143 research outputs found
Issuer-Free Oblivious Transfer with Access Control Revisited
Oblivious transfer with access control (OTAC) is an extension of oblivious transfer where each message is associated with an access control policy. A receiver can obtain a message only if her attributes satisfy the access control policy for that message. In most schemes, the receiver's attributes are certified by an issuer. Recently, two Issuer-Free OTAC protocols have been proposed. We show that the security definition for Issuer-Free OTAC fulfilled by those schemes poses a problem. Namely, the sender is not able to attest whether a receiver possesses a claimed attribute. Because of this problem, in both Issuer-Free OTAC protocols, any malicious receiver can obtain any message from the sender, regardless of the access control policy associated with the message. To address this problem, we propose a new security definition for Issuer-Free OTAC. Our definition requires the receiver to prove in zero-knowledge to the sender that her attributes fulfill some predicates. Our definition is suitable for settings with multiple issuers because it allows the design of OTAC protocols where the receiver, when accessing a record, can hide the identity of the issuer that certified her attributes
Oblivious Enforcement of Hidden Information Release Policies Using Online Certification Authorities
This thesis examines a new approach to attribute-based access control with hidden policies and hidden credentials. In this setting, a resource owner has an access control policy that is a function of Boolean-valued attributes of the resource requester. Access to the resource should be granted if and only if the resource owner's policy is satisfied, but we wish to hide the access control policy from the resource requester and the requester's attributes from the resource owner.Previous solutions to this problem involved the use of cryptographic credentials held by the resource requester, but it is obvious that if no information is provided about the access control policy, then the resource requester must try to satisfy the policy using every available credential. An initial contribution of this thesis is the first published empirical evaluation of the state-of-the-art protocol of Frikken, Atallah, and Li for access control with hidden policies and hidden credentials, demonstrating that the computational cost of the required cryptographic operations is highly burdensome.A new system model is then proposed that includes the active involvement of online certification authorities (CAs). These are entities that can provide authoritative information about the attributes in a resource owner's access control policy. Allowing the resource owner to query these online CAs immediately removes the need for the resource requester to guess which credentials to use.If the resource owner was allowed to learn the values of a requester's attributes from online CAs, however, the requester's credentials would no longer be private. This thesis examines cryptographic solutions in which the CAs' replies do not directly reveal any attribute information to the resource owner, but can nevertheless be used in the enforcement of an access control policy. The techniques considered involve scrambled circuit evaluation, homomorphic encryption, and secure multiparty computation using arithmetic circuits and Shamir secret sharing. Empirical experiments demonstrate that the proposed protocols can provide an order-of-magnitude performance improvement over existing solutions
Non-Interactive Blind Signatures for Random Messages
Blind signatures allow a signer to issue signatures on messages chosen by the signature recipient. The main property is that the recipient\u27s message is hidden from the signer. There are many applications, including Chaum\u27s e-cash system and Privacy Pass, where no special distribution of the signed message is required, and the message can be random. Interestingly, existing notions do not consider this practical use case separately.
In this paper, we show that constraining the recipient\u27s choice over the message distribution spawns a surprising new primitive that improves the well-established state-of-the-art. We formalize this concept by introducing the notion of non-interactive blind signatures (). Informally, the signer can create a presignature with a specific recipient in mind, identifiable via a public key. The recipient can use her secret key to finalize it and receive a blind signature on a random message determined by the finalization process. The key idea is that online interaction between the signer and recipient is unnecessary. We show an efficient instantiation of in the random oracle model from signatures on equivalence classes.
The exciting part is that, in this case, for the recipient\u27s public key, we can use preexisting keys for Schnorr, ECDSA signatures, El-Gamal encryption scheme, or even the Diffie-Hellman key exchange. Reusing preexisting public keys allows us to distribute anonymous tokens similarly to cryptocurrency airdropping. Additional contributions include tagged non-interactive blind signatures () and their efficient instantiation. A generic construction in the random oracle or common reference string model based on verifiable random functions, standard signatures, and non-interactive proof systems
Privacy Enhancing Protocols using Pairing Based Cryptography
This thesis presents privacy enhanced cryptographic constructions,
consisting of formal definitions, algorithms and motivating
applications. The contributions are a step towards the development of
cryptosystems which, from the design phase, incorporate privacy as a
primary goal. Privacy offers a form of protection over personal and
other sensitive data to individuals, and has been the subject of much
study in recent years.
Our constructions are based on a special type of algebraic group called
bilinear groups. We present existing cryptographic constructions which
use bilinear pairings, namely Identity-Based Encryption (IBE). We define
a desirable property of digital signatures, blindness, and present new
IBE constructions which incorporate this property.
Blindness is a desirable feature from a privacy perspective as it allows
an individual to obscure elements such as personal details in the data
it presents to a third party. In IBE, blinding focuses on obscuring
elements of the identity string which an individual presents to the key
generation centre. This protects an individual's privacy in a direct
manner by allowing her to blind sensitive elements of the identity
string and also prevents a key generation centre from subsequently
producing decryption keys using her full identity string. Using blinding
techniques, the key generation centre does not learn the full identity
string.
