vetKeys: How a Blockchain Can Keep Many Secrets

We propose a new cryptographic primitive called verifiably encrypted threshold key derivation (vetKD) that extends identity-based encryption with a decentralized way of deriving decryption keys. We show how vetKD can be leveraged on modern blockchains to build scalable decentralized applications (or dapps ) for a variety of purposes, including preventing front-running attacks on decentralized finance (DeFi) platforms, end-to-end encryption for decentralized messaging and social networks (SocialFi), cross-chain bridges, as well as advanced cryptographic primitives such as witness encryption and one-time programs that previously could only be built from secure hardware or using a trusted third party. And all of that by secret-sharing just a single secret key..

Minimizing Information Disclosure in Authentication Transactions with Attribute-Based Credentials (Minimalisatie van vrijgegeven informatie in authenticatietransacties met behulp van attribuut-gebaseerde credentials)

With the rise of information and communication technologies, the need to authenticate individuals to authorize their access to online services or to hold them accountable for their actions has induced the development of a wide variety of authentication systems. Although for determining sole authorization it is mostly sufficient to verify an individual's unlinkable non-identifying properties, virtually all of these systems involve the disclosure of personally identifiable information. This raises numerous security and privacy issues because an uncontrolled dissemination of these data makes individuals vulnerable to identity theft, financial fraud, profiling, monitoring, discredit, or embarrassment. These issues are greatly amplified by technologies that ease data collection, aggregation, analysis, and distribution, by legislation that stipulates the retention of communication data, and by increasingly frequent data breaches where vast amounts of (personal) data records are compromised.Although there exist cryptographic techniques---namely anonymous credentials---that allow individuals to authenticate in a secure and privacy-preserving manner without having to disclose any personal or identifying information, there are no authentication systems that utilize these techniques. While the reasons for this are manifold, there are two major technical inhibitors. On the one hand, the available implementations are very complex and only usable with cryptographic expert knowledge, and, on the other hand, the cryptographic mechanisms alone do not suffice for building an authentication system.In this work, we overcome these inhibitors and present a functional authentication system on the basis of anonymous credentials that is usable without expert knowledge. With our system, service providers can formulate authentication requirements in terms of the minimal properties that users' certified attributes must have, and users can prove that their attributes fulfill these properties without disclosing their values. In situations where accountability is required, users can disclose personally identifying information such that it is only accessible if they misbehave or cause damage---which allows honest users to remain unidentifiable.The main building block of our system is a language framework with formal semantics for expressing the service providers' minimal authentication requirements as well as users' cryptographically backed claims in terms of attribute-based credentials. The framework abstracts away from cryptographic details and focuses solely on easily intelligible concepts. We also provide algorithms for transforming claims expressed in our language into the complex input specifications of the cryptographic implementations---which significantly eases their use for application developers---and for verifying claims with respect to a given policy. On the basis of these results, we develop a full-fledged prototype implementation to prove the concept and its efficiency: we show that our algorithms entail negligible computational overhead with respect to the time needed to generate and verify the cryptographic evidence that supports users' claims.Our system allows for reducing the information that is disclosed in authentication transactions to the necessary minimum and thereby mitigates the aforementioned issues of excessive data release. Its use is advantageous for both users and service providers in that the former benefit from privacy preservation and the latter from reducing the risks associated with holding large sets of sensitive personal information.status: publishe

Access control policy editor and analyzer for policies on a business level

Zsfassung in dt. SparcheAccess control is extensively used as security technology to prevent unauthorized access to protected information and system resources in accordance with a policy. However, the formulation of such policies is a complicated task and requires a lot of technical knowledge. This task can therefore only be performed by security experts but not by the persons that are familiar with the business domain for which the access shall be controlled.This thesis introduces therefore a policy language that expresses the access control behavior on a higher abstraction level --- on the business level. The level elevation is achieved by formulating the policies around the data itself and its meaning to the business. The business meaning is introduced by formalizing business concepts and classifying the data according to these concepts.On top of this new policy language, this thesis proposes a number of analysis algorithms that may be performed by the policy authors in order to answer common questions that arise during the authoring process and to simulate an evaluation of the policies. Moreover, since the authoring of access control policies is not a task that is performed by a single person, a policy delegation mechanism is proposed that allows multiple authors to formulate their policies collaboratively. In order to enable an enforcement of the policies in an existing IT infrastructure without making any changes to the infrastructure, it is shown how the policies on the business level are translated into the standardized policy language XACML.Finally a prototype of a user friendly policy editor and analyzer is created that puts all the pieces together in one tool. This tool enables non-technical policy authors to formulate policies on the business level collaboratively and to perform the introduced analysis algorithms on the policies. To facilitate the formulation of the policies for the non-technical authors, significant emphasis was put on the usability aspect of the prototype.Zugriffskontrolle auf Basis von bestimmten Regelwerken, sogenannten Policies, ist eine weit verbreitete Technik um unautorisierten Zugriff auf geschützte Informationen und Systemressourcen zu verhindern. Die Formulierung von solchen Policies ist eine schwierige Aufgabe, welche ein hohes Maß an technischem Wissen erfordert. Diese Aufgabe kann demnach nur von Sicherheitsexperten durchgeführt werden, aber nicht von den eigentlichen Verantwortlichen, die mit dem jeweiligen Geschäftsbereich vertraut sind, in dem der Zugriff kontrolliert wird.Diese Arbeit stellt daher eine neue Sprache zur Formulierung von Policies vor, welche das Zugriffsverhalten auf höherer Abstraktionsebene, auf der Geschäftsebene, ausdrückt. Die Abstrahierung wird dadurch erreicht, dass die Policies nun mit Hilfe von Begriffen formuliert werden, die in dem jeweiligen Geschäftsbereich geläufig sind.Auf Basis dieser neuen Sprache wird eine Reihe von Analysealgorithmen eingeführt, die den Autoren von Policies dabei helfen, Fragen zu beantworten, die beim Verfassen der Policies häufig auftreten, und die ihn den Entscheidungsfindungsprozesses für einen Zugriff auf Informationen simulieren lassen. Da das Verfassen von Policies eine Aufgabe ist, die meist von mehreren Personen in Zusammenarbeit durchgeführt wird, beschreiben wir einen Mechanismus zur Delegierung der Zugriffskontrolle. Damit die Policies auf der Geschäftsebene auch in bereits bestehenden IT Infrastrukturen eingesetzt werden können ohne Änderungen an dieser Infrastruktur vornehmen zu müssen, wird gezeigt, wie eine Übersetzung der Policies in die standardisierte Sprache namens XACML ausgeführt werden kann.Schließlich wird ein Prototyp eines Policyeditors gezeigt, welcher alle zuvor erwähnten Konzepte und Funktionen in sich vereint. Dieses Werkzeug ermöglicht es auch Personen, die nicht über technisches Wissen verfügen, Policies auf der Geschäftsebene zu Verfassen und die erwähnten Analysefunktionen zur Beantwortung auftretender Fragen durchzuführen. Um für die technisch nicht versierten Autoren den Erstellungsprozess der Policies so einfach wie möglich zu gestalten, wurde bei dem erstellten Editor besonders Wert auf die Benutzerfreundlichkeit gelegt.10

