The upcoming EU Digital Identity (EUDI) Wallet aims to reshape identification and authentication across the EU. Users will receive attestations of identity data from issuers, store them in their wallet, and present them to relying parties. To prevent illegitimate access to sensitive attestation data, the EUDI framework introduces access control policies attached to attestations by their issuers; so-called embedded disclosure policies (EDPs). However, the current EDP framework is inflexible and restricted to simple whitelists and roots-of-trust logic. We improve on the EUDI’s EDP concept, adopting ideas from the literature to build a more dynamic policy-based access control mechanism. We introduce a generic model for flexible, attribute-based access control. We then instantiate such a scheme using DCQL, a query language already established in the EUDI ecosystem. Finally, we also implement and integrate this approach into a real-world EUDI wallet, demonstrating its feasibility.

Quantum computing threatens the foundations of current cryptography, posing critical risks to digital identity systems. This paper presents a modular framework to support the transition of European digital identities toward post-quantum cryptography (PQC). Developed within the context of the POSEIDON project, the framework considers both Self-Sovereign Identity (SSI) principles and the European Digital Identity (EUDI) Wallet ecosystem. Our framework centers on crypto-agile abstraction layers and decouples identity components from underlying cryptographic primitives, enabling the adoption of classical, hybrid, and post-quantum algorithms. The framework supports user-centric credential management, qualified signatures, and resource-constrained IoT environments through interoperable building blocks and lightweight components. Additionally, we complement our framework with the Quantum Risk Assessment Toolkit, a lifecycle-aware recommendation platform that rounds out the framework’s capabilities for achieving an agile PQC transition across different deployment scenarios.

User-centric, privacy-preserving identity wallets-such as those defined under the EU Digital Identity (EUDI) framework-control access to their ecosystem by requiring Relying Parties (RPs) to authenticate and declare their data access permissions. Under eIDAS 2, this is realized through two certificates: Access Certificates (RPACs), which authenticate individual service instances, and Registration Certificates (RPRCs), which specify the attributes a service is permitted to request. However, in the absence of auditability, misissuance and silent revocation of these certificates remain undetectable-undermining user trust and regulatory oversight. To address this gap, we propose RP Certificate Transparency (RPCT): a transparent logging architecture that records both issuance and revocation of RP certificates in an append-only, publicly auditable log. By adapting Certificate Transparency (CT) principles to the EUDI context, RPCT enables monitoring of issued certificates and detection of overly permissive authorizations. Our design addresses known CT limitations, such as lack of revocation transparency, privacy leakage, and monitoring overhead, and adds support for offline-verifiable proofs that preserve user unlinkability. We demonstrate that our architecture meets the regulation's accountability, auditability, and privacy goals. More generally, our architecture represents an efficient, general-purpose transparency service that can be applied to any user-centric credential system.

The increasing adoption of Internet of Things (IoT) in critical sectors like healthcare, energy, and industrial systems underscores the need for robust and secure data backup solutions. IoT devices often handle sensitive and mission-critical data, but they are prone to failure due to cyberattacks, hardware failures, and environmental factors. Ensuring end-to-end security and asynchronous recovery in such environments is essential to maintaining operational continuity and data integrity. We address the challenge of securely backing up sensitive data stored in Trusted Execution Environments (TEEs) on IoT devices. We focus on devices that fail without prior knowledge of replacement devices. Such scenarios necessitate secure backup mechanisms that work without requiring interaction between old and new devices. We formulate several architectures that leverage Key Encapsulation Mechanisms (KEM), Proxy Re-Encryption (PRE), and Attribute-Based Encryption (ABE). We address emerging quantum threats by also considering PQC primitives. We evaluate multiple backup schemes regarding security, flexibility, and resilience in a unified testbed. We further validate two approaches through a full proof-of-concept implementation and experimental evaluation using a real-world IoT testbed. Our findings show that secure, scalable, and quantum-resilient IoT backup systems can be built using modular cryptographic approaches. Our comparative evaluation of multiple backup schemes provides crucial insights for designing future-proof IoT Infrastructures, highlighting the trade-offs between different architectural approaches and cryptographic primitives.

