Anonymous token schemes are cryptographic protocols for limiting the access to online resources to credible users. The resource provider issues a set of access tokens to the credible user that they can later redeem anonymously, i.e., without the provider being able to link their redemptions. When combined with credibility tests such as CAPTCHAs, anonymous token schemes can significantly increase user experience and provider security, without exposing user access patterns to providers.Current anonymous token schemes such as the Privacy Pass protocol by Davidson et al. rely on oblivious pseudorandom functions (OPRFs), which let server and user jointly compute randomly looking access tokens. For those protocols, token issuing costs are linear in the number of requested tokens.In this work, we propose a new approach for building anonymous token schemes. Instead of relying on two-party computation to realize a privacy-preserving pseudorandom function evaluation, we propose to offload token generation to the user by using group verifiable random functions (GVRFs). GVRFs are a new cryptographic primitive that allow users to produce verifiable pseudorandomness. Opposed to standard VRFs, verification is anonymous within the group of credible users. We give a construction of group VRFs from the Dodis-Yampolskiy VRF and EquivalenceClass Signatures, based on pairings and a new Diffie-Hellman inversion assumption that we analyze in the Generic Group Model. Our construction enjoys compact public keys and proofs, while evaluation and verification costs are only slightly increased compared to the Dodis-Yampolskiy VRF.By deploying a group VRF instead of a OPRF, we obtain an anonymous token scheme where communication as well as server-side computation during the issuing phase is constant and independent of the number of tokens a user requests. Moreover, by means of our new concept of updatable token policies, the number of unspent tokens in circulation can retrospectively (i.e., even after the credibility check) be decreased or increased in order to react to the current or expected network situation. Our tokens are further countable and publicly verifiable. This comes at the cost of higher computational efforts for token redemption and verification as well as somewhat weaker unlinkability guarantees compared to Privacy Pass.

Password-protected key retrieval (PPKR) enables users to store and retrieve high-entropy keys from a server securely. The process is bootstrapped from a human-memorizable password only, addressing the challenge of how end-users can manage cryptographic key material. The core security requirement is protection against a corrupt server, which should not be able to learn the key or offline- attack it through the password protection. PPKR is deployed at a large scale with the WhatsApp Backup Protocol (WBP), allowing users to access their encrypted messaging history when switching to a new device. Davies et al. (Crypto'23) formally analyzed the WBP, proving that it satisfies most of the desired security. The WBP uses the OPAQUE protocol for password-based key exchange as a building block and relies on the server using a hardware security module (HSM) for most of its protection. In fact, the security analysis assumes that the HSM is incorruptible - rendering most of the heavy cryptography in the WBP obsolete.

A Single Sign-On (SSO) system allows users to access different remote services while authenticating only once. SSO can greatly improve the usability and security of online activities by dispensing with the need to securely remember or store tens or hundreds of authentication secrets. On the downside, today's SSO providers can track users' online behavior, and collect personal data that service providers want to see asserted before letting a user access their resources. In this work, we propose a new policy-based Single Sign-On service, i.e., a system that produces access tokens that are conditioned on the user's attributes fulfilling a specified policy. Our solution is based on multi-party computation and threshold cryptography, and generates access tokens of standardized format. The central idea is to distribute the role of the SSO provider among several entities, in order to shield user attributes and access patterns from each individual entity. We provide a formal security model and analysis in the Universal Composability framework, against proactive adversaries. Our implementation and benchmarking show the practicality of our system for many real-world use cases.

Cloud storage is becoming increasingly popular among end users that outsource their personal data to services such as Dropbox or Google Drive. For security, uploaded data should ideally be encrypted under a key that is controlled and only known by the user. Current solutions that support user-centric encryption either require the user to manage strong cryptographic keys, or derive keys from weak passwords. While the former has massive usability issues and requires secure storage by the user, the latter approach is more convenient but offers only little security since encrypted data is susceptible to offline attacks. The recent concept of password-authenticated secret-sharing (PASS) enables users to securely derive strong keys from weak passwords by leveraging a distributed server setup, and has been considered a promising step towards secure and usable encryption. However, using PASS for encryption is not as suitable as originally thought: it only considers the (re)construction of a single, static key -- whereas practical encryption will require the management of many, object-specific keys. Using a dedicated PASS instance for every key makes the solution vulnerable against online attacks, inherently leaks access patterns to the servers and poses the risk of permanent data loss when an incorrect password is used at encryption. We therefore propose a new protocol that directly targets the problem of boostrapping encryption from a single password: distributed password-authenticated symmetric encryption DPaSE. DPaSE offers strong security and usability, such as protecting the user's password against online and offline attacks, and ensuring message privacy and ciphertext integrity as long as at least one server is honest. We formally define the desired security properties in the UC framework and propose a provably secure instantiation. The core of our protocol is a new type of Oblivious Pseudorandom Function (OPRF) that allows to extend a previous partially-blind query with a follow-up request and will be used to blindly carry over passwords across evaluations and avoid online attacks. Our (proof-of-concept) implementation of DPaSE uses 10 exponentiations at the user, 4 exponentiations and 2 pairings at each server, and has a server throughput of 76 account creations and 37 (user authentication followed by) encryptions per second, when run between a user and 2-10 servers.

