International Association
for Cryptologic Research

A $1$-out-of-$N$ ring signature scheme, introduced by Rivest, Shamir, and Tauman-Kalai (ASIACRYPT '01), allows a signer to sign a message as part of a set of size $N$ (the so-called ``ring'') which are anonymous to any verifier, including other members of the ring. Threshold ring (or ``thring'') signatures generalize ring signatures to $t$-out-of-$N$ parties, with $t \geq 1$, who anonymously sign messages and show that they are distinct signers (Bresson et al., CRYPTO'02). Until recently, there was no construction of ring signatures that both $(i)$ had logarithmic signature size in $N$, and $(ii)$ was secure in the plain model. The work of Backes et al. (EUROCRYPT'19) resolved both these issues. However, threshold ring signatures have their own particular problem: with a threshold $t \geq 1$, signers must often reveal their identities to the other signers as part of the signing process. This is an issue in situations where a ring member has something controversial to sign; he may feel uncomfortable requesting that other members join the threshold, as this reveals his identity. Building on the Backes et al. template, in this work we present the first construction of a thring signature that is logarithmic-sized in $N$, in the plain model, and does not require signers to interact with each other to produce the thring signature. We also present a linkable counterpart to our construction, which supports a fine-grained control of linkability. Moreover, our thring signatures can easily be adapted to achieve the recent notions of claimability and repudiability (Park and Sealfon, CRYPTO'19).

Proving the security of complex protocols is a crucial and very challenging task. A widely used approach for reasoning about such protocols in a modular way is universal composability. A perfect model for universal composability should provide a sound basis for formal proofs and be very flexible in order to allow for modeling a multitude of different protocols. It should also be easy to use, including useful design conventions for repetitive modeling aspects, such as corruption, parties, sessions, and subroutine relationships, such that protocol designers can focus on the core logic of their protocols.While many models for universal composability exist, including the UC, GNUC, and IITM models, none of them has achieved this ideal goal yet. As a result, protocols cannot be modeled faithfully and/or using these models is a burden rather than a help, often even leading to underspecified protocols and formally incorrect proofs.Given this dire state of affairs, the goal of this work is to provide a framework for universal composability which combines soundness, flexibility, and usability in an unmatched way. Developing such a security framework is a very difficult and delicate task, as the long history of frameworks for universal composability shows.We build our framework, called iUC, on top of the IITM model, which already provides soundness and flexibility while lacking sufficient usability. At the core of iUC is a single simple template for specifying essentially arbitrary protocols in a convenient, formally precise, and flexible way. We illustrate the main features of our framework with example functionalities and realizations.

We revisit the notion of proxy re-encryption ($$\mathsf {PRE}$$PRE), an enhanced public-key encryption primitive envisioned by Blaze et al. (Eurocrypt’98) and formalized by Ateniese et al. (NDSS’05) for delegating decryption rights from a delegator to a delegatee using a semi-trusted proxy. $$\mathsf {PRE}$$PRE notably allows to craft re-encryption keys in order to equip the proxy with the power of transforming ciphertexts under a delegator’s public key to ciphertexts under a delegatee’s public key, while not learning anything about the underlying plaintexts.We study an attractive cryptographic property for $$\mathsf {PRE}$$PRE, namely that of forward secrecy. In our forward-secret $$\mathsf {PRE}$$PRE (fs-$$\mathsf {PRE}$$PRE) definition, the proxy periodically evolves the re-encryption keys and permanently erases old versions while he delegator’s public key is kept constant. As a consequence, ciphertexts for old periods are no longer re-encryptable and, in particular, cannot be decrypted anymore at the delegatee’s end. Moreover, delegators evolve their secret keys too, and, thus, not even they can decrypt old ciphertexts once their key material from past periods has been deleted. This, as we will discuss, directly has application in short-term data/message-sharing scenarios.Technically, we formalize fs-$$\mathsf {PRE}$$PRE. Thereby, we identify a subtle but significant gap in the well-established security model for conventional $$\mathsf {PRE}$$PRE and close it with our formalization (which we dub fs-$$\mathsf {PRE} ^+$$PRE+). We present the first provably secure and efficient constructions of fs-$$\mathsf {PRE}$$PRE as well as $$\mathsf {PRE}$$PRE (implied by the former) satisfying the strong fs-$$\mathsf {PRE} ^+$$PRE+ and $$\mathsf {PRE} ^+$$PRE+ notions, respectively. All our constructions are instantiable in the standard model under standard assumptions and our central building block are hierarchical identity-based encryption ($$\mathsf {HIBE}$$HIBE) schemes that only need to be selectively secure.