*04:17* [Pub][ePrint]
Two novel applications of bilinear groups to ABE encryption, by Riccardo Longo and Chiara Marcolla and Massimiliano Sala
Bilinear groups are often used to create Attribute-Based Encryption (ABE) algorithms.In particular, they have been used to create an ABE system with multi authorities, but limited to the ciphertext-policy instance.

Here, for the first time, we propose two multi-authority key-policy ABE systems.

In our first proposal, the authorities may be set up in any moment and without any coordination.

A party can simply act as an ABE authority by creating its own public parameters and issuing private keys to the users.

A user can thus encrypt data choosing both a set of attributes and a set of trusted authorities, maintaining full control unless all his chosen authorities collude against him.

In our second system, the authorities are allowed to collaborate to achieve shorter keys and parameters, enhancing the efficiency of encryption and decryption.

We prove our systems secure under algebraic assumptions on the bilinear groups: the bilinear Diffie-Hellmann assumption and an original variation of the former.

*04:17* [Pub][ePrint]
Partial Garbling Schemes and Their Applications, by Yuval Ishai and Hoeteck Wee
Garbling schemes (aka randomized encodings of functions) represent a function F by a \"simpler\" randomized function F^ such that F^(x) reveals F(x) and no additional information about x. Garbling schemes have found applications in many areas of cryptography. Motivated by the goal of improving the efficiency of garbling schemes, we make the following contributions:- We suggest a general new notion of partial garbling which unifies several previous notions from the literature, including standard garbling schemes, secret sharing schemes, and \"conditional disclosure of secrets\". This notion considers garbling schemes in which part of the input is public, in the sense that it can be leaked by F^.

- We present constructions of partial garbling schemes for (boolean and arithmetic) formulas and branching programs which take advantage of the public input to gain better efficiency.

- We demonstrate the usefulness of the new notion by presenting applications to efficient attribute-based encryption, delegation, and secure computation. In each of these applications, we obtain either new schemes for larger classes of functions or efficiency improvements from quadratic to linear. In particular, we obtain the first ABE scheme in bilinear groups for arithmetic formulas, as well as more efficient delegation schemes for boolean and arithmetic branching programs.

*04:17* [Pub][ePrint]
Ring ORAM: Closing the Gap Between Small and Large Client Storage Oblivious RAM, by Ling Ren and Christopher W. Fletcher and Albert Kwon and Emil Stefanov and Elaine Shi and Marten van Dijk and Sriniv
We present Ring ORAM, a simple and low-latency ORAM construction that can be parameterized for either small or large client storage.Simply by tuning parameters, Ring ORAM matches or exceeds the performance of the best-known small and large client storage schemes and can achieve a constant factor online bandwidth overhead over insecure systems.

We evaluate Ring ORAM in theory and in practice.

On the theory side, we prove that Ring ORAM matches the asymptotic bandwidth and client storage of Path ORAM, the prior-art scheme for small client storage.

Tuning parameters for small client storage, Ring ORAM reduces overall bandwidth relative to Path ORAM by a factor of $2.7\\times$ and reduces online bandwidth to constant (a $57\\times$ improvement over Path ORAM given realistic parameters).

Tuning parameters for large client storage, Ring ORAM outperforms Path ORAM (which is given equal storage) by $4.5\\times$ and SSS ORAM, the prior-art scheme for large client storage, by 16-33\\%.

Using secure processors as a case study for small client storage, Ring ORAM on average reduces ORAM response time by nearly $5\\times$ and improves workload completion time by $2.75\\times$, relative to Path ORAM.

In the large storage setting, running an enterprise file system trace with bursty requests, Ring ORAM adds negligible overhead to response time given a $>100$~Mbps network bandwidth.

By comparison, Burst ORAM incurs large overheads in response time unless network bandwidth $>350$~Mbps.

These results suggest that Ring ORAM is a compelling construction in both large client storage (e.g., file server) and small client storage (e.g., remote secure processor) settings.

*04:17* [Pub][ePrint]
Hierarchical deterministic Bitcoin wallets that tolerate key leakage, by Gus Gutoski and Douglas Stebila
A Bitcoin wallet is a set of private keys known to a user and which allow that user to spend any Bitcoin associated with those keys. In a hierarchical deterministic (HD) wallet, child private keys are generated pseudorandomly from a master private key, and the corresponding child public keys can be generated by anyone with knowledge of the master public key. These wallets have several interesting applications including Internet retail, trustless audit, and a treasurer allocating funds among departments. A specification of HD wallets has even been accepted as Bitcoin standard BIP32.Unfortunately, in all existing HD wallets---including BIP32 wallets---an attacker can easily recover the master private key given the master public key and any child private key. This vulnerability precludes use cases such as a combined treasurer-auditor, and some in the Bitcoin community have suspected that this vulnerability cannot be avoided.

