Privacy-Preserving Authentication: Theory vs. Practice

Abstract: With the increasing use of online services, the protection of the privacy of users becomes more and more important. This is particularly critical as authentication and authorization as realized on the Internet nowadays, typically relies on centralized identity management solutions. Although those are very convenient from a user's perspective, they are quite intrusive from a privacy perspective and are currently far from implementing the concept of data minimization. Fortunately, cryptography offers exciting primitives such as zero-knowledge proofs and advanced signature schemes to realize various forms of so-called anonymous credentials. Such primitives allow to realize online authentication and authorization with a high level of built-in privacy protection (what we call privacy-preserving authentication). Though these primitives have already been researched for various decades and are well understood in the research community, unfortunately, they lack widespread adoption. In this paper, we look at the problems, what cryptography can do, some deployment examples, and barriers to widespread adoption. Latter using the example of the EU Digital Identity Wallet (EUDIW) and the recent discussion and feedback from cryptography experts around this topic. We also briefly comment on the transition to post-quantum cryptography.

• The paper provides a comprehensive analysis of privacy‐preserving authentication, highlighting the potential of anonymous credentials to enhance digital privacy.
• It examines cryptographic techniques such as zero‐knowledge proofs and BBS signatures, demonstrating how they support unlinkability and minimal data disclosure.
• The paper identifies practical barriers including hardware limitations and standardization gaps that challenge the implementation of advanced privacy protocols.

Privacy-Preserving Authentication: Theory vs. Practice - An Analytical Overview

The paper "Privacy-Preserving Authentication: Theory vs. Practice" by Daniel Slamanig provides a comprehensive examination of privacy-preserving authentication mechanisms, exploring the theoretical underpinnings, practical implementations, and challenges that inhibit widespread adoption. The discussion juxtaposes current authentication practices with advanced cryptographic techniques, emphasizing the critical importance of privacy in digital identity management.

Current Authentication Landscape and Privacy Concerns

Authentication and authorization processes are fundamental to accessing digital resources. However, traditional methods, such as password-based and single sign-on (SSO) systems, often compromise privacy. Password-based authentication is convenient yet prone to weak password creation and reuse, while SSO centralizes identity management, posing significant privacy risks as Identity Providers (IdPs) can track user activities across multiple services. These approaches generally lack robust data minimization, where the principle stipulates revealing only minimal identifying information necessary for authentication.

The Promise of Anonymous Credentials

Anonymous credentials offer a cryptographic solution to the privacy challenges of conventional systems. These credentials utilize advanced techniques, including zero-knowledge proofs (ZKPs) and signatures, to enable users to authenticate while selectively disclosing attributes. This mechanism inherently supports unlinkability, making it difficult for interactions to be traced back to a user, even in cases of IdP and relying party collusion. Such a system not only enhances privacy but also aligns with regulatory mandates like the GDPR.

Practical Barriers to Adoption

Despite the maturity of the theoretical framework surrounding anonymous credentials, real-world deployment is limited. Several factors contribute to this gap. Technical barriers include the dependence on cryptographic primitives that are not yet standardized or widely supported by existing hardware security modules (HSMs), smart cards, or TPMs, which predominantly support legacy cryptography standards like RSA and ECDSA.

The transition to post-quantum cryptography introduces additional complexity. While current anonymous credential systems can employ post-quantum techniques such as lattice-based cryptography, these are not yet fully mature or standardized. Nonetheless, the inherent unconditional privacy of certain anonymous credential systems, like those based on BBS signatures, ensures continued privacy protection irrespective of quantum advancements.

The European Union Digital Identity Wallet (EUDIW)

The European Union's initiative to implement the Digital Identity Wallet exemplifies the potential integration of anonymous credentials in large-scale infrastructures. However, initial designs focused on conventional digital signatures with insufficient attention to unlinkability. The paper argues for employing anonymous credentials, like BBS-signature-based systems, as they provide the desired level of privacy. Despite expert recommendations, challenges remain due to existing system compatibilities and the nascent state of requisite standards.

Future Directions and Considerations

The future landscape of privacy-preserving authentication depends significantly on overcoming the technical and infrastructural challenges identified in the paper. This requires a concerted effort towards standardization and the development of cryptographic protocols that can seamlessly integrate with existing and forthcoming technologies. Additionally, as quantum resistance becomes a more pressing concern, ensuring cryptographic agility in design will be paramount.

In conclusion, while the theoretical benefits of privacy-preserving authentication mechanisms are well-established, practical implementation lags. Addressing the highlighted concerns can lead to a robust framework that not only protects user privacy but also paves the way for secure, trustworthy digital interactions in an evolving technological era.