International Association for Cryptologic Research

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2013-02-07
07:46 [Event][New]

From July 22 to July 26
Location: Tarragona, Spain

2013-02-06
19:17 [Pub][ePrint]

Dragonfly is a password authenticated key exchange protocol that has been submitted to the Internet Engineering Task Force as a candidate standard for general internet use. We analyzed the security of this protocol and devised an attack that is capable of extracting both the session key and password from an honest party. This attack was then implemented and experiments were performed to determine the time-scale required to successfully complete the attack.

19:17 [Pub][ePrint]

The ease of programming offered by the CUDA programming model attracted a lot of programmers to try the platform for acceleration of many non-graphics applications. Cryptography, being no exception, also found its share of exploration efforts, especially block ciphers. In this contribution we present a detailed walk-through of effective mapping of HC-128 and HC-256 stream ciphers on GPUs. Due to inherent inter-S-Box dependencies, intra-S-Box dependencies and a high number of memory accesses per keystream word generation, parallelization of HC series of stream ciphers remains challenging. For the first time, we present various optimization strategies for HC-128 and HC-256 speedup in tune with CUDA device architecture. The peak performance achieved with a single data-stream for HC-128 and HC-256 is 0.95 Gbps and 0.41 Gbps respectively. Although these throughput figures do not beat the CPU performance (10.9 Gbps for HC-128 and 7.5 Gbps for HC-256), our multiple parallel data-stream implementation is benchmarked to reach approximately 31 Gbps for HC-128 and 14 Gbps for HC-256 (with 32768 parallel data-streams). To the best of our knowledge, this is the first reported effort of mapping HC-Series of stream ciphers on GPUs.

19:17 [Pub][ePrint]

In this work we come up with two fully homomorphic schemes.

First, we propose an IND-CPA secure symmetric key homomorphic encryption scheme using multivariate polynomial ring over finite fields. This scheme gives a method of constructing a CPA secure homomorphic encryption scheme from another symmetric deterministic CPA secure scheme. We base the security of the scheme on information theoretic arguments and prove the scheme to be IND-CPA secure, rather than basing security on hard problems like Ideal Membership and Gr\\\"obner basis as seen in most polly cracker based schemes which also use multivariate polynomial rings. This scheme is not compact but has many interesting properties. Second, we also describe another similar symmetric key scheme which is compact, fully homomorphic and doesn\'t require bootstrapping. The scheme is on the lines of the work of Albrecht et.al. (Asiacrypt-2011) and is proven to be bounded CPA secure. Proof is based on Ideal Membership/ Ideal Remainder/Gr\\\"obner basis problem.

19:17 [Pub][ePrint]

The Advanced Encryption Standard (AES) is the most widely used block cipher. The high level structure of AES can be viewed as a (10-round) key-alternating cipher, where a t-round key-alternating cipher KA_t consists of a small number $t$ of fixed permutations P_i on n bits, separated by key addition:

KA_t(K,m)= k_t + P_t(... k_2 + P_2(k_1 + P_1(k_0 + m))...),

where (k_0,...,k_t) are obtained from the master key K using some key derivation function.

For t=1, KA_1 collapses to the well-known Even-Mansour cipher, which is known to be indistinguishable from a (secret) random permutation, if P_1 is modeled as a (public) random permutation. In this work we seek for stronger security of key-alternating ciphers --- indifferentiability from an ideal cipher --- and

ask the question under which conditions on the key derivation function and for how many rounds t is the key-alternating cipher KA_t indifferentiable from the ideal cipher, assuming P_1,...,P_t are (public) random permutations?

As our main result, we give an affirmative answer for t=5, showing that the 5-round key-alternating cipher KA_5 is indifferentiable from an ideal cipher, assuming P_1,...,P_5 are five independent random permutations, and the key derivation function sets all rounds keys

k_i=f(K), where 0

16:53 [Job][New]

PhD Positions in Applied Cryptology

The Vernam Lab at WPI in Worcester, MA has open PhD positions in applied cryptology. In particular there are two openings in side channel analysis and countermeasure design and implementation.

Candidates should have a Master’s degree in electronics, computer science or applied mathematics, with strong interest in algorithms and signal processing. Prior experience in side channel analysis and embedded software or hardware design is an asset.

We offer a competitive salary and an international cutting-edge research program in an attractive working environment in the greater Boston area. WPI is one of the highest-ranked technical colleges in the US.

