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2012-11-21
16:17 [Pub][ePrint]

In Africacrypt 2009, Galindo-Garcia [11] proposed a lightweight identity-based signature (IBS) scheme based on the Schnorr signature. The construction is simple and claimed to be the most efficient IBS till date. The security is based on the discrete-log assumption and the security argument consists of two reductions: B1 and B2, both of which use the multiple-forking lemma [4] to solve the discrete-log problem (DLP).

In this work, we revisit the security argument given in [11]. Our contributions are two fold: (i) we identify several problems in the original argument and (ii) we provide a detailed new security argument which allows significantly tighter reductions. In particular, we show that the reduction B1 in [11] fails in the standard security model for IBS [1], while the reduction B2 is incomplete. To remedy these problems, we adopt a two-pronged approach. First, we sketch ways to fill the gaps by making minimal changes to the structure of the original security argument; then, we provide a new security argument. The new argument consists of three reductions: R1, R2 and R3 and in each of them, solving the DLP is reduced to breaking the IBS. R1 uses the general forking lemma [2] together with the programming of the random oracles and Coron\'s technique [7]. Reductions R2 and R3, on the other hand, use the multiple-forking lemma along with the programming of the random oracles. We show that the reductions R1 and R2 are significantly tighter than their original counterparts.

16:17 [Pub][ePrint]

In this work we present a modification of a well-established measure of dependence appropriate for the analysis of stopping times for adversarial processes on cryptographic primitives. We apply this measure to construct generic criteria for the ideal behavior of fixed functions in both the random oracle and ideal permutation setting. More significantly, we provide a nontrivial extension of the notion of hash function indifferentiability, transporting the theory from the status of providing security arguments for protocols utilizing ideal primitives into the more realistic setting of protocol assurance with fixed functions. The methodology this measure introduces to indifferentiability analysis connects the security of a hash function with an indifferentiable mode to the security of the underlying compression function in a quantitative way; thus, we prove that dependence results on cryptographic primitives provide a direct means of determining the practical resistance or vulnerability of protocols employing such primitives.

16:17 [Pub][ePrint]

Recently, the concept of Search in Encrypted Data (SED) has become a highlight in cryptography. A SED scheme enables a client to have third-party server(s) to perform certain search functionalities on his encrypted data. In this book chapter, we aim at conducting a systematic study on SED schemes. Firstly, we describe three application scenarios and identify the desirable security requirements. Secondly, we provide two orthogonal categorizations and review the related security models for each category of SED schemes. Thirdly, we analyze the practical issues related to SED schemes and identify some future research directions.

16:17 [Pub][ePrint]

Adding a Schnorr signature to ElGamal encryption is a popular proposal aiming at thwarting chosen-ciphertext attacks by rendering the scheme plaintext-aware. However, there is no known security proof for the resulting scheme, at least not in a weaker model than the one obtained by combining the Random Oracle Model (ROM) and the Generic Group Model (Schnorr and Jakobsson, ASIACRYPT 2000). In this paper, we propose a very simple modification to Schnorr-Signed ElGamal encryption that leaves keys and ciphertexts size unchanged, for which the resulting scheme is semantically secure under adaptive chosen-ciphertext attacks (IND-CCA2-secure) in the ROM under the Decisional Diffie-Hellman assumption. In fact, we even prove that our new scheme is plaintext-aware in the ROM as defined by Bellare et al. (CRYPTO\'98). Interestingly, we also observe that Schnorr-Signed ElGamal is not plaintext-aware (again, for the definition of Bellare et al.) under the Computational Diffie-Hellman assumption. We show that our new scheme additionally achieves anonymity as well as robustness, a notion formalized by Abdalla et al. (TCC 2010) which captures the fact that it is hard to create a ciphertext that is valid under two different public keys. Finally, we study the hybrid variant of our new proposal, and show that it is IND-CCA2-secure in the ROM under the Computational Diffie-Hellman assumption when used with a symmetric encryption scheme satisfying the weakest security notion, namely ciphertext indistinguishability under one-time attacks (IND-OT-security).

15:41 [Event][New]

Submission: 1 May 2013
From September 16 to September 17
Location: Bucharest, Romania

10:28 [Event][New]

Submission: 15 February 2013
From July 15 to July 17
Location: Newark, NJ, USA

2012-11-20
14:47 [Job][New]

The Horst Görtz Institute for IT-Security (HGI) at Ruhr-University Bochum is one of Europe’s leading research centers in IT security. The DFG, or German Research Foundation, awarded more than €4 million to the HGI for the establishment of the interdisciplinary research training group “New Challenges for Cryptography in Ubiquitous Computing”. We are looking for candidates with an outstanding Ph.D. in the fields of computer science, electrical engineering, mathematics or related areas.

