Registration-Based Encryption from Standard Assumptions

The notion of Registration-Based Encryption (RBE) was recently introduced by Garg, Hajiabadi, Mahmoody, and Rahimi [TCC\u2718] with the goal of removing the private-key generator (PKG) from IBE. Specifically, RBE allows encrypting to identities using a (compact) master public key, like how IBE is used, with the benefit that the PKG is substituted with a weaker entity called key curator who has no knowledge of any secret keys. Here individuals generate their secret keys on their own and then publicly register their identities and their corresponding public keys to the key curator. Finally, individuals obtain rare decryption-key updates from the key curator as the population grows. In their work, they gave a construction of RBE schemes based on the combination of indistinguishability obfuscation and somewhere statistically binding hash functions. However, they left open the problem of constructing RBE schemes based on standard assumptions. In this work, we resolve the above problem and construct RBE schemes based on standard assumptions (e.g., CDH or LWE). Furthermore, we show a new application of RBE in a novel context. In particular, we show that anonymous variants of RBE (which we also construct under standard assumptions) can be used for realizing abstracts forms of anonymous messaging tasks in simple scenarios in which the parties communicate by writing messages on a shared board in a synchronized way

Verifiable Registration-Based Encryption

In a recent work, Garg, Hajiabadi, Mahmoody, and Rahimi (TCC 18) introduced a new encryption framework, which they referred to as Registration-Based Encryption (RBE). The central motivation behind RBE was to provide a novel methodology for solving the well-known key-escrow problem in Identity-Based Encryption (IBE) systems. Informally, in an RBE system there is no private-key generator unlike IBE systems, but instead it is replaced with a public key accumulator. Every user in an RBE system samples its own public-secret key pair, and sends the public key to the accumulator for registration. The key accumulator has no secret state, and is only responsible for compressing all the registered user identity-key pairs into a short public commitment. Here the encryptor only requires the compressed parameters along with the target identity, whereas a decryptor requires supplementary key material along with the secret key associated with the registered public key. The initial construction by Garg et al. (TCC 18) based on standard assumptions only provided weak efficiency properties. In a follow-up work by Garg, Hajiabadi, Mahmoody, Rahimi, and Sekar (PKC 19), they gave an efficient RBE construction from standard assumptions. However, both these works considered the key accumulator to be honest which might be too strong an assumption in real-world scenarios. In this work, we initiate a formal study of RBE systems with malicious key accumulators. To that end, we introduce a strengthening of the RBE framework which we call Verifiable RBE (VRBE). A VRBE system additionally gives the users an extra capability to obtain short proofs from the key accumulator proving correct (and unique) registration for every registered user as well as proving non-registration for any yet unregistered identity. We construct VRBE systems which provide succinct proofs of registration and non-registration from standard assumptions (such as CDH, Factoring, LWE). Our proof systems also naturally allow a much more efficient audit process which can be perfomed by any non-participating third party as well. A by-product of our approach is that we provide a more efficient RBE construction than that provided in the prior work of Garg et al. (PKC 19). And, lastly we initiate a study on extension of VRBE to a wider range of access and trust structures

Data Minimisation in Communication Protocols: A Formal Analysis Framework and Application to Identity Management

With the growing amount of personal information exchanged over the Internet, privacy is becoming more and more a concern for users. One of the key principles in protecting privacy is data minimisation. This principle requires that only the minimum amount of information necessary to accomplish a certain goal is collected and processed. "Privacy-enhancing" communication protocols have been proposed to guarantee data minimisation in a wide range of applications. However, currently there is no satisfactory way to assess and compare the privacy they offer in a precise way: existing analyses are either too informal and high-level, or specific for one particular system. In this work, we propose a general formal framework to analyse and compare communication protocols with respect to privacy by data minimisation. Privacy requirements are formalised independent of a particular protocol in terms of the knowledge of (coalitions of) actors in a three-layer model of personal information. These requirements are then verified automatically for particular protocols by computing this knowledge from a description of their communication. We validate our framework in an identity management (IdM) case study. As IdM systems are used more and more to satisfy the increasing need for reliable on-line identification and authentication, privacy is becoming an increasingly critical issue. We use our framework to analyse and compare four identity management systems. Finally, we discuss the completeness and (re)usability of the proposed framework

A Practical Set-Membership Proof for Privacy-Preserving NFC Mobile Ticketing

To ensure the privacy of users in transport systems, researchers are working on new protocols providing the best security guarantees while respecting functional requirements of transport operators. In this paper, we design a secure NFC m-ticketing protocol for public transport that preserves users' anonymity and prevents transport operators from tracing their customers' trips. To this end, we introduce a new practical set-membership proof that does not require provers nor verifiers (but in a specific scenario for verifiers) to perform pairing computations. It is therefore particularly suitable for our (ticketing) setting where provers hold SIM/UICC cards that do not support such costly computations. We also propose several optimizations of Boneh-Boyen type signature schemes, which are of independent interest, increasing their performance and efficiency during NFC transactions. Our m-ticketing protocol offers greater flexibility compared to previous solutions as it enables the post-payment and the off-line validation of m-tickets. By implementing a prototype using a standard NFC SIM card, we show that it fulfils the stringent functional requirement imposed by transport operators whilst using strong security parameters. In particular, a validation can be completed in 184.25 ms when the mobile is switched on, and in 266.52 ms when the mobile is switched off or its battery is flat