*21:17* [Pub][ePrint]
Randomizing Scalar Multiplication Using Exact Covering Systems of Congruences, by Eleonora Guerrini and Laurent Imbert and Théo Winterhalter
In this paper we present a generic, uniformly randomized scalar multiplication algorithm based on covering systems of congruences, with built-in protections against various side-channel attacks. It has been tailored to resist a recent class of attacks called horizontal attacks. These very powerful attacks exploit some unsuspected weaknesses hidden in most, if not all, highly regular and constant time algorithms.We provide a thorough complexity analysis, several arguments to support its robustness and some encouraging numerical experiments.

*21:17* [Pub][ePrint]
XPX: Generalized Tweakable Even-Mansour with Improved Security Guarantees, by Bart Mennink
We present XPX, a tweakable blockcipher based on a single permutation P. On input of a tweak (t_{11},t_{12},t_{21},t_{22}) in mathcal{T} and a message m, it outputs ciphertext c=P(m xor Delta_1) xor Delta_2, where Delta_1=t_{11}k xor t_{12}P(k) and Delta_2=t_{21}k xor t_{22}P(k). Here, the tweak space mathcal{T} is required to satisfy a certain set of trivial conditions (such as (0,0,0,0) not in mathcal{T}). We prove that XPX with any such tweak space is a strong tweakable pseudorandom permutation. Next, we consider the security of XPX under related-key attacks, where the adversary can freely select a key-deriving function upon every evaluation. We prove that XPX achieves various levels of related-key security, depending on the set of key-deriving functions and the properties of mathcal{T}. For instance, if t_{12},t_{22} neq 0 and (t_{21},t_{22}) neq (0,1) for all tweaks, XPX is XOR-related-key secure.XPX generalizes Even-Mansour (EM), but also Rogaway\'s XEX based on EM, and tweakable EM used in Minalpher. As such, XPX finds a wide range of applications. We show how our results on XPX directly imply related-key security of the authenticated encryption schemes Pr{\\o}st-COPA and Minalpher, and how a straightforward adjustment to the MAC function Chaskey and to keyed Sponges makes them provably related-key secure.

*21:17* [Pub][ePrint]
How to Build Time-Lock Encryption, by Tibor Jager
Time-lock encryption is a method to encrypt a message such that it can only be decrypted after a certain deadline has passed. A computationally powerful adversary should not be able to learn the message before the deadline. However, even receivers with relatively weak computational resources should immediately be able to decrypt after the deadline, without any interaction with the sender, other receivers, or a trusted third party.We show how to realize this strong notion of secure encryption by making the additional, very realistic assumption that intermediate results of an iterative, public, large-scale computation --- like the computations performed by users of the popular cryptocurrency Bitcoin --- are publicly available. We use these computations as a \"computational reference clock\", which mimics a physical clock in a computational setting, and show how the computations performed by the reference clock can be \"reused\" to build secure time-lock encryption. A nice feature of this approach is that it can be based on a public computation which is performed \"anyway\" and independent of the time-lock encryption scheme.

We provide the first formal definitions of computational reference clocks and time-lock encryption, and give a proof-of-concept construction which combines a computational reference clock with witness encryption (Garg et al., STOC 2013). We also explain how to construct a computational reference clock based on Bitcoins.

*09:17* [Pub][ePrint]
Shadow-Bitcoin: Scalable Simulation via Direct Execution of Multi-threaded Applications, by Andrew Miller and Rob Jansen
We describe a new methodology that enables the di-rect execution of multi-threaded applications inside of

Shadow, an existing parallel discrete-event network sim-

ulation framework. Our methodology utilizes function

interposition and an application-layer thread library to

emulate the ordinary thread interface to the application.

Using this methodology, we implement a new Shadow

plug-in that directly executes the Bitcoin reference client

software. We describe optimizations that enable scalable

execution of thousands of Bitcoin nodes on a single ma-

chine, and discuss how to model the Bitcoin network for

experimental purposes. Finally, we present novel denial-

of-service attacks against the Bitcoin software, which

exploit low-level implementation artifacts in the Bitcoin

reference client. We demonstrate these attacks using our

methodology, tools, and models.

*09:17* [Pub][ePrint]
On the power of Public-key Functional Encryption with Function Privacy, by Vincenzo Iovino and Qiang Tang and Karol Zebrowski
In CRYPTO 2014 Bitansky et al. introduced a natural strengthening of indistinguishability obfuscation (iO) called strong iO (siO) and showed candidate constructions of such primitive from reasonable assumptions. In this paper, assuming quasi-siO, a natural weakening of siO, for a class of circuits C we construct a public-key functional encryption (FE) scheme with function privacy (FPFE) for the same class C. In the public-key setting known constructions of FPFE were limited to very restricted classes of functionalities like inner-product [Agrawal et al. - PKC 2015] whereas ours can be instantiated for general functionalities. Then, inspired by the Naor\'s transformation from IBE to signature schemes, we construct from FPFE a natural generalization of a signature scheme endowed with functional properties, that we call functional anonymous signature (FAS) scheme. In a FAS (that we show to be equivalent to quasi-siO and FPFE), Alice can sign a circuit C chosen from some distribution D to get a signature $\\sigma$ and can publish a verification key that allows anybody holding a message m to verify that (1) $\\sigma$ is a valid signature of Alice for some (possibly unknown to him) circuit C and (2) C(m) = 1. Beyond unforgeability the security of FAS guarantees that the signature hide as much information as possible about C except what can be inferred from knowledge of D.

