Provable Security Analysis of FIDO2

We carry out the first provable security analysis of the new FIDO2 protocols, the promising FIDO Alliance\u27s proposal for a standard for passwordless user authentication. Our analysis covers the core components of FIDO2: the W3C’s Web Authentication (WebAuthn) specification and the new Client-to-Authenticator Protocol (CTAP2). Our analysis is modular. For WebAuthn and CTAP2, in turn, we propose appropriate security models that aim to capture their intended security goals and use the models to analyze their security. First, our proof confirms the authentication security of WebAuthn. Then, we show CTAP2 can only be proved secure in a weak sense; meanwhile we identify a series of its design flaws and provide suggestions for improvement. To withstand stronger yet realistic adversaries, we propose a generic protocol called sPACA and prove its strong security; with proper instantiations sPACA is also more efficient than CTAP2. Finally, we analyze the overall security guarantees provided by FIDO2 and WebAuthn+sPACA based on the security of its components. We expect that our models and provable security results will help clarify the security guarantees of the FIDO2 protocols. In addition, we advocate the adoption of our sPACA protocol as a substitute of CTAP2 for both stronger security and better performance

Malware-Resistant Protocols for Real-World Systems

Cryptographic protocols are widely used to protect real-world systems from attacks. Paying for goods in a shop, withdrawing money or browsing the Web; all these activities are backed by cryptographic protocols. However, in recent years a potent threat became apparent. Malware is increasingly used in attacks to bypass existing security mechanisms. Many cryptographic protocols that are used in real-world systems today have been found to be susceptible to malware attacks. One reason for this is that most of these protocols were designed with respect to the Dolev-Yao attack model that assumes an attacker to control the network between computer systems but not the systems themselves. Furthermore, most real-world protocols do not provide a formal proof of security and thus lack a precise definition of the security goals the designers tried to achieve. This work tackles the design of cryptographic protocols that are resilient to malware attacks, applicable to real-world systems, and provably secure. In this regard, we investigate three real-world use cases: electronic payment, web authentication, and data aggregation. We analyze the security of existing protocols and confirm results from prior work that most protocols are not resilient to malware. Furthermore, we provide guidelines for the design of malware-resistant protocols and propose such protocols. In addition, we formalize security notions for malware-resistance and use a formal proof of security to verify the security guarantees of our protocols. In this work we show that designing malware-resistant protocols for real-world systems is possible. We present a new security notion for electronic payment and web authentication, called one-out-of-two security, that does not require a single device to be trusted and ensures that a protocol stays secure as long as one of two devices is not compromised. Furthermore, we propose L-Pay, a cryptographic protocol for paying at the point of sale (POS) or withdrawing money at an automated teller machine (ATM) satisfying one-out-of-two security, FIDO2 With Two Displays (FIDO2D) a cryptographic protocol to secure transactions in the Web with one-out-of-two security and Secure Aggregation Grouped by Multiple Attributes (SAGMA), a cryptographic protocol for secure data aggregation in encrypted databases. In this work, we take important steps towards the use of malware-resistant protocols in real-world systems. Our guidelines and protocols can serve as templates to design new cryptographic protocols and improve security in further use cases

Challenges with Passwordless FIDO2 in an Enterprise Setting: A Usability Study

Fast Identity Online 2 (FIDO2), a modern authentication protocol, is gaining popularity as a default strong authentication mechanism. It has been recognized as a leading candidate to overcome limitations (e.g., phishing resistance) of existing authentication solutions. However, the task of deprecating weak methods such as password-based authentication is not trivial and requires a comprehensive approach. While security, privacy, and end-user usability of FIDO2 have been addressed in both academic and industry literature, the difficulties associated with its integration with production environments, such as solution completeness or edge-case support, have received little attention. In particular, complex environments such as enterprise identity management pose unique challenges for any authentication system. In this paper, we identify challenging enterprise identity lifecycle use cases (e.g., remote workforce and legacy systems) by conducting a usability study, in which over 100 cybersecurity professionals shared their perception of challenges to FIDO2 integration from their hands-on field experience. Our analysis of the user study results revealed serious gaps such as account recovery (selected by over 60% of our respondents), and identify priority development areas for the FIDO2 community.Comment: to be published in the IEEE Secure Development Conference 202

Fast IDentity Online with Anonymous Credentials (FIDO-AC)

