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22:17 [Pub][ePrint] Multi-Client Verifiable Computation with Stronger Security Guarantees, by S. Dov Gordon and Jonathan Katz and Feng-Hao Liu and Elaine Shi and Hong-Sheng Zhou

  Choi et al. (TCC 2013) introduced the notion of multi-client verifiable computation (MVC) in which a set of clients outsource to an untrusted server the computation of a function f over their collective inputs in a sequence of time periods. In that work, the authors defined and realized multi-client verifiable computation satisfying soundness against a malicious server and privacy against the semi-honest corruption of a single client. Very recently, Goldwasser et al. (Eurocrypt 2014) provided an alternative solution relying on multi-input functional encryption.

Here we conduct a systematic study of MVC, with the goal of satisfying stronger security requirements. We begin by introducing a simulation-based notion of security that provides a unified way of defining soundness and privacy, and automatically captures several attacks not addressed in previous work. We then explore the feasibility of achieving this notion of security. Assuming no collusion between the server and the clients, we demonstrate a protocol for multi-client verifiable computation that achieves strong security in several respects. When server-client collusion is possible, we show (somewhat surprisingly) that simulation-based security cannot be achieved in general, even assuming semi-honest behavior.

22:17 [Pub][ePrint] Harder, Better, Faster, Stronger - Elliptic Curve Discrete Logarithm Computations on FPGAs, by Erich Wenger and Paul Wolfger

  Computing discrete logarithms takes time. It takes time to develop new algorithms, choose the best algorithms, implement these algorithms correctly and efficiently, keep the system running for several months, and, finally, publish the results. In this paper, we present a highly performant architecture that can be used to compute discrete logarithms of Weierstrass curves defined over binary fields and Koblitz curves using FPGAs. We used the architecture to compute for the first time a discrete logarithm of the elliptic curve \\texttt{sect113r1}, a previously standardized binary curve, using 10 Kintex-7 FPGAs. To achieve this result, we investigated different iteration functions, used a negation map, dealt with the fruitless cycle problem, built an efficient FPGA design that processes 900 million iterations per second, and we tended for several months the optimized implementations running on the FPGAs.

22:17 [Pub][ePrint] Security of the AES with a Secret S-box, by Tyge Tiessen and Lars R. Knudsen and Stefan Kölbl and Martin M. Lauridsen

  How does the security of the AES change when the S-box is replaced

by a secret S-box, about which the adversary has no knowledge? Would it be safe to reduce the number of encryption rounds?

In this paper, we demonstrate attacks based on integral cryptanalysis

which allows to recover both the secret key and the secret S-box for respectively four, five,

and six rounds of the AES. Despite the significantly larger amount of secret information which an

adversary needs to recover, the attacks are very efficient with

time/data complexities of $2^{17}/2^{16}$, $2^{38}/2^{40}$ and $2^{90}/2^{64}$, respectively.

Another interesting aspect of our attack is that it works both as chosen plaintext and as chosen ciphertext attack. Surprisingly, the chosen ciphertext variant has a significantly lower time complexity in the attacks on four and five round, compared to the respective chosen plaintext attacks.

22:17 [Pub][ePrint] Observations on the SIMON block cipher family, by Stefan Kölbl and Gregor Leander and Tyge Tiessen

  In this paper we analyze the general class of functions underlying the SIMON block cipher. In particular, we derive efficiently computable and easy to implement expressions for the exact differential and linear behavior of SIMON-like round function. Using those expressions we investigate a large set of natural SIMON variants with respect to the most important cryptographic criteria. Interestingly, the NSA\'s choice for the parameters are not always optimal.

Using a computer aided approach based on SAT/SMT solvers we are able to find both the optimal differential and linear characteristics for variants of SIMON and can also give better estimates on the

probability of differentials. As a result of this analysis we propose different sets of rotation constants, which feature better properties on some criteria, and might be interesting for further analysis.

22:17 [Pub][ePrint] New Attacks on Feistel Structures with Improved Memory Complexities, by Itai Dinur and Orr Dunkelman and Nathan Keller and Adi Shamir

  Feistel structures are an extremely important and extensively researched type of cryptographic schemes. In this paper we describe improved attacks on Feistel structures with more than 4 rounds.

We achieve this by a new attack that combines the main benefits of

meet-in-the-middle attacks (which can reduce the time complexity by

comparing only half blocks in the middle) and dissection attacks (which can reduce the memory complexity but have to guess full blocks in the middle in order to perform independent attacks above and below it). For example, for a 7-round Feistel structure on n-bit inputs with seven independent round keys of n/2 bits each), a MITM attack can use (2^1.5n ,2^1.5n) time and memory, while dissection requires (2^2n, 2^n) time and memory. Our new attack requires only (2^1.5n, 2^n) time and memory, using a few known plaintext/ciphertext pairs. When we are allowed to use more known plaintexts, we develop new techniques which rely on the existence of multicollisions and differential properties deep in the structure in order to further reduce the memory complexity.

Our new attacks are not just theoretical generic constructions - in fact, we can use them to improve the best known attacks on several concrete cryptosystems such as CAST-128 (where we reduce the memory complexity from 2^111 to 2^64) and DEAL-256 (where we reduce the memory complexity from 2^200 to 2^144), without affecting their time and data complexities. An extension of our techniques applies even to some non-Feistel structures - for example, in the case of FOX, we reduce the memory complexity of all the best known attacks by a factor of 2^16.