In recent years, oblivious pseudorandom functions (OPRFs) have become a ubiquitous primitive used in cryptographic protocols and privacy-preserving technologies. The growing interest in OPRFs, both theoretical and applied, has produced a vast number of different constructions and functionality variations. In this paper, we provide a systematic overview of how to build and use OPRFs. We first categorize existing OPRFs into essentially four families based on their underlying PRF (Naor-Reingold, Dodis-Yampolskiy, Hashed Diffie-Hellman, and generic constructions). This categorization allows us to give a unified presentation of all oblivious evaluation methods in the literature, and to understand which properties OPRFs can (or cannot) have. We further demonstrate the theoretical and practical power of OPRFs by visualizing them in the landscape of cryptographic primitives, and by providing a comprehensive overview of how OPRFs are leveraged for improving the privacy of internet users. Our work systematizes 15 years of research on OPRFs and provides inspiration for new OPRF constructions and applications thereof.

With the advent of the Internet of Things (IoT), establishing a secure channel between smart devices becomes crucial. Recent research proposes zero-interaction pairing (ZIP), which enables pairing without user assistance by utilizing devices' physical context (e.g., ambient audio) to obtain a shared secret key. The state-of-the-art ZIP schemes suffer from three limitations: (1) prolonged pairing time (i.e., minutes or hours), (2) vulnerability to brute-force offline attacks on a shared key, and (3) susceptibility to attacks caused by predictable context (e.g., replay attack) because they rely on limited entropy of physical context to protect a shared key. We address these limitations, proposing FastZIP, a novel ZIP scheme that significantly reduces pairing time while preventing offline and predictable context attacks. In particular, we adapt a recently introduced Fuzzy Password-Authenticated Key Exchange (fPAKE) protocol and utilize sensor fusion, maximizing their advantages. We instantiate FastZIP for intra-car device pairing to demonstrate its feasibility and show how the design of FastZIP can be adapted to other ZIP use cases. We implement FastZIP and evaluate it by driving four cars for a total of 800 km. We achieve up to three times shorter pairing time compared to the state-of-the-art ZIP schemes while assuring robust security with adversarial error rates below 0.5%.

Single Sign-On (SSO) is becoming an increasingly popular authentication method for users that leverages a trusted Identity Provider (IdP) to bootstrap secure authentication tokens from a single user password. It alleviates some of the worst security issues of passwords, as users no longer need to memorize individual passwords for all service providers, and it removes the burden of these service to properly protect huge password databases. However, SSO also introduces a single point of failure. If compromised, the IdP can impersonate all users and learn their master passwords. To remedy this risk while preserving the advantages of SSO, Agrawal et al. (CCS'18) recently proposed a distributed realization termed PASTA (password-authenticated threshold authentication) which splits the role of the IdP across n servers. While PASTA is a great step forward and guarantees security as long as not all servers are corrupted, it uses a rather inflexible corruption model: servers cannot be corrupted adaptively and - even worse - cannot recover from corruption. The latter is known as proactive security and allows servers to re-share their keys, thereby rendering all previously compromised information useless. In this work, we improve upon the work of PASTA and propose a distributed SSO protocol with proactive and adaptive security (PESTO), guaranteeing security as long as not all servers are compromised at the same time. We prove our scheme secure in the UC framework which is known to provide the best security guarantees for password-based primitives. The core of our protocol are two new primitives we introduce: partially-oblivious distributed PRFs and a class of distributed signature schemes. Both allow for non-interactive refreshing of the secret key material and tolerate adaptive corruptions. We give secure instantiations based on the gap one-more BDH and RSA assumption respectively, leading to a highly efficient 2-round PESTO protocol. We also present an implementation and benchmark of our scheme in Java, realizing OAuth-compatible bearer tokens for SSO, demonstrating the viability of our approach.