Anonymous authentication of membership in dynamic groups

Thesis (S.B. and M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (leaves 34-36).by Todd C. Parnell.S.B.and M.Eng

Fair Exchange with Guardian Angels

In this paper we propose a new probabilistic Fair Exchange Protocol which requires no central Trusted Third Party. Instead, it relies on a virtually distributed and decentralized Trusted Third Party which is formalized as a Guardian Angel: a kind of Observer e.g. a tamper proof security device. We thus introduce a network model with Pirates and Guardian Angels which is well suited for Ad Hoc networks. In this setting we reduce the Fair Exchange Problem to a Synchronization Problem in which honest parties need to eventually decide whether or not a protocol succeeded in a synchronous way through a hostile network which does not guaranty that sent messages will be eventually received. This problem can be of independent interest in order to add reliability of protocol termination in secure channels

SoK: A Taxonomy for Layer-2 Scalability Related Protocols for Cryptocurrencies

Blockchain based systems, in particular cryptocurrencies, face a serious limitation: scalability. This holds, especially, in terms of number of transactions per second. Several alternatives are currently being pursued by both the research and practitioner communities. One venue for exploration is on protocols that do not constantly add transactions on the blockchain and therefore do not consume the blockchain\u27s resources. This is done using off-chain transactions, i.e., protocols that minimize the interaction with the blockchain, also commonly known as Layer-2 approaches. This work relates several existing off-chain channel methods, also known as payment and state channels, channel network constructions methods, and other components as channel and network management protocols, e.g., routing nodes. All these components are crucial to keep the usability of the channel, and are often overlooked. For the best of our knowledge, this work is the first to propose a taxonomy for all the components of the Layer-2. We provide an extensive coverage on the state-of-art protocols available. We also outline their respective approaches, and discuss their advantages and disadvantages

HyperDegrade: From GHz to MHz Effective CPU Frequencies

acceptedVersionPeer reviewe

Securing Abe\u27s Mix-net Against Malicious Verifiers via Witness Indistinguishability

We show that the simple and appealing unconditionally sound mix-net due to Abe (Asiacrypt\u2799) can be augmented to further guarantee anonymity against malicious verifiers. This additional guarantee implies, in particular, that when applying the Fiat-Shamir transform to the mix-net\u27s underlying sub-protocols, anonymity is provably guaranteed for {\em any} hash function. As our main contribution, we demonstrate how anonymity can be attained, even if most sub-protocols of a mix-net are merely witness indistinguishable (WI). We instantiate our framework with two variants of Abe\u27s mix-net. In the first variant, ElGamal ciphertexts are replaced by an alternative, yet equally efficient, lossy encryption scheme. In the second variant, new dummy vote ciphertexts are injected prior to the mixing process, and then removed. Our techniques center on new methods to introduce additional witnesses to the sub-protocols within the proof of security. This, in turn, enables us to leverage the WI guarantees against malicious verifiers. In our first instantiation, these witnesses follow somewhat naturally from the lossiness of the encryption scheme, whereas in our second instantiation they follow from leveraging combinatorial properties of the Benes-network. These approaches may be of independent interest. Finally, we demonstrate cases in Abe\u27s original mix-net (without modification) where only one witness exists, such that if the WI proof leaks information on the (single) witness in these cases, then the system will not be anonymous against malicious verifiers

Fully Collusion Resistant Trace-and-Revoke Functional Encryption for Arbitrary Identities

Functional Encryption (FE) has been extensively studied in the recent years, mainly focusing on the feasibility of constructing FE for general functionalities, as well as some realizations for restricted functionalities of practical interest, such as inner-product. However, little consideration has been given to the issue of key leakage on FE. The property of FE that allows multiple users to obtain the same functional keys from the holder of the master secret key raises an important problem: if some users leak their keys or collude to create a pirated decoder, how can we identify at least one of those users, given some information about the compromised keys or the pirated decoder? Moreover, how do we disable the decryption capabilities of those users (i.e. traitors)? Two recent works have offered potential solutions to the above traitor scenario. However, the two solutions satisfy weaker notions of security and traceability, can only tolerate bounded collusions (i.e., there is an a priori bound on the number of keys the pirated decoder obtains), or can only handle a polynomially large universe of possible identities. In this paper, we study trace-and-revoke mechanism on FE and provide the first construction of trace-and-revoke FE that supports arbitrary identities, is both fully collusion resistant and fully anonymous. Our construction relies on a generic transformation from revocable predicate functional encryption with broadcast (RPFE with broadcast, which is an extension of revocable predicate encryption with broadcast proposed by Kim and J. Wu at ASIACRYPT\u272020) to trace-and-revoke FE. Since this construction admits a generic construction of trace-and-revoke inner-product FE (IPFE), we instantiate the trace-and-revoke IPFE from the well-studied Learning with Errors (LWE). This is achieved by proposing a new LWE-based attribute-based IPFE (ABIPFE) scheme to instantiate RPFE with broadcast

Group Signatures with Message-Dependent Opening: Formal Definitions and Constructions

This paper introduces a new capability for group signatures called message-dependent opening. It is intended to weaken the high trust placed on the opener; i.e., no anonymity against the opener is provided by an ordinary group signature scheme. In a group signature scheme with message-dependent opening (GS-MDO), in addition to the opener, we set up an admitter that is not able to extract any user’s identity but admits the opener to open signatures by specifying messages where signatures on the specified messages will be opened by the opener. The opener cannot extract the signer’s identity from any signature whose corresponding message is not specified by the admitter. This paper presents formal definitions of GS-MDO and proposes a generic construction of it from identity-based encryption and adaptive non-interactive zero-knowledge proofs. Moreover, we propose two specific constructions, one in the standard model and one in the random oracle model. Our scheme in the standard model is an instantiation of our generic construction but the message-dependent opening property is bounded. In contrast, our scheme in the random oracle model is not a direct instantiation of our generic construction but is optimized to increase efficiency and achieves the unbounded message-dependent opening property. Furthermore, we also demonstrate that GS-MDO implies identity-based encryption, thus implying that identity-based encryption is essential for designing GS-MDO schemes