*21:17* [Pub][ePrint]
Automated Analysis and Synthesis of Authenticated Encryption Schemes, by Viet Tung Hoang and Jonathan Katz and Alex J. Malozemoff
Authenticated encryption (AE) schemes are symmetric-key encryption schemes ensuring strong notions of confidentiality and integrity. Although various AE schemes are known, there remains significant interest in developing schemes that are more efficient, meet even stronger security notions (e.g., misuse-resistance), or satisfy certain non-cryptographic properties (e.g., being patent-free).We present an automated approach for analyzing and synthesizing blockcipher-based AE schemes, significantly extending prior work by Malozemoff et al. (CSF 2014) who synthesize encryption schemes satisfying confidentiality only. Our main insight is to restrict attention to a certain class of schemes that is expressive enough to capture several known constructions yet also admits automated reasoning about security. We use our approach to generate thousands of AE schemes with provable security guarantees, both known (e.g., variants of OCB and CCM) and new. Implementing two of these new schemes, we find their performance competitive with state-of-the-art AE schemes.

*21:17* [Pub][ePrint]
Ed448-Goldilocks, a new elliptic curve, by Mike Hamburg
Many papers have proposed elliptic curves which are faster and easier to implement than the NIST prime-order curves. Most of these curves have had fields of size around $2^256$, and thus security estimates of around 128 bits. Recently there has been interest in a stronger curve, prompting designs such as Curve41417 and Microsoft\'s pseudo-Mersenne-prime curves.Here I report on the design of another strong curve, called Ed448-Goldilocks. Implementations of this curve can perform very well for its security level on many architectures. As of this writing, this curve is favored by IRTF CFRG for inclusion in future versions of TLS along with Curve25519.

*21:17* [Pub][ePrint]
Practical Round-Optimal Blind Signatures in the Standard Model, by Georg Fuchsbauer and Christian Hanser and Daniel Slamanig
Round-optimal blind signatures are notoriously hard to construct in the standard model, especially in the malicious-signer model, where blindness must hold under adversarially chosen keys. This is substantiated by several impossibility results. The only construction that can be termed theoretically efficient, by Garg and Gupta (Eurocrypt\'14), requires complexity leveraging, inducing an exponential security loss. We present a construction of practically efficient round-optimal blind signatures in the standard model. It is conceptually simple and builds on the recent structure-preserving signatures on equivalence classes (SPS-EQ) from Asiacrypt\'14. While the traditional notion of blindness follows from standard assumptions, we prove blindness under adversarially chosen keys under an interactive variant of DDH. However, we neither require non-uniform assumptions nor complexity leveraging.

We then show how to extend our construction to partially blind signatures and to blind signatures on message vectors, which yield a construction of one-show anonymous credentials à la \"anonymous credentials light\" (CCS\'13) in the standard model.

Furthermore, we give the first SPS-EQ construction under non-interactive assumptions and show how SPS-EQ schemes imply conventional structure-preserving signatures, which allows us to apply optimality results for the latter to SPS-EQ.

*21:17* [Pub][ePrint]
On Necessary Padding with IO, by Justin Holmgren
We show that the common proof technique of padding a circuit before IO obfuscation is sometimes necessary. That is, assuming indistinguishability obfuscation (IO) and one-way functions exist, we define samplers Sam_0, which outputs (aux_0, C_0), and Sam_1, which outputs (aux_1, C_1) such that: - The distributions (aux_0, iO(C_0)) and (aux_1, iO(C_1)) are perfectly distinguishable.

- For padding s = poly(lambda)$, the distributions (aux_0, iO(C_0||0^s)) and (aux_1, iO(C_1||0^s)) are computationally indistinguishable.

We note this refutes the recent \"Superfluous Padding Assumption\" of Brzuska and Mittelbach.

*21:17* [Pub][ePrint]
An Unconditionally Hiding and Long-Term Binding Post-Quantum Commitment Scheme, by Daniel Cabarcas and Denise Demirel and Florian Göpfert and Jean Lancrenon and Thomas Wunderer
Commitment schemes are among cryptography\'s most important building blocks. Besides their basic properties, hidingness and bindingness, for many applications it is important that the schemes applied support proofs of knowledge. However, all existing solutions which have been proven to provide these protocols are only computationally hiding or are not resistant against quantum adversaries. This is not suitable for long-lived systems, such as long-term archives, where commitments have to provide security also in the long run. Thus, in this work we present a new post-quantum unconditionally hiding commitment scheme that supports (statistical) zero-knowledge protocols and allows to refreshes the binding property over time. The bindingness of our construction relies on the approximate shortest vector problem, a lattice problem which is conjectured to be hard for polynomial approximation factors, even for a quantum adversary. Furthermore, we provide a protocol that allows the committer to prolong the bindingness property of a given commitment, while showing in zero-knowledge fashion that the value committed to did not change. In addition, our construction yields two more interesting features: 1) the ability to \"convert\" a Pedersen commitment into a lattice-based one, and 2) the construction of a hybrid approach whose bindingness relies on the discrete logarithmand approximate shortest vector problems.