16:17 [Pub][ePrint]

Akleylek et al. [S. Akleylek, L. Emmungil and U. Nuriyev, Algorithm for peer-to-peer security, journal of Appl. Comput. Math., Vol. 6(2), pp.258-264, 2007.], introduced a modified algorithm with steganographic approach for security in peer-to-peer (P2P) network. In this cryptosystem, Akleylek et al. attempt to increase the security of P2P network by connecting the ElGamal cryptosystem with knapsack problem. We show that this combination leak the security and makes the hybrid cryptosystem‎ vulnerable to \"ciphertext only attack\". Thus, in the network, an attacker can apply this attack and simply can recover the original message (plaintext) from any {\\it challenge ciphertext}. Moreover, we show that the receiver cannot decrypt the ciphertext in polynomial time and so, the proposed cryptosystem is completely impractical. We modify this cryptosystem to increase security and efficiency.

16:17 [Pub][ePrint]

Applying cut-and-choose techniques to Yao\'s garbled circuit protocol has been a promising approach for designing efficient Two-Party Computation (2PC) with malicious and covert security, as is evident from various optimizations and software implementations in the recent years. We revisit the security and efficiency properties of this popular approach and propose alternative constructions and definitions that are more suitable for use in practice.

* We design an efficient fully-secure malicious 2PC protocol for two-output functions that only requires $O(t|C|)$ symmetric-key operations (with small constant factors) where $|C|$ is the circuit size and $t$ is a statistical security parameter. This is essentially the {\\em optimal} complexity for protocols based on cut-and-choose, resolving a main question left open by the previous work on the subject.

Our protocol utilizes novel techniques for enforcing \\emph{garbler\'s input consistency} and handling \\emph{two-output functions} that are more efficient than all prior solutions.

* Motivated by the goal of eliminating the \\emph{all-or-nothing} nature of 2PC with covert security (that privacy and correctness are fully compromised if the adversary is not caught in the challenge phase), we propose a new security definition for 2PC that strengthens the guarantees provided by the standard covert model, and offers a smoother security vs. efficiency tradeoff to protocol designers in choosing the \\emph{right deterrence factor}. In our new notion, correctness is always guaranteed, privacy is fully guaranteed with probability ($1-\\epsilon$), and with probability $\\epsilon$ (i.e. the event of undetected cheating), privacy is only partially compromised\" with at most a {\\em single bit} of information leaked, in \\emph{case of an abort}.

We present two efficient 2PC constructions achieving our new notion. Both protocols are competitive with the previous 2PC based on cut-and-choose. E.g., the price of strengthening a covert 2PC to satisfy our notion (to obtain full correctness and maximum leakage of a single bit), is only $\\frac{1}{\\epsilon}$ additional garbled circuits.

A distinct feature of the techniques we use in all our constructions is to check consistency of inputs and outputs using new gadgets that are themselves \\emph{garbled circuits}, and to verify validity of these gadgets using \\emph{multi-stage} cut-and-choose openings.

These techniques may be of an independent interest.

16:17 [Pub][ePrint]

In this paper, first, it is proved that finding the approximate shortest vector could be Karp-reduced to GapSVP.

Second, it is proved that shortest vector problem itself could be reduced to GapSVP with a quite small gap.

Third, we improve the complexity results of uSVP, proving uSVP could be reduced from SVP (our results are better than any known result). What\'s more, we prove that the search version of uSVP could be reduced to decisional version of uSVP with almost the same gap.

16:17 [Pub][ePrint]

Certificateless public key cryptography (CLPKC), with properties of no key escrow and no certificate, has received a lot of attention since its invention. However, membership revocation in certificateless cryptosystem still remains a non-trivial problem: the existing solutions are not practical for use due to either a costly mediator or enormous computation (secret channel). In this paper, we present a new approach to revocation in CLPKC with a concrete construction of a revocable certificateless signature (RCLS) scheme. In our scheme, a user\'s private key is composed of three parts: an initial partial private key, a time key and a secret value. The transmission of updated-key requires only a public channel, which makes our RCLS scheme more efficient than other methods. We first provide formal definition and security model for a RCLS scheme. The new scheme is proved secure in the random oracle model, based on the Computational Diffie-Hellman problem.

16:17 [Pub][ePrint]

In this paper, we introduce the notion of a joint compartmented threshold access structure (JCTAS). We study the necessary conditions for the existence of an ideal and perfect secret sharing scheme and give a characterization of almost all ideal JCTASes. Then we give an ideal and almost surely perfect construction that realizes such access structures. We prove the asymptotic perfectness of this construction by the Schwartz-Zippel Lemma.