Short Group Signature without Random Oracles

We construct a short group signature which is proven secure without random oracles. By making certain reasonable assumptions and applying the technique of non-interactive proof system, we prove that our scheme is full anonymity and full traceability. Compared with other related works, such as BW06, BW07, ours is more practical due to the short size of both public key and group signature

On the Random-Oracle Methodology as Applied to Length-Restricted Signature Schemes

In earlier work, we described a "pathological" example of a signature scheme that is secure in the Random Oracle Model, but for which no secure implementation exists. For that example, however, it was crucial that the scheme is able to sign "long messages" (i.e., messages whose length is not a-priori bounded). This left open the possibility that the Random Oracle Methodology is sound with respect to signature schemes that sign only "short" messages (i.e., messages of a-priori bounded length, smaller than the length of the keys in use), and are "memoryless" (i.e., the only thing kept between dierent signature generations is the initial signing-key). In this work, we extend our negative result to address such signature schemes. A key ingredient in our proof is a new type of interactive proof systems, which may be of independent interest

Correcting Subverted Random Oracles

The random oracle methodology has proven to be a powerful tool for designing and reasoning about cryptographic schemes. In this paper, we focus on the basic problem of correcting faulty—or adversarially corrupted—random oracles, so that they can be confidently applied for such cryptographic purposes. We prove that a simple construction can transform a “subverted” random oracle—which disagrees with the original one at a small fraction of inputs—into an object that is indifferentiable from a random function, even if the adversary is made aware of all randomness used in the transformation. Our results permit future designers of cryptographic primitives in typical kleptographic settings (i.e., those permitting adversaries that subvert or replace basic cryptographic algorithms) to use random oracles as a trusted black box

Input-shrinking functions: theory and application

In this thesis, we contribute to the emerging field of the Leakage-Resilient Cryptography by studying the problem of secure data storage on hardware that may leak information, introducing a new primitive, a leakage-resilient storage, and showing two different constructions of such storage scheme provably secure against a class of leakage functions that can depend only on some restricted part of the memory and against a class of computationally weak leakage functions, e.g. functions computable by small circuits, respectively. Our results come with instantiations and analysis of concrete parameters. Furthermore, as second contribution, we present our implementation in C programming language, using the cryptographic library of the OpenSSL project, of a two-party Authenticated Key Exchange (AKE) protocol, which allows a client and a server, who share a huge secret file, to securely compute a shared key, providing client-to-server authentication, also in the presence of active attackers. Following the work of Cash et al. (TCC 2007), we based our construction on a Weak Key Exchange (WKE) protocol, developed in the BRM, and a Password-based Authenticated Key Exchange (PAKE) protocol secure in the Universally Composable (UC) framework. The WKE protocol showed by Cash et al. uses an explicit construction of averaging sampler, which uses less random bits than the random choice but does not seem to be efficiently implementable in practice. In this thesis, we propose a WKE protocol similar but simpler than that one of Cash et al.: our protocol uses more randomness than the Cash et al.'s one, as it simply uses random choice instead of averaging sampler, but we are able to show an efficient implementation of it. Moreover, we formally adapt the security analysis of the WKE protocol of Cash et al. to our WKE protocol. To complete our AKE protocol, we implement the PAKE protocol showed secure in the UC framework by Abdalla et al. (CT-RSA 2008), which is more efficient than the Canetti et al.'s UC-PAKE protocol (EuroCrypt 2005) used in Cash et al.'s work. In our implementation of the WKE protocol, to achieve small constant communication complexity and amount of randomness, we rely on the Random Oracle (RO) model. However, we would like to note that in our implementation of the AKE protocol we need also a UC-PAKE protocol which already relies on RO, as it is impossible to achieve UC-PAKE in the standard model. In our work we focus not only on the theoretical aspects of the area, providing formal models and proofs, but also on the practical ones, analyzing instantiations, concrete parameters and implementation of the proposed solutions, to contribute to bridge the gap between theory and practice in this field

A New Family of Implicitly Authenticated Diffie-Hellman Protocols

Cryptography algorithm standards play a key role both to the practice of information security and to cryptography theory research. Among them, the MQV and HMQV protocols ((H)MQV, in short) are a family of implicitly authenticated Diffie-Hellman key-exchange (DHKE) protocols that are among the most efficient and are widely standardized. In this work, from some new perspectives and under some new design rationales, and also inspired by the security analysis of HMQV, we develop a new family of practical implicitly authenticated DHKE (IA-DHKE) protocols, which enjoy notable performance among security, efficiency, privacy, fairness and easy deployment. We make detailed comparisons between our new protocols and (H)MQV, showing that the newly developed protocols outperform HMQV in most aspects. Very briefly speaking, we achieve: 1. The most efficient provably secure IA-DHKE protocol to date, and the first online-optimal provably secure IA-DHKE protocols. 2. The first IA-DHKE protocol that is provably secure, resilience to the leakage of DH components and exponents, under merely standard assumptions without additionally relying on the knowledge-of-exponent assumption (KEA). 3. The first provably secure privacy-preserving and computationally fair IA-DHKE protocol, with privacy-preserving properties of reasonable deniability and post-ID computability and the property of session-key computational fairness. Guided by our new design rationales, in this work we also formalize and introduce some new concept, say session-key computational fairness (as a complement to session-key security), to the literature

Exploring Trusted Platform Module Capabilities: A Theoretical and Experimental Study

Trusted platform modules (TPMs) are hardware modules that are bound to a computer's motherboard, that are being included in many desktops and laptops. Augmenting computers with these hardware modules adds powerful functionality in distributed settings, allowing us to reason about the security of these systems in new ways. In this dissertation, I study the functionality of TPMs from a theoretical as well as an experimental perspective. On the theoretical front, I leverage various features of TPMs to construct applications like random oracles that are impossible to implement in a standard model of computation. Apart from random oracles, I construct a new cryptographic primitive which is basically a non-interactive form of the standard cryptographic primitive of oblivious transfer. I apply this new primitive to secure mobile agent computations, where interaction between various entities is typically required to ensure security. I prove these constructions are secure using standard cryptographic techniques and assumptions. To test the practicability of these constructions and their applications, I performed an experimental study, both on an actual TPM and a software TPM simulator which has been enhanced to make it reflect timings from a real TPM. This allowed me to benchmark the performance of the applications and test the feasibility of the proposed extensions to standard TPMs. My tests also show that these constructions are practical

Cryptography based on the Hardness of Decoding

This thesis provides progress in the fields of for lattice and coding based cryptography. The first contribution consists of constructions of IND-CCA2 secure public key cryptosystems from both the McEliece and the low noise learning parity with noise assumption. The second contribution is a novel instantiation of the lattice-based learning with errors problem which uses uniform errors