22:17 [Pub][ePrint] High Precision Fault Injections on the Instruction Cache of ARMv7-M Architectures, by Lionel Rivière and Zakaria Najm and Pablo Rauzy and Jean-Luc Danger and Julien Bringer and Laurent Sauvage

  Hardware and software of secured embedded systems are prone to physical attacks.

In particular, fault injection attacks revealed vulnerabilities on the data and the control flow

allowing an attacker to break cryptographic or secured algorithms implementations.

While many research studies concentrated on successful attacks on the data flow, only a few targets the instruction flow.

In this paper, we focus on electromagnetic fault injection (EMFI) on the control flow,

especially on the instruction cache.

We target the very widespread (smartphones, tablets, settop-boxes, health-industry monitors and sensors, etc.) ARMv7-M architecture.

We describe a practical EMFI platform and present a methodology providing high control level and high reproducibility over fault injections.

Indeed, we observe that a precise fault model occurs in up to 96\\% of the cases.

We then characterize and exhibit this practical fault model on the cache that is not yet considered in the literature.

We comprehensively describe its effects and show how it can be used to reproduce well known fault attacks.

Finally, we describe how it can benefits attackers to mount new powerful attacks or simplify existing ones.

22:17 [Pub][ePrint] On the Effectiveness of the Remanence Decay Side-Channel to Clone Memory-based PUFs, by Yossef Oren and Ahmad-Reza Sadeghi and Christian Wachsmann

  We present a side-channel attack based on remanence decay in volatile memory and show how it can be exploited effectively to launch a non-invasive cloning attack against SRAM PUFs --- an important class of PUFs typically proposed as lightweight security primitive with low overhead by using the existing memory of the underlying device. We validate our approach against two SRAM PUF implementations in 65~nm CMOS ASICs. We discuss countermeasures against our attack and propose the constructive use of remanence decay to improve the cloning-resistance of SRAM PUFs.

Moreover, as a further contribution of independent interest, we show how to use our evaluation results to significantly improve the performance of the recently proposed TARDIS scheme, which is based on remanence decay in SRAM and used as a time-keeping mechanism for low-power clock-less devices.

22:17 [Pub][ePrint] Cryptanalysis of HMAC/NMAC-Whirlpool, by Jian Guo and Yu Sasaki and Lei Wang and Shuang Wu

  In this paper, we present universal forgery and key recovery attacks on the most popular hash-based MAC constructions, e.g., HMAC and NMAC, instantiated with an AES-like hash function Whirlpool. These attacks work with Whirlpool reduced to 6 out of 10 rounds in single-key setting. To the best of our knowledge, this is the first result on ``original\'\' key recovery for HMAC (previous works only succeeded in recovering the equivalent keys). Interestingly, the number of attacked rounds is comparable with that for collision and preimage attacks on Whirlpool hash function itself. Lastly, we present a distinguishing-H attack against the full HMAC- and NMAC-Whirlpool.

22:17 [Pub][ePrint] Insynd: Privacy-Preserving Secure One-Way Messaging Using Balloons, by Tobias Pulls and Roel Peeters

  Insynd is a cryptographic scheme for secure and privacy-preserving one-way messaging. Insynd is ideally suited for sending personalised breach notifications to end-users, enabling services to provide positive evidence to a third party that they have complied with their obligations (and conversely, for end-users to prove that the notification was not timely). Insynd provides i) secrecy of messages, ii) message integrity and authenticity, iii) protection against recipient profiling, and iv) publicly verifiable proofs of who sent what message to which recipient at what particular time. Our scheme is built on an authenticated data structure, named Balloon, enabling the safe outsourcing of storage of messages to an untrusted server (such as commodity cloud services). The author of messages is in the forward-security model. Insynd uses modern cryptographic primitives, making it also a suitable secure logging system or evidence store, despite the use of \"slow\" public-key cryptography. Our prototype implementation shows improved performance over related work and competitive performance for more data-intensive settings like secure logging.

22:17 [Pub][ePrint] Bad directions in cryptographic hash functions, by Daniel J. Bernstein and Andreas Hülsing and Tanja Lange and Ruben Niederhagen

  A 25-gigabyte \"point obfuscation\" challenge \"using security parameter 60\" was announced at the Crypto 2015 rump session; \"point obfuscation\" is another name for password hashing. This paper shows that the particular matrix-multiplication hash function used in the challenge is much less secure than previous password-hashing functions are believed to be. This paper\'s attack algorithm broke the challenge in just 19 minutes using a cluster of 21 PCs.

22:17 [Pub][ePrint] Inverting the Fnal exponentiation of Tate pairings on ordinary elliptic curves using faults, by Ronan Lashermes and Jacques Fournier and Louis Goubin

  The calculation of the Tate pairing on ordinary curves involves two major steps: the Miller Loop (ML) followed by the Final Exponentiation (FE). The first step for achieving a full pairing inversion would be to invert this FE, which in itself is a mathematically difficult problem. To our best knowledge, most fault attack schemes proposed against pairing algorithms have mainly focussed on the ML. They solved, if at all, the inversion of the FE in some special `easy\' cases or even showed that the complexity of the FE is an intrinsic countermeasure against a successful full fault attack on the Tate pairing. In this paper, we present a fault attack on the FE whereby the inversion of the final exponentiation becomes feasible using $3$ independent faults.