*15:17* [Pub][ePrint]
A heuristic for finding compatible differential paths with application to HAS-160, by Aleksandar Kircanski and Riham AlTawy and Amr M. Youssef
The question of compatibility of differential paths plays a central role in second ordercollision attacks on hash functions. In this context, attacks typically proceed by starting from the

middle and constructing the middle-steps quartet in which the two paths are enforced on the respec-

tive faces of the quartet structure. Finding paths that can fit in such a quartet structure has been

a major challenge and the currently known compatible paths extend over a suboptimal number of

steps for hash functions such as SHA-2 and HAS-160. In this paper, we investigate a heuristic that

searches for compatible differential paths. The application of the heuristic in case of HAS-160 yields

a practical second order collision over all of the function steps, which is the first practical result that

covers all of the HAS-160 steps. An example of a colliding quartet is provided

*21:17* [Pub][ePrint]
Trapdoor Smooth Projective Hash Functions, by Fabrice Benhamouda and David Pointcheval
Katz and Vaikuntanathan recently improved smooth projective hash functions in order to build one-round password-authenticated key exchange protocols (PAKE). To achieve security in the UC framework they allowed the simulator to extract the hashing key, which required simulation-sound non-interactive zero-knowledge proofs that are unfortunately inefficient.We improve the way the latter extractability is obtained by introducing the notion of trapdoor smooth projective hash function (TSPHF). A TSPHF is an SPHF with a trapdoor, which may not allow to recover the complete hashing key, but which still allows to compute the hash value, which is enough for an application to PAKE with UC-security against static corruptions. We additionally show that TSPHFs yield zero-knowledge proofs in two flows, with straight-line extractability.

Besides those quite interesting applications of TSPHF, we also show how to generically build them on languages of ciphertexts, using any ElGamal-like encryption. Our concrete instantiations lead to efficient one-round UC-secure PAKE, extractable zero-knowledge arguments, and verifiable encryption of Waters signatures. In the case of the PAKE, our construction is the most efficient one-round UC-secure PAKE to date.

*21:17* [Pub][ePrint]
Quantum one-time programs, by Anne Broadbent and Gus Gutoski and Douglas Stebila
A one-time program is a hypothetical device by which a user may evaluate a circuit on exactly one input of his choice, before the device self-destructs. One-time programs cannot be achieved by software alone, as any software can be copied and re-run. However, it is known that every circuit can be compiled into a one-time program using a very basic hypothetical hardware device called a one-time memory. At first glance it may seem that quantum information, which cannot be copied, might also allow for one-time programs. But it is not hard to see that this intuition is false: one-time programs for classical or quantum circuits based solely on quantum information do not exist, even with computational assumptions. This observation raises the question, \"what assumptions are required to achieve one-time programs for quantum circuits?\" Our main result is that any quantum circuit can be compiled into a one-time program assuming only the same basic one-time memory devices used for classical circuits. Moreover, these quantum one-time programs achieve statistical universal composability (UC-security) against any malicious user. Our construction employs methods for computation on authenticated quantum data, and we present a new quantum authentication scheme called the trap scheme for this purpose. As a corollary, we establish UC-security of a recent protocol for delegated quantum computation.

*21:17* [Pub][ePrint]
Limits of provable security for homomorphic encryption, by Andrej Bogdanov and Chin Ho Lee
We show that public-key bit encryption schemes which support weak (i.e., compact) homomorphic evaluation of any sufficiently \"sensitive\" collection of functions cannot be proved message indistinguishable beyond AM intersect coAM via general (adaptive) reductions, and beyond statistical zero-knowledge via reductions of constant query complexity. Examples of sensitive collections include parities, majorities, and the class consisting of all AND and OR functions.Our techniques also give a method for converting a strong (i.e., distribution-preserving) homomorphic evaluator for essentially any boolean function (except the trivial ones, the NOT function, and the AND and OR functions) into a rerandomization algorithm: This is a procedure that converts a ciphertext into another ciphertext which is statistically close to being independent and identically distributed with the original one. Our transformation preserves negligible statistical error.

*21:17* [Pub][ePrint]
Analysis and Improvement of the Generic Higher-Order Masking Scheme of FSE 2012, by Arnab Roy and Srinivas Vivek
Masking is a well-known technique used to prevent block cipher implementations from side-channel attacks. Higher-order side channel attacks (e.g. higher-order DPA attack) on widely used block cipher like AES have motivated the design of efficient higher-order masking schemes. Indeed, it is known that as the masking order increases, the difficulty of side-channel attack increases exponentially. However, the main problem in higher-order masking is to design an efficient and secure technique for S-box computations in block cipher implementations. At FSE 2012, Carlet et al. proposed a generic masking scheme that can be applied to any S-box at any order. This is the first generic scheme for efficient software implementations. Analysis of the running time, or \\textit{masking complexity}, of this scheme is related to a variant of the well-known problem of efficient exponentiation (\\textit{addition chain}), and evaluation of polynomials. In this paper we investigate optimal methods for exponentiation

in $\\mathbb{F}_{2^{n}}$ by studying a variant of addition chain,

which we call \\textit{cyclotomic-class addition chain}, or \\textit{CC-addition chain}. Among several interesting properties, we prove lower bounds on min-length CC-addition

chains. We define the notion of \\GFn-polynomial chain, and use it to count the number of \\textit{non-linear} multiplications required while evaluating polynomials over $\\mathbb{F}_{2^{n}}$. We also give a lower bound on the length of such a chain for any polynomial. As a consequence, we show that a lower bound for the masking complexity of DES S-boxes is three, and that of PRESENT S-box is two. We disprove a claim previously made by Carlet et al. regarding min-length CC-addition chains. Finally, we give a polynomial evaluation method, which results into an improved masking scheme (compared to the technique of Carlet et al.) for DES S-boxes. As an illustration we apply this method to several other S-boxes and show significant improvement for them.

*21:17* [Pub][ePrint]
STES: A Stream Cipher Based Low Cost Scheme for Securing Stored Data, by Debrup Chakraborty and Cuauhtemoc Mancillas-Lopez and Palash Sarkar
The problem of securing data present on USB memories and SD cards has not been adequately addressed in the cryptography literature. While the formal notion of a tweakable enciphering scheme (TES) is well accepted as the proper primitive for secure data storage, the real challenge is to design a low cost TES which can perform at the data rates of the targeted memory devices. In this work, we provide the first answer to this problem. Our solution, called STES, combines a stream cipher with a XOR universal hash function. The securityof STES is rigorously analyzed in the usual manner of provable security approach. By carefully defining appropriate variants of the multi-linear hash function and the pseudo-dot product based

hash function we obtain controllable trade-offs between area and throughput. We combine the hash function with the recent hardware oriented stream ciphers, namely Mickey, Grain and Trivium. Our implementations are targeted towards two low cost FPGAs -- Xilinx Spartan~3 and Lattice ICE40. Simulation results demonstrate

that the speed of encryption/decryption matches the data rates of different USB and SD memories. We believe that our work opens up the possibility of actually putting FPGAs within controllers of such memories to perform low-level in-place encryption.