Web authentication is a critical component of today's Internet and the digital world we interact with. The FIDO2 protocol enables users to leverage common devices to easily authenticate to online services in both mobile and desktop environments following the passwordless authentication approach based on cryptography and biometric verification. However, there is little to no connection between the authentication process and users' attributes. More specifically, the FIDO protocol does not specify methods that could be used to combine trusted attributes with the FIDO authentication process generically and allows users to disclose them to the relying party arbitrarily. In essence, applications requiring attributes verification (e.g. age or expiry date of a driver's license, etc.) still rely on ad-hoc approaches, not satisfying the data minimization principle and not allowing the user to vet the disclosed data. A primary recent example is the data breach on Singtel Optus, one of the major telecommunications providers in Australia, where very personal and sensitive data (e.g. passport numbers) were leaked. This paper introduces FIDO-AC, a novel framework that combines the FIDO2 authentication process with the user's digital and non-shareable identity. We show how to instantiate this framework using off-the-shelf FIDO tokens and any electronic identity document, e.g., the ICAO biometric passport (ePassport). We demonstrate the practicality of our approach by evaluating a prototype implementation of the FIDO-AC system.Comment: to be published in the 32nd USENIX Security Symposium(USENIX 2023

Towards secure communication and authentication: Provable security analysis and new constructions

Secure communication and authentication are some of the most important and practical topics studied in modern cryptography. Plenty of cryptographic protocols have been proposed to accommodate all sorts of requirements in different settings and some of those have been widely deployed and utilized in our daily lives. It is a crucial goal to provide formal security guarantees for such protocols. In this thesis, we apply the provable security approach, a standard method used in cryptography to formally analyze the security of cryptographic protocols, to three problems related to secure communication and authentication. First, we focus on the case where a user and a server share a secret and try to authenticate each other and establish a session key for secure communication, for which we propose the first user authentication and key exchange protocols that can tolerate strong corruptions on the client-side. Next, we consider the setting where a public-key infrastructure (PKI) is available and propose models to thoroughly compare the security and availability properties of the most important low-latency secure channel establishment protocols. Finally, we perform the first provable security analysis of the new FIDO2 protocols, the promising proposed standard for passwordless user authentication from the Fast IDentity Online (FIDO) Alliance to replace the world's over-reliance on passwords to authenticate users, and design new constructions to achieve stronger security.Ph.D

Accountable authentication with privacy protection: The Larch system for universal login

Credential compromise is hard to detect and hard to mitigate. To address this problem, we present larch, an accountable authentication framework with strong security and privacy properties. Larch protects user privacy while ensuring that the larch log server correctly records every authentication. Specifically, an attacker who compromises a user's device cannot authenticate without creating evidence in the log, and the log cannot learn which web service (relying party) the user is authenticating to. To enable fast adoption, larch is backwards-compatible with relying parties that support FIDO2, TOTP, and password-based login. Furthermore, larch does not degrade the security and privacy a user already expects: the log server cannot authenticate on behalf of a user, and larch does not allow relying parties to link a user across accounts. We implement larch for FIDO2, TOTP, and password-based login. Given a client with four cores and a log server with eight cores, an authentication with larch takes 150ms for FIDO2, 91ms for TOTP, and 74ms for passwords (excluding preprocessing, which takes 1.23s for TOTP).Comment: This is an extended version of a paper appearing at OSDI 202

Token meets Wallet: Formalizing Privacy and Revocation for FIDO2

The FIDO2 standard is a widely-used class of challenge-response type protocols that allows to authenticate to an online service using a hardware token. Barbosa et al. (CRYPTO `21) provided the first formal security model and analysis for the FIDO2 standard. However, their model has two shortcomings: (1) It does not include privacy, one of the key features claimed by FIDO2. (2) It only covers tokens that store {all secret keys locally}. In contrast, due to limited memory, most existing FIDO2 tokens either derive all secret keys from a common seed or store keys on the server (the latter approach is also known as {key wrapping}). In this paper, we revisit the security of the WebAuthn component of FIDO2 as implemented in practice. Our contributions are as follows. (1) We adapt the model of Barbosa et al. so as to capture authentication tokens using key derivation or key wrapping. (2) We provide the {first formal definition of privacy for the WebAuthn component of FIDO2}. We then prove the privacy of this component in common FIDO2 token implementations if the underlying building blocks are chosen appropriately. (3) We address the unsolved problem of {global key revocation} in FIDO2. To this end, we introduce and analyze a simple revocation procedure that builds on the popular BIP32 standard used in cryptocurrency wallets and can efficiently be implemented with existing FIDO2 servers

