A threshold signature scheme splits the signing key among$\ell$parties, such that any$t$-subset of parties can jointly generate signatures on a given message. Designing concretely efficient post-quantum threshold signatures is a pressing question, as evidenced by NIST's recent call. In this work, we propose, implement, and evaluate a lattice-based threshold signature scheme, Ringtail, which is the first to achieve a combination of desirable properties: (i) The signing protocol consists of only two rounds, where the first round is message-independent and can thus be preprocessed offline. (ii) The scheme is concretely efficient and scalable to$t\leq 1024$parties. For 128-bit security and$t=1024$parties, we achieve 13.4 KB signature size and 10.5 KB of online communication. (iii) The security is based on the standard learning with errors (LWE) assumption in the random oracle model. This improves upon the state-of-the-art (with comparable efficiency) which either has a three-round signing protocol [Eurocrypt'24] or relies on a new non-standard assumption [Crypto'24]. To substantiate the practicality of our scheme, we conduct the first WAN experiment deploying a lattice-based threshold signature, across 8 countries in 5 continents. We observe that an overwhelming majority of the end-to-end latency is consumed by network latency, underscoring the need for round-optimized schemes.

As billions of people rely on end-to-end encrypted messaging, the exposure of metadata, such as communication timing and participant relationships, continues to deanonymize users. Asynchronous metadata-hiding solutions with strong cryptographic guarantees have historically been bottlenecked by quadratic$O(N^{2})$server computation in the number of users$N$due to reliance on private information retrieval (PIR). We present Myco, a metadata-private messaging system that preserves strong cryptographic guarantees while achieving$O(N\log^{2}N)$efficiency. To achieve this, we depart from PIR and instead introduce an oblivious data structure through which senders and receivers privately communicate. To unlink reads and writes, we instantiate Myco in an asymmetric two-server distributed-trust model where clients write messages to one server tasked with obliviously transmitting these messages to another server, from which clients read. Myco achieves throughput improvements of up to 302x over multi-server and 2,219x over single-server state-of-the-art systems based on PIR.

A verifiable delay function (VDF) requires a specified number of sequential steps to compute, yet the validity of its output can be verified efficiently, much faster than recomputing the function from scratch. VDFs are a versatile cryptographic tool, with many industrial applications, such as blockchain consensus protocols, lotteries and verifiable randomness. Unfortunately, without exceptions, all known practical VDF constructions are broken by quantum algorithms. In this work, we investigate the practicality of VDFs with plausible post-quantum security. We propose Papercraft, a working implementation of a VDF based entirely on lattice techniques and thus plausibly post-quantum secure. Our VDF is based on new observations on lattice-based succinct argument systems with many low-level optimisations, yielding the first lattice-based VDF that is implementable on today's hardware. As an example, our Papercraft implementation can verify a computation of over 6 minutes in just 7 seconds. Overall, our work demonstrates that lattice-based VDFs are not just a theoretical construct, paving the way for their practical deployment.

Instagram infographics are a digital activism tool that have redefined action frames for technology-facilitated social movements. From the 1960s through the 1980s, United States ethnic movements practiced collective action: ideologically unified, resource-intensive activism. Researchers have argued that modern technologically mediated movements, in contrast, practice connective action: individualized, low-resource online activism. We argue that Instagram infographics are both connective and collective. We conducted a qualitative interview study juxtaposing the insights of past and present U.S. ethnic movement activists and analyzed Black Lives Matter Instagram data over the course of 7 years (2014-2020). We find that Instagram infographic activism bridges connective and collective action in three ways: (1) Scope for Education: Visually enticing and digestible infographics reduce the friction of information dissemination, facilitating collective movement education while preserving customizability. (2) Reconciliation for Credibility: Activists use connective features to combat infographic misinformation and resolve internal differences, creating a trusted collective movement front. (3) High-Resource Efforts for Transformative Change: Instagram infographic activism has been paired with boots on the ground and action-oriented content, curating a connective-to-collective pipeline that expends movement resources. Our work unveils the vitality of evaluating digital activism action frames at the movement integration level, exemplifies the powerful coexistence of connective and collective action, and offers design implications for activists seeking to leverage this novel tool.