*10:17* [Pub][ePrint]
Completeness for Symmetric Two-Party Functionalities - Revisited, by Yehuda Lindell and Eran Omri and Hila Zarosim
Understanding the minimal assumptions required for carrying out cryptographic tasks is one of the fundamental goals of theoretical cryptography. A rich body of work has been dedicated to understanding the complexity of cryptographic tasks in the context of (semi-honest) secure two-party computation. Much of this work has focused on the characterization of trivial and complete functionalities (resp., functionalities that can be securely implemented unconditionally, and functionalities that can be used to securely compute all functionalities).All previous works define reductions via an ideal implementation of the functionality; \\ie $f$ reduces to $g$ if one can implement $f$ using an ideal box (or oracle) that computes the function $g$ and returns the output to both parties. Such a reduction models the computation of $f$ as an \\emph{atomic operation}. However, in the real-world, protocols proceed in rounds, and the output is not learned by the parties simultaneously. In this paper we show that this distinction is significant. Specifically, we show that there exist symmetric functionalities (where both parties receive the same outcome), that are neither trivial nor complete under ``ideal-box reductions\'\', and yet the existence of a constant-round protocol for securely computing such a functionality implies infinitely-often oblivious transfer (meaning that it is secure for infinitely-many $n$\'s).

In light of the above, we propose an alternative definitional infrastructure for studying the triviality and completeness of functionalities.

*15:42* [PhD][New]
Kwangsu Lee: Efficient Hidden Vector Encryptions and Its Applications
Name: Kwangsu Lee

Topic: Efficient Hidden Vector Encryptions and Its Applications

Category: public-key cryptography

Description: \r\nPredicate encryption is a new paradigm of public key encryption that enables searches on encrypted data. Using the predicate encryption, we can search keywords or attributes on encrypted data without decrypting the ciphertexts. In predicate encryption, a ciphertext is associated with attributes and a token corresponds to a predicate. The token that corresponds to a predicate $f$ can decrypt the ciphertext associated with attributes $\\vect{x}$ if and only if $f(\\vect{x})=1$.\r\n

\r\nHidden vector encryption (HVE) is a special kind of predicate encryption. HVE supports the evaluation of conjunctive equality, comparison, and subset operations between attributes in ciphertexts and attributes in tokens. Currently, several HVE schemes were proposed where the ciphertext size, the token size, and the decryption cost are proportional to the number of attributes in the ciphertext. In this thesis, we consider the efficiency, the generality, and the security of HVE schemes. The results of this thesis are described as follows.\r\n

\r\nThe first results of this thesis are efficient HVE schemes where the token consists of just four group elements and the decryption only requires four bilinear map computations, independent of the number of attributes in the ciphertext. The construction uses composite order bilinear groups and is selectively secure under the well-known assumptions.\r\n

\r\nThe second results are efficient HVE schemes that are secure under any kind of pairing types. To achieve our goals, we proposed a general framework that converts HVE schemes from composite order bilinear groups to prime order bilinear groups. Using the framework, we convert the previous HVE schemes from composite order bilinear groups to prime order bilinear groups.\r\n

\r\nThe third results are fully secure HVE schemes with short tokens. Previous HVE schemes were proven to be secure only in the selective security model where the capabilities of the adversaries are[...]

*15:34* [PhD][New]
Zachary Kissel: Verifiable Symmetric Searchable Encryption
Name: Zachary Kissel

Topic: Verifiable Symmetric Searchable Encryption

Category: secret-key cryptography

Description: Cloud storage has become increasingly prevalent in recent years. It provides a convenient platform for users to store data that can be accessed from anywhere at anytime without the cost of maintaining a storage infrastructure. However, cloud storage is inherently insecure, hindering general acceptance of the paradigm shift. To make use of storage services provided by a cloud, users would need to place their trust, at least implicitly, in the provider. There have been a number of attempts to alleviate the need for this trust through cryptographic methods. An immediate approach would be to encrypt each file before uploading it to the cloud. This approach, calls for a new searching mechanism over encrypted data stored in the cloud.\r\n

\r\nThis dissertation considers a solution to this problem using Symmetric Searchable Encryption (SSE). SSE allows users to offload search queries to the cloud. The cloud is then responsible for returning the encrypted files that match the search queries (also encrypted). Most previous work was focused on keyword search in the Honest-but-Curious (HBC) cloud model, while some more recent work has considered searching on phrases. Recently, a new cloud model was introduced that supersedes the HBC model. This new model, called Semi-Honest but Curious (SHBC), is less restrictive over the actions a cloud can take. In this dissertation, we present three systems that are secure under this new SHBC model. Two systems provide phrase search and the other provides hierarchical access control over keyword search.

[...]

*13:17* [Pub][ePrint]
Efficient Non-Interactive Zero Knowledge Arguments for Set Operations, by Prastudy Fauzi and Helger Lipmaa and Bingsheng Zhang
We propose a non-interactive zero knowledge \\emph{pairwise multiset sum equality test (PMSET)} argument in the common reference string (CRS) model that allows a prover to show that the given committed multisets $\\AAA_j$ for $j \\in \\set{1, 2, 3, 4}$ satisfy $\\AAA_1 \\uplus \\AAA_2 = \\AAA_3 \\uplus \\AAA_4$, i.e., every element is contained in $\\AAA_1$ and $\\AAA_2$ exactly as many times as in $\\AAA_3$ and $\\AAA_4$.As a corollary to the $\\PUTME$ argument, we present arguments that enable to efficiently verify the correctness of various (multi)set operations, for example, that one committed set is the intersection or union of two other committed sets.

The new arguments have constant communication and verification complexity (in group elements and group operations, respectively), whereas the CRS length and the prover\'s computational complexity are both proportional to the cardinality of the (multi)sets.

We show that one can shorten the CRS length at the cost of a small increase of the communication and the verifier\'s computation.