Token meets Wallet: Formalizing Privacy and Revocation for FIDO2

The FIDO2 standard is a widely-used class of challenge-response type protocols that allows to authenticate to an online service using a hardware token. Barbosa et al. (CRYPTO ‘21) provided the first formal security model and analysis for the FIDO2 standard. However, their model has two shortcomings: (1) It does not include privacy, one of the key features claimed by FIDO2. (2) It only covers tokens that store all secret keys locally. In contrast, due to limited memory, most existing FIDO2 tokens either derive all secret keys from a common seed or store keys on the server (the latter approach is also known as key wrapping). In this paper, we revisit the security of the WebAuthn component of FIDO2 as implemented in practice. Our contributions are as follows. (1) We adapt the model of Barbosa et al. so as to capture authentication tokens using key derivation or key wrapping. (2) We provide the first formal definition of privacy for the WebAuthn component of FIDO2. We then prove the privacy of this component in common FIDO2 token implementations if the underlying building blocks are chosen appropriately. (3) We address the unsolved problem of global key revocation in FIDO2. To this end, we introduce and analyze a simple revocation procedure that builds on the popular BIP32 standard used in cryptocurrency wallets and can efficiently be implemented with existing FIDO2 servers

How Not to Handle Keys: Timing Attacks on FIDO Authenticator Privacy

This paper presents a timing attack on the FIDO2 (Fast IDentity Online) authentication protocol that allows attackers to link user accounts stored in vulnerable authenticators, a serious privacy concern. FIDO2 is a new standard specified by the FIDO industry alliance for secure token online authentication. It complements the W3C WebAuthn specification by providing means to use a USB token or other authenticator as a second factor during the authentication process. From a cryptographic perspective, the protocol is a simple challenge-response where the elliptic curve digital signature algorithm is used to sign challenges. To protect the privacy of the user the token uses unique key pairs per service. To accommodate for small memory, tokens use various techniques that make use of a special parameter called a key handle sent by the service to the token. One of the most popular techniques used by leading token manufacturers (e.g. Yubico), termed key wrapping, stores the encrypted secret key in the server’s database and provides it to the token via the key handle parameter. We identify and analyse a vulnerability in the way the processing of key handles is implemented that allows attackers to remotely link user accounts on multiple services. We show that for vulnerable authenticators there is a difference between the time it takes to process a key handle for a different service but correct authenticator, and for a different authenticator but correct service. This difference can be used to perform a timing attack allowing an adversary to link the same authenticator across different services. Two of the eight hardware authenticators we tested were vulnerable despite FIDO level 1 certification, indicating a not insignificant problem. This vulnerability cannot be easily mitigated on authenticators because, for security reasons, they usually do not allow firmware updates. In addition, we show that due to the way existing browsers implement the WebAuthn standard, the attack can be executed remotely. However, we discuss countermeasures that can be implemented by browser providers to mitigate the remote form of the attac

Unlinkable Delegation of WebAuthn Credentials

The W3C\u27s WebAuthn standard employs digital signatures to offer phishing protection and unlinkability on the web using authenticators which manage keys on behalf of users. This introduces challenges when the account owner wants to delegate certain rights to a proxy user, such as to access their accounts or perform actions on their behalf, as delegation must not undermine the decentralisation, unlinkability, and attestation properties provided by WebAuthn. We present two approaches, called remote and direct delegation of WebAuthn credentials, maintaining the standard\u27s properties. Both approaches are compatible with Yubico\u27s recent Asynchronous Remote Key Generation (ARKG) primitive proposed for backing up credentials. For remote delegation, the account owner stores delegation credentials at the relying party on behalf of proxies, whereas the direct variant uses a delegation-by-warrant approach, through which the proxy receives delegation credentials from the account owner and presents them later to the relying party. To realise direct delegation we introduce Proxy Signature with Unlinkable Warrants (PSUW), a new proxy signature scheme that extends WebAuthn\u27s unlinkability property to proxy users and can be constructed generically from ARKG. We discuss an implementation of both delegation approaches, designed to be compatible with WebAuthn, including extensions required for CTAP, and provide a software-based prototype demonstrating overall feasibility. On the performance side, we observe only a minor increase of a few milliseconds in the signing and verification times for delegated WebAuthn credentials based on ARKG and PSUW primitives. We also discuss additional functionality, such as revocation and permissions management, and mention usability considerations