Digital identity ecosystems are rapidly transforming the landscape of identity management. Self-Sovereign Identity (SSI) promises to enhance the individual’s agency over their identity; and related concepts form core parts of the EU’s upcoming eIDAS 2.0 regulation. Yet, this privacy-preserving technology must become less privacy-preserving for one overlooked party – credential issuers. Issuers are trusted to validate users’ attributes and attest to them. Thus, a compromised or misbehaving issuer is an immense threat, being able to issue credentials that allow them to impersonate anyone. We address this concern by introducing Credential Issuance Transparency (CIT), a transparency framework for the issuance of identifying credentials. We take concepts from the Web PKI’s Certificate Transparency (CT), such as using public append-only logs, but adapt them to a privacy-preserving SSI world. In contrast to CT, the public logs of CIT disclose no information about a credential or its subject. Still, genuine subjects can monitor the log to discover mis-issued credentials that would allow an attacker to impersonate them; and empowered by non-interactive zero-knowledge proofs, verifiers can mandate correct logging. CIT is practical. It adds a neglectable overhead of less than 2 ms to credential showing. Daily monitoring for mis-issuance requires less than 1 GB of data to be downloaded, and less than 10 s of computation to be invested. This makes CIT an important step towards SSI’s organizational acceptance and real-world feasibility.

The developers of critical networked applications currently relied on kernel-level protections to ensure the timely delivery of critical messages like Wireless Emergency Alert (WEA) or multi-factor authentication (MFA) notifications. However, the increasing complexity of mobile operating system kernels and network stacks increases the attack surface for adversaries to exploit. In this paper, we introduce a system which safeguards network availability for critical mobile applications against powerful attackers. We achieve this by using the Trusted Execution Environment (TEE) found in most mobile devices to host minimal network drivers. Further, we utilize Trusted I/O to ensure that critical messages reach the end-user even if the device’s kernel is compromised. To demonstrate the feasibility of our approach, we provide a PoC implementation that mimics multi-factor authentication. Our Evaluation demonstrates that latency for all applications is reduced by around $21 \%$ on a representative mobile platform (ARM Cortex A9), though a significant throughput performance is observed.

Delegating authentication to identity providers like Google or Facebook, while convenient, compromises user privacy. These identity providers can record users' every move; the global identifiers they provide also enable internet-wide tracking. We show that neither is a necessary evil by presenting the BISON pseudonym derivation protocol, inspired by Oblivious Pseudorandom Functions. It hides the service provider's identity from the identity provider yet produces a trusted, scoped, immutable pseudonym. Colluding service providers cannot link BISON pseudonyms; this prevents user tracking. BISON does not require a long-lived state on the user device and does not add additional actors to the authentication process. BISON is practical. It is easy to understand, implement, and reason about, and is designed to integrate into existing authentication protocols. To demonstrate this, we provide an OpenID Connect extension that allows OIDC's PPID pseudonyms to be derived using BISON while remaining fully backwards compatible. Additionally, BISON uses only lightweight cryptography. Pseudonym derivation requires a total of four elliptic curve scalar-point multiplications and four hash function evaluations, taking ≈3 ms in our proof of concept implementation. Thus, BISON's privacy guarantees can be realized in practice. This makes BISON a crucial stepping stone towards the privacy-preserving internet of tomorrow.

In credential-based authentication systems (wallets), users transmit personally identifiable and potentially sensitive data to Service Providers (SPs). Here, users must often trust that they are communicating with a legitimate SP and that the SP has a lawful reason for requesting the information that it does. In the event of data misuse, identifying and holding the SP accountable can be difficult. In this paper, we first enumerate the privacy requirements of electronic wallet systems. For this, we explore applicable legal frameworks and user expectations. Based on this, we argue that forcing each user to evaluate each SP individually is not a tractable solution. Instead, we outline technical measures in the form of an SP accreditation system. We delegate trust decisions to an authorized Accreditation Body (AB), which equips each SP with a machine-readable set of data permissions. These permissions are checked and enforced by the user’s wallet software, preventing over-sharing sensitive data. The accreditation body we propose is publicly auditable. By enabling the detection of misconduct, our accreditation system increases user trust and thereby fosters the proliferation of the system.

ERATOSTHENES is an EC, co-funded, research project strongly considering modern security challenges in the domain of Internet of Things in mind of their huge penetration into our day to day lives. There are a series of recent challenges that recently have been converted into obstacles or risk points that could block the secure operation of IoT networks in all day to day activities, from home to office, to leisure and security. These include examples such as the highly increased number of connected devices (at all network levels) that are on top forming inhomogeneous networks and systems of systems. Different vendor characteristics further increase the attack surface that is expected to further rise in the upcoming years. Such, highly critical, characteristics, dramatically increase the needs for confidentiality access control, user and things’ privacy, devices’ trustworthiness and compliance that require lifecycle considerations. The ERATOSTHENES project orchestrates a novel distributed, automated, auditable, yet privacy-respectful, Trust and Identity Management Framework and Reference Architecture with the ultimate scope to dynamically and holistically manage IoT devices in a lifecycle approach, strengthening trust, identities, and resilience in the entire IoT ecosystem while supporting the enforcement of the NIS directive, GDPR and Cybersecurity Act. This publication describes the ERATOSTHENES technical concept and reference architecture as well as design considerations, architecture characteristics, connectivity and interoperability.