*21:17* [Pub][ePrint]
Unconditionally Secure Computation with Reduced Interaction, by Ivan Damgård and Jesper Buus Nielsen
We study the question of how much interaction is needed for unconditionally secure multiparty computation. We first consider the number of messages that need to be sent to compute a non-trivial function (such as the AND of several input bits), assuming that all players have input and get output. We show that for $n$ players and $t$ corruptions, $n(t+3)/2$ messages is necessary, this holds already for semi-honest and static corruption. Note that for functions that can be securely computed in constant round, this bound is tight up to a constant factor. For the case $t=1$ and semi-honest security, we show that $2 n$ messages is also sufficient to compute a rich class of functions efficiently, showing that the bound is exact for $t=1$.

Next, we consider round complexity.

It is a long-standing open problem to determine whether all efficiently computable functions can also be efficiently computed in constant-round with {\\em unconditional} security. Providing a positive answer seems to require completely new ideas for protocol design. Motivated by this, we consider the question of whether we can compute any function securely, while minimizing the interaction of {\\em some of} the players? And if so, how many players can this apply to? Note that we still want the standard security guarantees (correctness, privacy, termination) and we consider the standard communication model with secure point-to-point channels. We answer the questions as follows: for passive security, with $n=2t+1$ players and $t$ corruptions, up to $t$ players can have minimal interaction, i.e., they send 1 message in the first round to each of the $t+1$ remaining players and receive one message from each of them in the last round. Using our result on message complexity, we show that this is (unconditionally) optimal. For malicious security with $n=3t+1$ players and $t$ corruptions, up to $t$ players can have minimal interaction, also this is shown to be optimal.

*21:17* [Pub][ePrint]
More on Impossibility of Virtual Black-Box Obfuscation in Idealized Models, by Mohammad Mahmoody and Ameer Mohammed and Soheil Nematihaji
The celebrated work of Barak et al. (Crypto\'01) ruled out the possibility of virtual black-box (VBB) obfuscation for general circuits. The recent work of Canetti, Kalai, and Paneth (TCC\'15) extended this impossibility to the random oracle model as well assuming the existence of trapdoor permutations (TDPs). On the other hand, the works of Barak et al. (Crypto\'14) and Brakerski-Rothblum (TCC\'14) showed that general VBB obfuscation is indeed possible in idealized graded encoding models. The recent work of Pass and Shelat (Cryptology ePrint 2015/383) complemented this result by ruling out general VBB obfuscation in idealized graded encoding models that enable evaluation of constant-degree polynomials in finite fields.In this work extend the above two impossibly results for general VBB obfuscation in idealized models. In particular we prove the following two results both assuming the existence of TDPs:

* There is no general VBB obfuscation in the generic group model of Shoup (Eurocrypt\'97) for {any abelian} group. By applying our techniques to the setting of Pass and Shelat we extend their result to any (even noncommutative) finite {ring}.

* There is no general VBB obfuscation even in the {random trapdoor permutation oracle} model. Our proof extends to any number of levels of hierarchical trapdoors.

*21:17* [Pub][ePrint]
Phasing: Private Set Intersection using Permutation-based Hashing, by Benny Pinkas and Thomas Schneider and Gil Segev and Michael Zohner
Private Set Intersection (PSI) allows two parties to compute the intersection of private sets while revealing nothing more than the intersection itself. PSI needs to be applied to large data sets in scenarios such as measurement of ad conversion rates, data sharing, or contact discovery. Existing PSI protocols do not scale up well, and therefore some applications use insecure solutions instead.We describe a new approach for designing PSI protocols based on permutation-based hashing, which enables to reduce the length of items mapped to bins while ensuring that no collisions occur. We denote this approach as Phasing, for Permutation-based Hashing Set Intersection. Phasing can dramatically improve the performance

of PSI protocols whose overhead depends on the length of the representations of input items.

We apply Phasing to design a new approach for circuit-based PSI protocols. The resulting protocol is up to 5 times faster than the previously best Sort-Compare-Shuffle circuit of Huang et al. (NDSS 2012). We also apply Phasing to the OT-based PSI protocol of Pinkas et

al. (USENIX Security 2014), which is the fastest PSI protocol to date. Together with additional improvements that reduce the computation complexity by a logarithmic factor, the resulting protocol improves run-time by a factor of up to 20 and can also have better communication overhead than the previously best PSI protocol in that respect. The new protocol is only moderately less efficient

than an insecure PSI protocol that is currently used by real-world applications, and is therefore the first secure PSI protocol that is scalable to the demands and the constraints of current real-world settings.