We propose a new HD wallet that is not subject to this vulnerability. Our HD wallet can tolerate the leakage of up to m private keys with a master public key size of O(m). We prove that breaking our HD wallet is at least as hard as the so-called ``one more\'\' discrete logarithm problem.

*04:17* [Pub][ePrint]
Complete Characterization of Fairness in Secure Two-Party Computation of Boolean Functions, by Gilad Asharov and Amos Beimel and Nikolaos Makriyannis and Eran Omri
Fairness is a desirable property in secure computation; informally it means that if one party gets the output of the function, then all parties get the output. Alas, an implication of Cleve\'s result (STOC 86) is that when there is no honest majority, in particular in the important case of the two-party setting, there exist

functions that cannot be computed with fairness. In a surprising result, Gordon et al.\\ (JACM 2011) showed that some interesting functions can be computed with fairness in the two-party setting, and re-opened the question of understanding which Boolean functions can be computed with fairness, and which cannot.

Our main result in this work is a complete characterization of the (symmetric)

Boolean functions that can be computed with fairness in the two-party setting; this settles an open problem of Gordon et al. The characterization is quite simple: A function can be computed with fairness \\emph{if and only if} the all one-vector or the all-zero vector are in the affine span of either the rows or the columns of the matrix describing the function. This is true for both deterministic and randomized functions. To prove the possibility result, we modify the protocol of Gordon et al.\\ (JACM 2011); the resulting protocol computes with full security (and in particular with fairness) all functions that are computable with fairness.

We extend the above result in two directions. First, we completely characterize the Boolean functions that can be computed with fairness in the multiparty case, when the number of parties is constant and at most half of the parties can be malicious.

Second, we consider the two-party setting with asymmetric Boolean functionalities, that is, when the output of each party is one bit, but the outputs are not necessarily the same. We generalize our aforementioned protocol for symmetric functions to handle asymmetric functions, and obtain a sufficient condition for computing such functions with fairness. In addition, we provide a necessary condition for fairness; however, a gap is left between these two conditions.

We then consider a specific asymmetric function in this gap area, and by designing a new protocol, we show that it is computable with fairness. However, we do not give a complete characterization for all functions that lie in this gap, and their classification remains open.

*04:17* [Pub][ePrint]
Robustly Secure Two-Party Authenticated Key Exchange from Ring-LWE, by Xiaopeng Yang, Wenping Ma and Chengli Zhang
Using the hard assumption of Ring-Decision Learning With Errors (DLWE) in the lattice, we propose a new authenticated key exchange (AKE) scheme which is based on Peikert\'s reconciliation technique. Under the CK model, the proposed scheme is provably secure. Compared with the traditional Diffie-Hellman (DH) authenticated key exchange (AKE) schemes, the proposed scheme not only has better efficiency andstronger security but also resists quantum attacks because of the hard assumption on lattice problem. The comparisons between Ring-LWE based ones shows that the proposed scheme protects the shared session key with balanced key derivation function (KDF) compared with those current AKE schemes from LWE.

*04:17* [Pub][ePrint]
COFFE: Ciphertext Output Feedback Faithful Encryption, by Christian Forler and David McGrew and Stefan Lucks and Jakob Wenzel
In this paper we introduce the first authenticated encryption schemebased on a hash function, called COFFE. This research has been

motivated by the challenge to fit secure cryptography into constrained

devices -- some of these devices have to use a hash function, anyway,

and the challenge is to avoid the usage of an additional block cipher

to provide authenticated encryption. COFFE satisfies the common

security requirements regarding authenticated encryption, i.e., IND-CPA-

and INT-CTXT-security. Beyond that, it provides the following

additional security features: resistance against side-channel attacks

and INT-CTXT security in the nonce-misuse scenario. It also support

failure-friendly authentication under reasonable assumptions.

*04:17* [Pub][ePrint]
CONIKS: A Privacy-Preserving Consistent Key Service for Secure End-to-End Communication, by Marcela S. Melara and Aaron Blankstein and Joseph Bonneau and Michael J. Freedman and Edward W. Felten
Recent revelations about government surveillance have significantlyincreased the demand for end-to-end secure communications. However, key management remains a major barrier to adoption. Current

systems are often either vulnerable to a malicious or coerced key directory or they make unrealistic assumptions about user behavior,

for example, that users will verify key fingerprints out of band.

We present CONIKS, a system that provides automated key management for end users capable of seamless integration into existing secure messaging applications. In CONIKS, key servers maintain consistent directories of username-to-public key bindings that

allow participants to detect any equivocation or unexpected key

changes by malicious key servers. CONIKS also preserves user\'s

privacy by ensuring that adversaries cannot harvest large numbers

of usernames from the directories. Our prototype chat application

extends the Off-the-Record Messaging plug-in for Pidgin. A single

commodity server can support up to 10 million users and clients

need only download less than 100 kB per day of additional data.