The growing interconnectedness of computer systems has led to the need for a flexible approach to trust management. Many countries operate trust schemes to enable the automated assessment of the trustworthiness of information. But this assessment remains a challenge if the information was issued in a foreign trust scheme. An issue is the lack of a root of trust shared between the trust schemes. Other challenges are the heterogeneity of trust models used by entities operating in different legal and cultural environments. In this paper, we present a novel approach to facilitate the interoperability between different trust schemes. In our approach, trust scheme operators take legal agreements that exist between two countries and publish them as a machine-readable trust recognition. Additionally, a scheme operator codifies the rules for trust recognition of the other scheme in the form of a trust translation. Using this information, a trust verifier maps trust data from the other scheme into its own scheme. This allows a verifier to automatically process transactions from other trust schemes in a trustworthy way.

Authentication, authorization, and trust verification are central parts of an access control system. The conditions for granting access in such a system are collected in access policies. Since access conditions are often complex, dedicated languages – policy languages – for defining policies are in use.However, current policy languages are unable to express such conditions having privacy of users in mind. With privacy-preserving technologies, users are enabled to prove information to the access system without revealing it.In this work, we present a generic design for supporting privacy-preserving technologies in policy languages. Our design prevents unnecessary disclosure of sensitive information while still allowing the formulation of expressive rules for access control. For that we make use of zero-knowledge proofs (NIZKs). We demonstrate our design by applying it to the TPL policy language, while using SNARKs. Also, we evaluate the resulting ZK-TPL language and its associated toolchain. Our evaluation shows that for regular-sized credentials communication and verification overhead is negligible.

Background: Recent advances in the design of blockchain-based personal data sharing platforms bring the benefit of empowering users with more control and privacy-preserving measures in sharing data products. However, so far very little is known about users’ intentions to adopt such platforms for providing or consuming data products. Objective: This study aims to investigate users’ main expectations, preferences, and concerns regarding the adoption of blockchain-based personal data sharing platforms in the health and education domains. Methods: Fifteen participants were involved in a multidimensional evaluation of a prototyped release of the KRAKEN blockchain-based data sharing platform and asked to assess it in the health or education pilot domains. Data collected during online group interviews with participants were analyzed by applying the micro interlocutor technique to provide a descriptive overview of participant responses. Results: Participants showed a marginal acceptance of the prototype usability, asking for some improvements of the user experience and for a more transparent presentation of the platform security and privacy preserving capabilities. Participants expressed interest in using the platform as data providers and consumers as well as setting privacy policies for sharing data products with third parties, including the possibility of revoking access to data. Conclusions: Blockchain-based data sharing platforms are more likely to engage target users when technical design is informed by a deeper knowledge of their needs, expectations, and relevant concerns.

Personal data is an attractive source of insights for a diverse field of research and business. While our data is highly valuable, it is often privacy-sensitive. Thus, regulations like the GDPR restrict what data can be legally published, and what a buyer may do with this sensitive data. While personal data must be protected, we can still sell some insights gathered from our data that do not hurt our privacy. A data marketplace is a platform that helps users to sell their data while assisting buyers in discovering relevant datasets. The major challenge such a marketplace faces is balancing between offering valuable insights into data while preserving privacy requirements. Private data marketplaces try to solve this challenge by offering privacy-preserving computations on personal data. Such computations allow for calculating statistics or training machine learning models on personal data without accessing the data in plain. However, the user selling the data cannot restrict who can buy or what type of computation the data is allowed. We close the latter gap by proposing a flexible access control architecture for private data marketplaces, which can be applied to existing data markets. Our architecture enables data sellers to define detailed policies restricting who can buy their data. Furthermore, a seller can control what computation a specific buyer can purchase on the data, and make constraints on its parameters to mitigate privacy breaches. The data market’s computation system then enforces the policies before initiating a computation. To demonstrate the feasibility of our approach, we provide an implementation for the KRAKEN marketplace, a distributed data market using MPC. We show that our approach is practical since it introduces a negligible performance overhead and is secure against several adversaries.