The research training group will study problems which are fundamental for securing the Internet of Things. The research is structured in three levels: cryptographic primitives, device and system level. The research topics range from cryptographic foundations such as fully homomorphic encryption for privacy in cloud computing, over security for medical implants to internet security solutions involving new national ID cards.

Beside the own research, the main task of the Post-Doc is to work with the UbiCrypt Ph.D. students, and to encourage collaboration between them. Thus, an interest in working with doctoral students and a broad interest in current research are required.

- Start: 01.04.2013

- Competitive salary

- Application: Send your documents by January 15, 2013, to grako (at) hgi.rub.de

- Required documents: CV, certificates (Bachelor, Master/Diplom, Ph.D.), transcripts , motivation for applying (1 page), names of at least two people who can provide reference letters (email addresses are sufficient)

A group of internationally renowned researchers together with excellent funding provides an extremely interesting scientific environment. The HGI is known for its good working atmosphere.

13:17 [Pub][ePrint]

Coin-tossing protocols are protocols that generate a random bit with uniform distribution. These protocols are used as a building block in many cryptographic protocols. Cleve [STOC 1986] has shown that if at least half of the parties can be malicious, then, in any r-round coin-tossing protocol, the malicious parties can cause a bias of Omega(1/r) to the bit that the honest parties output. However, for more than two decades the best known protocols had bias t/\\sqrt{r}, where t is the number of corrupted parties. Recently, in a surprising result, Moran, Naor, and Segev [TCC 2009] have shown that there is an r-round two-party coin-tossing protocol with the optimal bias of O(1/r). We extend Moran et al. results to the multiparty model when less than 2/3 of the parties are malicious. The bias of our protocol is proportional to 1/r and depends on the gap between the number of malicious parties and the number of honest parties in the protocol. Specifically, for a constant number of parties or when the number of malicious parties is somewhat larger than half, we present an r-round m-party coin-tossing protocol with optimal bias of O(1/r).

13:17 [Pub][ePrint]

The indifferentiability framework of Maurer, Renner, and Holenstein (MRH) has gained immense popularity in recent years and has proved to be a powerful way to argue security of cryptosystems that enjoy proofs in the random oracle model. Recently, however, Ristenpart, Shacham, and Shrimpton (RSS) showed that the composition theorem of MRH has a more limited scope than originally thought, and that extending its scope required the introduction of reset-indifferentiability, a notion which no practical domain extenders satisfy with respect to random oracles.

In light of the results of RSS, we set out to rigorously tackle the specifics of indifferentiability and reset-indifferentiability by viewing the notions as special cases of a more general definition. Our contributions are twofold. Firstly, we provide the necessary formalism to refine the notion of indifferentiability regarding composition. By formalizing the definition of stage minimal games we expose new notions lying in between regular indifferentiability (MRH) and reset-indifferentiability (RSS).

Secondly, we answer the open problem of RSS by showing that it is impossible to build any domain extender which is reset-indifferentiable from a random oracle. This result formally confirms the intuition that reset-indifferentiability is too strong of a notion to be satisfied by any hash function. As a consequence we look at the weaker notion of single-reset-indifferentiability, yet there as well we demonstrate that there are no meaningful\'\' domain extenders which satisfy this notion. Not all is lost though, as we also view indifferentiability in a more general setting and point out the possibility for different variants of indifferentiability.

13:17 [Pub][ePrint]

Hierarchical Key Assignment Schemes can be used to enforce access control policies by cryptographic means. In this paper, we present a new, enhanced security model for such schemes. We also give simple, efficient, and strongly-secure constructions for Hierarchical Key Assignment Schemes for arbitrary hierarchies using pseudorandom functions and forward-secure pseudorandom generators. We compare instantiations of our constructions with state-of-the-art Hierarchical Key Assignment Schemes, demonstrating that our new schemes possess an attractive trade-off between storage requirements and efficiency of key derivation.

2012-11-19
13:41 [Job][New]

The Government Communications Headquarters (GCHQ) in Cheltenham has agreed in principle to sponsor two PhD/Doctoral Studentships at Bristol University in the area of Cryptography.

The studentships are only open to UK nationals and the successful candidate will be required to spend in the region of 2 - 4 weeks per year at GCHQ headquarters in Cheltenham. To be considered for this studentship, candidates must therefore be prepared to undergo GCHQ\\\'s security clearance procedures.

The two studentships will be in the following areas:

Title: Authentication, Ciphers, and Encryption

Supervisors: Dan Page and Martijn Stam

Title: Information Leakage aware Verification

Supervisors: Elisabeth Oswald and Kerstin Eder