In this thesis, we study selected provably-secure cryptographic
constructions. Our contribution is to reconsider the design of such
constructions with a view to incorporating privacy. We present the new,
privacy-enhanced cryptographic protocols using these constructions as
primitives. We refine useful existing security notions and present
feasible security definitions and proofs for these constructions
Privacy-Preserving Electronic Ticket Scheme with Attribute-based Credentials
Electronic tickets (e-tickets) are electronic versions of paper tickets,
which enable users to access intended services and improve services'
efficiency. However, privacy may be a concern of e-ticket users. In this paper,
a privacy-preserving electronic ticket scheme with attribute-based credentials
is proposed to protect users' privacy and facilitate ticketing based on a
user's attributes. Our proposed scheme makes the following contributions: (1)
users can buy different tickets from ticket sellers without releasing their
exact attributes; (2) two tickets of the same user cannot be linked; (3) a
ticket cannot be transferred to another user; (4) a ticket cannot be double
spent; (5) the security of the proposed scheme is formally proven and reduced
to well known (q-strong Diffie-Hellman) complexity assumption; (6) the scheme
has been implemented and its performance empirically evaluated. To the best of
our knowledge, our privacy-preserving attribute-based e-ticket scheme is the
first one providing these five features. Application areas of our scheme
include event or transport tickets where users must convince ticket sellers
that their attributes (e.g. age, profession, location) satisfy the ticket price
policies to buy discounted tickets. More generally, our scheme can be used in
any system where access to services is only dependent on a user's attributes
(or entitlements) but not their identities.Comment: 18pages, 6 figures, 2 table
Privacy Enhancing Technologies for solving the privacy-personalization paradox : taxonomy and survey
Personal data are often collected and processed in a decentralized fashion, within
different contexts. For instance, with the emergence of distributed applications,
several providers are usually correlating their records, and providing personalized services to their clients. Collected data include geographical and indoor
positions of users, their movement patterns as well as sensor-acquired data that
may reveal usersâ physical conditions, habits and interests. Consequently, this
may lead to undesired consequences such as unsolicited advertisement and even
to discrimination and stalking. To mitigate privacy threats, several techniques
emerged, referred to as Privacy Enhancing Technologies, PETs for short.
On one hand, the increasing pressure on service providers to protect usersâ privacy resulted in PETs being adopted. One the other hand, service providers
have built their business model on personalized services, e.g. targeted ads and
news. The objective of the paper is then to identify which of the PETs have the
potential to satisfy both usually divergent - economical and ethical - purposes.
This paper identifies a taxonomy classifying eight categories of PETs into three
groups, and for better clarity, it considers three categories of personalized services. After defining and presenting the main features of PETs with illustrative
examples, the paper points out which PETs best fit each personalized service
category.
Then, it discusses some of the inter-disciplinary privacy challenges that may
slow down the adoption of these techniques, namely: technical, social, legal and
economic concerns. Finally, it provides recommendations and highlights several
research directions
Cryptographic techniques for privacy and access control in cloud-based applications
Digitization is one of the key challenges for todayâs industries and society. It affects more and more business areas and also user data and, in particular, sensitive information. Due to its sensitivity, it is important to treat personal information as secure and private as possible yet enabling cloud-based software to use that information when requested by the user. In this thesis, we focus on the privacy-preserving outsourcing and sharing of data, the querying of outsourced protected data, and the usage of personal information as an access control mechanism for rating platforms, which should be protected from coercion attacks. In those three categories, we present cryptographic techniques and protocols that push the state of the art. In particular, we first present multi-client oblivious RAM (ORAM), which augments standard ORAM with selective data sharing through access control, confidentiality, and integrity. Second, we investigate on recent work in frequency-hiding order-preserving encryption and show that the state of the art misses rigorous treatment, allowing for simple attacks against the security of the existing scheme. As a remedy, we show how to fix the security definition and that the existing scheme, slightly adapted, fulfills it. Finally, we design and develop a coercion-resistant rating platform. Coercion-resistance has been dealt with mainly in the context of electronic voting yet also affects other areas of digital life such as rating platforms.Die Digitalisierung ist eine der gröĂten Herausforderungen fĂŒr Industrie und Gesellschaft. Neben vielen GeschĂ€ftsbereichen betrifft diese auch, insbesondere sensible, Nutzerdaten. Daher sollten persönliche Informationen so gut wie möglich gesichert werden. Zugleich brauchen Cloud-basierte Software-Anwendungen, die der Nutzer verwenden möchte, Zugang zu diesen Daten. Diese Dissertation fokussiert sich auf das sichere Auslagern und Teilen von Daten unter Wahrung der PrivatsphĂ€re, auf das Abfragen von geschĂŒtzten, ausgelagerten Daten und auf die Nutzung persönlicher Informationen als Zugangsberechtigung fĂŒr erpressungsresistente Bewertungsplattformen. Zu diesen drei Themen prĂ€sentieren wir kryptographische Techniken und Protokolle, die den Stand der Technik voran treiben. Der erste Teil stellt Multi-Client Oblivious RAM (ORAM) vor, das ORAM durch die Möglichkeit, Daten unter Wahrung von Vertraulichkeit und IntegritĂ€t mit anderen Nutzern zu teilen, erweitert. Der zweite Teil befasst sich mit Freuquency-hiding Order-preserving Encryption. Wir zeigen, dass dem Stand der Technik eine formale Betrachtung fehlt, was zu Angriffen fĂŒhrt. Um Abhilfe zu schaffen, verbessern wir die Sicherheitsdefinition und beweisen, dass das existierende VerschlĂŒsselungsschema diese durch minimale Ănderung erfĂŒllt. AbschlieĂend entwickeln wir ein erpressungsresistentes Bewertungsportal. Erpressungsresistenz wurde bisher hauptsĂ€chlich im Kontext von elektronischen Wahlen betrachtet
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