating the Resilience of Random Key Predistribution Schemes, by Ed Kendall and Michelle Kendall and Wilfrid S. Kendall

  A commonly used metric for comparing the resilience of key predistribution schemes is $\\fail_s$, which measures the proportion of network connections which are `broken\' by an adversary which has compromised $s$ nodes. In `Random key predistribution schemes for sensor networks\', Chan, Perrig and Song present a formula for measuring the resilience in a class of random key predistribution schemes called $q$-composite schemes. We present a correction to this formula for schemes where more than one key may be used to secure a link between a pair of nodes. Our corrected formula features an additional parameter which makes it applicable to a wider variety of random key predistribution schemes, including the original Eschenauer Gligor scheme. We also present a simplification of the formula for calculating connectivity.

We refer to the recent paper by Yum and Lee which also claims to correct the original formula for the $q$-composite scheme. However the resulting formula is complicated, computationally demanding, and hard to understand. The formula which we propose and prove is easily computable and can be applied to a wider range of schemes.



15:17 [Pub][ePrint] Constructing Pairing-Friendly Genus 2 Curves with Split Jacobian, by Robert Drylo

  Genus 2 curves with simple but not absolutely simple jacobians can be used to construct pairing-based cryptosystems more efficient than for a generic genus 2 curve. We show that there is a full analogy between methods for constructing ordinary pairing-friendly elliptic curves and simple abelian varieties, which are iogenous over some extension to a product of elliptic curves. We extend the notion of complete, complete with variable discriminant, and sparse families introduced in by Freeman, Scott and Teske for elliptic curves, and we generalize the Cocks-Pinch method and the Brezing-Weng method to construct families of each type. To realize abelian surfaces as jacobians we use of genus 2 curves of the form $y^2=x^5+ax^3+bx$ or $y^2=x^6+ax^3+b$, and apply the method of Freeman and Satoh. As applications we find some families of abelian surfaces with recorded $\\rho$-value $\\rho=2$ for embedding degrees $k=3,4,6,12$, or $\\rho=2.1$ for $k=27,54$. We also give variable-discriminant families with best $\\rho$-values.