A card requirements language enabling privacy-preserving access control

We address the problem of privacy-preserving access control in distributed systems. Users commonly reveal more personal data than strictly necessary to be granted access to online resources, even though existing technologies, such as anonymous credential systems, offer functionalities that would allow for privacy-friendly authorization. An important reason for this lack of technology adoption is, as we believe, the absence of a suitable authorization language offering adequate expressiveness to address the privacy-friendly functionalities. To overcome this problem, we propose an authorization language that allows for expressing access control requirements in a privacy-preserving way. Our language is independent from concrete technology, thus it allows for specifying requirements regardless of implementation details while it is also applicable for technologies designed without privacy considerations. We see our proposal as an important step towards making access control systems privacy-preserving.

Fine-Grained Disclosure of Access Policies

In open scenarios, where servers may receive requests to access their services from possibly unknown clients, access control is typically based on the evaluation of (certified or uncertified) properties, that clients can present. Since assuming the client to know a-priori the properties she should present to acquire access is clearly limiting, servers should be able to respond to client requests with information on the access control policies regulating access to the requested services. In this paper, we present a simple, yet flexible and expressive, approach for allowing servers to specify disclosure policies, regulating if and how access control policies on services can be communicated to clients. Our approach allows fine-grain specifications, thus capturing different ways in which policies, and portions thereof, can be communicated. We also define properties that can characterize the client view of the access control policy

Empagliflozin in Patients with Chronic Kidney Disease

Background The effects of empagliflozin in patients with chronic kidney disease who are at risk for disease progression are not well understood. The EMPA-KIDNEY trial was designed to assess the effects of treatment with empagliflozin in a broad range of such patients. Methods We enrolled patients with chronic kidney disease who had an estimated glomerular filtration rate (eGFR) of at least 20 but less than 45 ml per minute per 1.73 m(2) of body-surface area, or who had an eGFR of at least 45 but less than 90 ml per minute per 1.73 m(2) with a urinary albumin-to-creatinine ratio (with albumin measured in milligrams and creatinine measured in grams) of at least 200. Patients were randomly assigned to receive empagliflozin (10 mg once daily) or matching placebo. The primary outcome was a composite of progression of kidney disease (defined as end-stage kidney disease, a sustained decrease in eGFR to < 10 ml per minute per 1.73 m(2), a sustained decrease in eGFR of & GE;40% from baseline, or death from renal causes) or death from cardiovascular causes. Results A total of 6609 patients underwent randomization. During a median of 2.0 years of follow-up, progression of kidney disease or death from cardiovascular causes occurred in 432 of 3304 patients (13.1%) in the empagliflozin group and in 558 of 3305 patients (16.9%) in the placebo group (hazard ratio, 0.72; 95% confidence interval [CI], 0.64 to 0.82; P < 0.001). Results were consistent among patients with or without diabetes and across subgroups defined according to eGFR ranges. The rate of hospitalization from any cause was lower in the empagliflozin group than in the placebo group (hazard ratio, 0.86; 95% CI, 0.78 to 0.95; P=0.003), but there were no significant between-group differences with respect to the composite outcome of hospitalization for heart failure or death from cardiovascular causes (which occurred in 4.0% in the empagliflozin group and 4.6% in the placebo group) or death from any cause (in 4.5% and 5.1%, respectively). The rates of serious adverse events were similar in the two groups. Conclusions Among a wide range of patients with chronic kidney disease who were at risk for disease progression, empagliflozin therapy led to a lower risk of progression of kidney disease or death from cardiovascular causes than placebo