As other application of FPFE, we show that it can be used to construct in a black-box way (without using obfuscation directly) FE for randomized functionalities (RFE). Previous constructions of (public-key) RFE relied on iO [Goyal et al. - TCC 2015]. Combining properties of RFE and FAS, we obtain what we call signed probabilistic programs, in which Bob can verify that a (possibly hidden to him) probabilistic program P was signed by Alice and run P on any input but can not maliciously choose its random coins. Furthermore, our constructions of FPFE and RFE naturally generalize to the multi-inputs setting. Finally, we present a general picture of the relations among all these related primitives. One of the key points that such implications draw is that Attribute-based Encryption with function privacy implies FE, a notable fact that sheds light on the importance and power of function privacy for FE.

*09:17* [Pub][ePrint]
A Challenge Obfuscation Method for Thwarting Model Building Attacks on PUFs, by Yansong Gao, Damith C. Ranasinghe, Gefei Li, Said F. Al-Sarawi, Omid Kavehei, and Derek Abbott
Physical Unclonable Functions (PUFs), as novel lightweighthardware security primitives, provide a higher level security with lower power and area overhead in comparison with traditional cryptographic solutions. However, it has been demonstrated that PUFs are vulnerable to model building attacks, especially those using linear additive functions such as Arbiter PUF (APUF) and k-sum PUF as building units. Nevertheless, both APUFs and k-sum PUFs are highly desirable security primitives, especially for authentication, because they are capable of producing a huge number of challenge response pairs (CRPs) and can be easily integrated into silicon. In this paper, we actually rely on the

demonstrated vulnerability of PUFs to model building attacks as well as the relative ease with which this can be achieved to develop a new parameter-based authentication protocol based on obfuscating challenges sent to PUFs and their subsequent recovery. We show, using statistical analysis and model building attacks using published approaches, that constructing a model using machine learning techniques are infeasible when our proposed method is employed. Finally, we also demonstrate that our challenge obfuscation and recovery method can be successfully

used for secure key exchange between two parties.

*09:17* [Pub][ePrint]
The Oblivious Machine - or: How to Put the C into MPC, by Marcel Keller
We present the oblivious machine, a concrete notion for multipartyrandom access machine (RAM) computation and a toolchain to allow the

efficient execution of general programs written in a subset of C that

allows RAM-model computation over integers. The machine only leaks the

list of possible instructions and the running time. Our work is based

on the oblivious array for secret-sharing-based multiparty computation

by Keller and Scholl (Asiacrypt `14). This means that we only incur a

polylogarithmic overhead over the execution on a normal CPU.

We implemented our construction using the clang compiler from the LLVM

project and the SPDZ protocol by Damgård et al. (Crypto `12). The

latter provides active security against a dishonest majority and works

in the preprocessing model. The online phase clock rate of the

resulting machine is 41 Hz for a memory size of 1024 64-bit integers

and 2.2 Hz for a memory of 2^20 integers. Both timings have been taken

for two parties in a local network.

To further showcase our toolchain, we implemented and benchmarked

private regular expression matching. Matching a string of length 1024

against a regular expression with 69270 transitions as a finite state

machine takes seven hours online time, of which more than six hours

are devoted to loading the reusable program.

*09:17* [Pub][ePrint]
Practical Fully Homomorphic Encryption without Noise Reduction, by Dongxi Liu
We present a new fully homomorphic encryption (FHE) scheme that is efficient for practical applications. The main feature of our scheme is that noise reduction considered essential in current FHE schemes, such as boot strapping and modulus switching, is not needed in our scheme, because it allows arbitrarily large noises in its ciphertexts.A ciphertext in our scheme is a vector with its dimension specified as a security parameter of the encryption key. The dimension of ciphertexts does not change with homomorphic operations and all ciphertext elements are in a finite domain, so our scheme is compact. In addition, our scheme can directly encrypt big integers, rather than only bit messages.

We proved the hardness of recovering encryption keys from any number of ciphertexts with chosen plaintexts and then the semantic security of our scheme. The hardness of recovering keys from ciphertexts is based on the approximate greatest common divisors problem.

We implemented a prototype of our scheme and evaluated its concrete performance extensively from the aspects of encryption, decryption, homomorphic operations,

and bitwise operators over ciphertexts.

The efficiency of our scheme is confirmed by the evaluation result.