Finding Safety in Numbers with Secure Allegation Escrows

For fear of retribution, the victim of a crime may be willing to report it only if other victims of the same perpetrator also step forward. Common examples include 1) identifying oneself as the victim of sexual harassment, especially by a person in a position of authority or 2) accusing an influential politician, an authoritarian government, or ones own employer of corruption. To handle such situations, legal literature has proposed the concept of an allegation escrow: a neutral third-party that collects allegations anonymously, matches them against each other, and de-anonymizes allegers only after de-anonymity thresholds (in terms of number of co-allegers), pre-specified by the allegers, are reached. An allegation escrow can be realized as a single trusted third party; however, this party must be trusted to keep the identity of the alleger and content of the allegation private. To address this problem, this paper introduces Secure Allegation Escrows (SAE, pronounced "say"). A SAE is a group of parties with independent interests and motives, acting jointly as an escrow for collecting allegations from individuals, matching the allegations, and de-anonymizing the allegations when designated thresholds are reached. By design, SAEs provide a very strong property: No less than a majority of parties constituting a SAE can de-anonymize or disclose the content of an allegation without a sufficient number of matching allegations (even in collusion with any number of other allegers). Once a sufficient number of matching allegations exist, the join escrow discloses the allegation with the allegers' identities. We describe how SAEs can be constructed using a novel authentication protocol and a novel allegation matching and bucketing algorithm, provide formal proofs of the security of our constructions, and evaluate a prototype implementation, demonstrating feasibility in practice.Comment: To appear in NDSS 2020. New version includes improvements to writing and proof. The protocol is unchange

Shield: Secure Allegation Escrow System with Stronger Guarantees

The rising issues of harassment, exploitation, corruption, and other forms of abuse have led victims to seek comfort by acting in unison against common perpetrators (e.g., #MeToo movement). One way to curb these issues is to install allegation escrow systems that allow victims to report such incidents. The escrows are responsible for identifying victims of a common perpetrator and taking the necessary action to bring justice to them. However, users hesitate to participate in these systems due to the fear of such sensitive reports being leaked to perpetrators, who may further misuse them. Thus, to increase trust in the system, cryptographic solutions are being designed to realize secure allegation escrow (SAE) systems. In the work of Arun et al. (NDSS\u2720), which presents the state-of-the-art solution, we identify attacks that can leak sensitive information and compromise victim privacy. We also report issues present in prior works that were left unidentified. To arrest all these breaches, we put forth an SAE system that prevents the identified attacks and retains the salient features from all prior works. The cryptographic technique of secure multi-party computation (MPC) serves as the primary underlying tool in designing our system. At the heart of our system lies a new duplicity check protocol and an improved matching protocol. We also provide additional features such as allegation modification and deletion, which were absent in the state of the art. To demonstrate feasibility, we benchmark the proposed system with state-of-the-art MPC protocols and report the cost of processing an allegation. Different settings that affect system performance are analyzed, and the reported values showcase the practicality of our solution

Collusion-Deterrent Threshold Information Escrow

An information escrow (IE) service allows its users to encrypt a message such that the message is unlocked only when a user-specified condition is satisfied. Its instantiations include timed-release encryption and allegation escrows with applications ranging from e-auctions to the #metoo movement. The proposed IE systems typically employ threshold cryptography towards mitigating the single-point-of-failure problem. Here, a set of escrow agents securely realize the IE functionality as long as a threshold or more agents behave honestly. Nevertheless, these threshold information escrow (TIE) protocols are vulnerable to premature and undetectable unlocking of messages through collusion among rational agents offering the IE service. This work presents a provably secure TIE scheme in the mixed-behavior model consisting of rational and malicious escrow agents.; any collusion attempt among the agents towards premature decryption results in penalization through a loss of (crypto-)currency and getting banned from the system. The proposed collusion-deterrent escrow (CDE) scheme introduces a novel incentive-penalty mechanism among the agents to stay honest until the user-specified decryption condition is met. In particular, each agent makes a cryptocurrency deposit before the start of the protocol instance such that the deposit amount is returned to the agent when the user-specified condition is met or can be transferred by anyone who holds a secret key corresponding to a public key associated with the instance. Using a novel combination of oblivious transfer, robust bit watermarking, and secure multi-party computation, CDE ensures that whenever the agents collude to decrypt the user data prematurely, one or more whistle-blower agents can withdraw/transfer the deposits of all other agents, thereby penalizing them. We model collusion as a game induced among rational agents offering the CDE service and show that the agents do not collude at equilibrium in game-theoretic terms. We also present a prototype implementation of the CDE protocol and demonstrate its efficiency towards use in practice. While this work does not aim to solve the collusion problem fully, it significantly raises the bar for collusion. It offers an important step towards weakening the strong non-collusion assumption pervasive across multi-party computation applications

PentaGOD: Stepping beyond Traditional GOD with Five Parties

Secure multiparty computation (MPC) is increasingly being used to address privacy issues in various applications. The recent work of Alon et al. (CRYPTO\u2720) identified the shortcomings of traditional MPC and defined a Friends-and-Foes (FaF) security notion to address the same. We showcase the need for FaF security in real-world applications such as dark pools. This subsequently necessitates designing concretely efficient FaF-secure protocols. Towards this, keeping efficiency at the center stage, we design ring-based FaF-secure MPC protocols in the small-party honest-majority setting. Specifically, we provide (1,1)-FaF secure 5 party computation protocols (5PC) that consider one malicious and one semi-honest corruption and constitutes the optimal setting for attaining honest-majority. At the heart of it lies the multiplication protocol that requires a single round of communication with 8 ring elements (amortized). To facilitate having FaF-secure variants for several applications, we design a variety of building blocks optimized for our FaF setting. The practicality of the designed (1,1)-FaF secure 5PC framework is showcased by benchmarking dark pools. In the process, we also improve the efficiency and security of the dark pool protocols over the existing traditionally secure ones. This improvement is witnessed as a gain of up to $62\times$ in throughput compared to the existing ones. Finally, to demonstrate the versatility of our framework, we also benchmark popular deep neural networks