*19:17* [Pub][ePrint]
Adaptively Secure Fully Homomorphic Signatures Based on Lattices, by Xavier Boyen and Xiong Fan and Elaine Shi
In a homomorphic signature scheme, given the public key and a vector of signatures $\\vec{\\sigma}:= (\\sigma_1, \\ldots, \\sigma_l)$ over $l$ messages $\\vec{\\mu}:= (\\mu_1, \\ldots, \\mu_l)$, there exists an efficient algorithm to produce a signature $\\sigma\'$ for $\\mu = f(\\vec{\\mu})$. Given the tuple $(\\sigma\', \\mu, f)$, anyone can then publicly verify the validity of the signature $\\sigma\'$.Inspired by the recent (selectively secure) key-homomorphic functional encryption for circuits, recent works propose fully homomorphic signature schemes in the selective security model.

However, in order to gain adaptive security, one must rely on generic complexity leveraging, which is not only very inefficient but also leads to reductions that are ``unfalsifiable\'\'.

In this paper, we construct the first \\emph{adaptively secure} homomorphic signature scheme that can evaluate any circuit over signed data. For {\\it poly-logarithmic depth} circuits, our scheme achieves adaptive security under the standard {\\it Small Integer Solution} (SIS) assumption. For {\\it polynomial depth} circuits, the security of our scheme relies on sub-exponential SIS --- but unlike complexity leveraging, the security loss in our reduction depends only on circuit depth and on neither message length nor dataset size.

*13:17* [Pub][ePrint]
Practical UC security with a Global Random Oracle, by Ran Canetti and Abhishek Jain and Alessandra Scafuro
We show that there exist commitment, zero-knowledge and general function evaluation protocols with universally composable security, in a model where all parties and all protocols have access to a single, global, random oracle and no other trusted setup. This model provides significantly stronger composable security guarantees than the traditional random oracle model of Bellare and Rogaway [CCS\'93] or even the common reference string model. Indeed, these latter models provide no security guarantees in the presence of arbitrary protocols that use the same random oracle (or reference string or hash function).Furthermore, our protocols are highly efficient. Specifically, in the interactive setting, our commitment and general computation protocols are much more efficient than the best known ones due to Lindell [Crypto\'11,\'13] which are secure in the common reference string model. In the non-interactive setting, our protocols are slightly less efficient than the best known ones presented by Afshar et al. [Eurocrypt \'14] but do away with the need to rely on a non-global (programmable) reference string.

*13:17* [Pub][ePrint]
Robust Secret Sharing Schemes Against Local Adversaries, by Allison Bishop Lewko and Valerio Pastro
We study robust secret sharing schemes in which between one third and one half of the players are corrupted. In this scenario, robust secret sharing is possible only with a share size larger than the secrets, and allowing a positive probability of reconstructing the wrong secret. In the standard model, it is known that at least $m+k$ bits per share are needed to robustly share a secret of bit-length $m$ with an error probability of $2^{-k}$; however, to the best of our knowledge, the efficient scheme that gets closest to this lower bound has share size $m+\\widetilde O (n+k)$, where $n$ is the number of players in the scheme.We show that it is possible to obtain schemes with close to minimal share size in a model of local adversaries, i.e. in which corrupt players cannot communicate between receiving their respective honest shares and submitting corrupted shares to the reconstruction procedure, but may coordinate before the execution of the protocol and can also gather information afterwards.

In this limited adversarial model, we prove a lower bound of roughly $m+k$ bits on the minimal share size, which is (somewhat surprisingly) similar to the lower bound in the standard model, where much stronger adversaries are allowed.

We then present an efficient secret sharing scheme that essentially meets our lower bound, therefore improving upon the best known constructions in the standard model by removing a linear dependence on the number of players. For our construction, we introduce a novel procedure that compiles an error correcting code into a new randomized one, with the following two properties: a single local portion of a codeword leaks no information on the encoded message itself, and any set of portions of a codeword reconstructs the message with error probability exponentially low in the set size.

*13:17* [Pub][ePrint]
Adaptive Multiparty Non-interactive Key Exchange Without Setup In The Standard Model, by Vanishree Rao
Non-interactive key exchange (NIKE) is a fundamental notion in Cryptography. This notion was introduced by Diffie and Hellman in 1976. They proposed the celebrated 2-party NIKE protocol and left open as a fascinating question, whether NIKE could be realized in the multiparty setting. NIKE has since then been an active area of research with an ultimate goal of obtaining best possible security in the multiparty setting. Although this has evaded researchers for many decades, advancements have been made through relaxations in multiple directions such as restricting to 3-parties, static/semi-static model (where the adversary needs to commit to the set of parties he wishes to be challenged upon ahead of time), random-oracle model, allowing initial setup, etc.In this work, we settle the longstanding open question: we present the first multiparty NIKE protocol that is adaptively secure with no setup and in the standard model.

Our construction is based on indistinguishability obfuscation and obliviously-patchable puncturable pseudorandom functions, a new notion that we introduce.

We employ novel techniques of using indistinguishability obfuscation, which are interesting in their own right and which we believe would find wider applications in other settings. One such technique pertains overcoming, the somewhat inherent, drawback of non-adaptivity of the puncturing technique introduced by Sahai and Waters [STOC\'14]. Central to this technique is our new notion of obliviously-patchable puncturable pseudorandom functions. We present a concrete construction of these pseudorandom functions using multilinear maps.

Note that pseudorandom functions amounts to an interactive assumption. We shall establish via a meta-reduction technique that, in natural settings, an interactive assumption is necessary (even with setup).

*13:17* [Pub][ePrint]
A Denial of Service Attack against Fair Computations using Bitcoin Deposits, by Jethro Beekman
Bitcoin supports complex transactions where the recipient of a transaction can be programmatically determined.Using these transactions, multi-party computation protocols that aim to ensure fairness among participants have been designed.

We present a Denial of Service attack against these protocols that results in a net loss for some or all of the honest parties involved, violating those fairness goals.

*13:17* [Pub][ePrint]
Low-Cost Concurrent Error Detection for GCM and CCM, by Xiaofei Guo and Ramesh Karri
In many applications, encryption alone does not provide enough security. To enhance security, dedicated authenticated encryption (AE) mode are invented. Galios Counter Mode (GCM) and Counter with CBC-MAC mode (CCM) are the AE modes recommended by the National Institute of Standards and Technology. To support high data rates, AE modes are usually implemented in hardware. However, natural faults reduce its reliability and may undermine both its encryption andauthentication capability. We present a low-cost concurrent error detection (CED) scheme for 7 AE architectures. The proposed technique explores idle cycles of the AE mode architectures. Experimental results shows that the performance overhead can be lower than 100% for all architectures depending on the workload. FPGA implementation results show that the hardware overhead in the 0.1-23.3% range and

the power overhead is in the 0.2-23.2% range. ASIC implementation results show that the hardware overhead in the 0.1-22.8% range and the power overhead is in the 0.3-12.6% range. The underlying block cipher and hash module need not have CED built in. Thus, it allows system designers to integrate block cipher and hash function intellectual property from different vendors.