Data marketplaces are online platforms that provide a way for individuals to monetize their (personal) data. In traditional data marketplaces, the data is uploaded to the marketplace platform in plain-text. Since the broker has access to all data, such marketplaces are risk to the users' privacy. We present a privacy-preserving marketplace that allows data owners to keep control over their data. We use secure multi-party computation to enable data consumers to evaluate expressive functions on a set of data. In our marketplace, the broker has neither access to the data nor the results of a function evaluation. Furthermore, our design ensures data-origin authenticity and enables data owners to define data-usage policies that are enforced by the computation nodes. In addition to the architecture of this private marketplace, we discuss a reference implementation. We also provide an evaluation of our approach, demonstrating its practicability.

Trust management systems often use registries to authenticate data, or form trust decisions. Examples are revocation registries and trust status lists. By introducing distributed ledgers (DLs), it is also possible to create decentralized registries. A verifier then queries a node of the respective ledger, e.g., to retrieve trust status information during the verification of a credential. While this ensures trustworthy information, the process requires the verifier to be online and the ledger node available. Additionally, the connection from the verifier to the registry poses a privacy issue, as it leaks information about the user's behavior. In this paper, we resolve these issues by extending existing ledger APIs to support results that are trustworthy even in an offline setting. We do this by introducing attestations of the ledger's state, issued by ledger nodes, aggregatable into a collective attestation by all nodes. This attestation enables a user to prove the provenance of DL-based data to an offline verifier. Our approach is generic. So once deployed it serves as a basis for any use case with an offline verifier. We also provide an implementation for the Ethereum stack and evaluate it, demonstrating the practicability of our approach.

Digital identities play a vital role in an increasingly digital world. These identities often rely on central authorities to issue and manage them. Central authorities have the drawback of being a central trusted party, representing a bottleneck and single point of failure with exclusive control of identity-related data. Self-sovereign identity (SSI) tackles those problems by utilizing distributed ledger technology and making users the sovereign owners of their identity data. Nevertheless, SSI, as recent technology, still lacks qualified identity data. This is especially a problem since sensitive services like eGovernment or banking services require identity data issued by a qualified identity provider; thus, SSI - based identities cannot be used for these services. In this paper, we propose a concept for deriving identity data from an existing identity system into an SSI in a fully privacy-preserving way by additionally supporting offline verification. This way, we enable a chain of trust from the existing identity system to the SSI system by introducing a novel trust model. Our concept utilizes novel cryptographic primitives to support efficient and privacy-preserving identity showing as well as revo-cation. To underline the feasibility of our concept, we implement a proof system and benchmark the related use cases.

Assurance in digital authentication means represents a fundamental requirement in the authentication process of digital identities. Different level-of-assurance (LoA) describe the trustworthiness of the authentication specified by various standards. Some traditional governmental identity systems achieve a high LoA. Nevertheless, the recent self-sovereign identity (SSI) model, which utilizes identity wallets to ensure that the identity data control remains with the related user, still lacks a high LoA, detaining the full potential of SSI such as using it for sensitive use-cases like for eGovernment or public administration services. This work tackles this problem by starting with assessing related LoA standards. Based on this assessment are requirements defined to achieve an LoA high. These requirements are utilized in the process of defining and evaluating our proposed concept. Our generic serves as the foundation for other developers, aiming to elevate the LoA in their SSI systems. The implementation of a proof-of-concept showcases the feasibility and practicability of our concept. In the evaluation, we identify measures provided by our concept, used to meet the defined requirements, and discuss the design decisions.

Trust policies enable the automated processing of trust decisions for electronic transactions. We consider the Trust Policy Language TPL of the LIGHTest project [Mö19] that was designed for businesses and organizations to formulate their trust policies. Using TPL, organizations can decide if and how they want to rely on existing trust schemes like Europe’s eIDAS or trust scheme translations endorsed by them. While the LIGHTest project is geared towards classical approaches like PKI-based trust infrastructures and X.509 certificates, novel concepts are on the rise: one example is the self-sovereign identity (SSI) model that enables users better control of their credentials, offers more privacy, and supports decentralized solutions. Since SSI is based on distributed ledger (DL) technology, it is a question of how TPL can be adapted so that organizations can continue to enjoy the benefits of flexible policy descriptions with automated evaluation at a very high level of reliability. Our contribution is a first step towards integrating SSI and the interaction with a DL into a Trust Policy Language. We discuss this on a more conceptual level and also show required TPL modifications. We demonstrate that we can integrate SSI concepts into TPL without changing the syntax and semantics of TPL itself and have to add new formats and introduce a new built-in predicate for interacting with the DL. Another advantage of this is that the “business logic” aspect of a policy does not need to change, enable re-use of existing policies with the new trust model.