Conscript Your Friends into Larger Anonymity Sets with JavaScript

We present the design and prototype implementation of ConScript, a framework for using JavaScript to allow casual Web users to participate in an anonymous communication system. When a Web user visits a cooperative Web site, the site serves a JavaScript application that instructs the browser to create and submit "dummy" messages into the anonymity system. Users who want to send non-dummy messages through the anonymity system use a browser plug-in to replace these dummy messages with real messages. Creating such conscripted anonymity sets can increase the anonymity set size available to users of remailer, e-voting, and verifiable shuffle-style anonymity systems. We outline ConScript's architecture, we address a number of potential attacks against ConScript, and we discuss the ethical issues related to deploying such a system. Our implementation results demonstrate the practicality of ConScript: a workstation running our ConScript prototype JavaScript client generates a dummy message for a mix-net in 81 milliseconds and it generates a dummy message for a DoS-resistant DC-net in 156 milliseconds.Comment: An abbreviated version of this paper will appear at the WPES 2013 worksho

Emerge: Self-Emerging Data Release Using Cloud Data Storage

In the age of Big Data, advances in distributed technologies and cloud storage services provide highly efficient and cost-effective solutions to large scale data storage and management. Supporting self-emerging data using clouds is a challenging problem. While straight-forward centralized approaches provide a basic solution to the problem, unfortunately they are limited to a single point of trust. Supporting attack-resilient timed release of encrypted data stored in clouds requires new mechanisms for self emergence of data encryption keys that enables encrypted data to become accessible at a future point in time. Prior to the release time, the encryption key remains undiscovered and unavailable in a secure distributed system, making the private data unavailable. In this paper, we propose Emerge, a self-emerging timed data release protocol for securely hiding data encryption keys of private encrypted data in a large-scale Distributed Hash Table (DHT) network that makes the data available and accessible only at the defined release time. We develop a suite of erasure-coding-based routing path construction schemes for securely storing and routing encryption keys in DHT networks that protect an adversary from inferring the encryption key prior to the release time (release-ahead attack) or from destroying the key altogether (drop attack). Through extensive experimental evaluation, we demonstrate that the proposed schemes are resilient to both release-ahead attack and drop attack as well as to attacks that arise due to traditional churn issues in DHT networks

From Ephemerizer to Timed-Ephemerizer: Achieve Assured Lifecycle Enforcement for Sensitive Data

The concept of Ephemerizer, proposed by Perlman, is a cryptographic primitive for assured data deletion. With an Ephemerizer protocol, data in persistent storage devices will always be encrypted simultaneously using an ephemeral public key of the Ephemerizer (an entity which will publish a set of ephemeral public keys and periodically delete the expired ones) and the long-term public key of a user. An Ephemerizer protocol enables the user to securely decrypt the encrypted data without leaking any information to the Ephemerizer. So far, no security model has ever been proposed for this primitive and existing protocols have not been studied formally. Not surprisingly, we show that some existing Ephemerizer protocols possess security vulnerabilities. In this paper, we introduce the notion of Timed-Ephemerizer, which can be regarded as a hybrid primitive by combining Ephemerizer and Timed-Release Encryption. Compared with an Ephemerizer protocol, a Timed-Ephemerizer protocol further guarantees that data will only be released after a pre-defined disclosure time. Moreover, we propose a security model for Timed-Ephemerizer and formalize relevant security properties. We also propose a new Timed-Ephemerizer protocol and prove its security in the security model

Weakening Assumptions for Publicly-Verifiable Deletion

We develop a simple compiler that generically adds publicly-verifiable deletion to a variety of cryptosystems. Our compiler only makes use of one-way functions (or one-way state generators, if we allow the public verification key to be quantum). Previously, similar compilers either relied on the use of indistinguishability obfuscation (Bartusek et. al., ePrint:2023/265) or almost-regular one-way functions (Bartusek, Khurana and Poremba, arXiv:2303.08676).Comment: 13 pages. arXiv admin note: text overlap with arXiv:2303.0867

Sequential Multiple Encryption: Security and Application

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Publicly-Verifiable Deletion via Target-Collapsing Functions

We build quantum cryptosystems that support publicly-verifiable deletion from standard cryptographic assumptions. We introduce target-collapsing as a weakening of collapsing for hash functions, analogous to how second preimage resistance weakens collision resistance; that is, target-collapsing requires indistinguishability between superpositions and mixtures of preimages of an honestly sampled image. We show that target-collapsing hashes enable publicly-verifiable deletion (PVD), proving conjectures from [Poremba, ITCS'23] and demonstrating that the Dual-Regev encryption (and corresponding fully homomorphic encryption) schemes support PVD under the LWE assumption. We further build on this framework to obtain a variety of primitives supporting publicly-verifiable deletion from weak cryptographic assumptions, including: - Commitments with PVD assuming the existence of injective one-way functions, or more generally, almost-regular one-way functions. Along the way, we demonstrate that (variants of) target-collapsing hashes can be built from almost-regular one-way functions. - Public-key encryption with PVD assuming trapdoored variants of injective (or almost-regular) one-way functions. We also demonstrate that the encryption scheme of [Hhan, Morimae, and Yamakawa, Eurocrypt'23] based on pseudorandom group actions has PVD. - $X$ with PVD for $X \in \{$attribute-based encryption, quantum fully-homomorphic encryption, witness encryption, time-revocable encryption$\}$, assuming $X$ and trapdoored variants of injective (or almost-regular) one-way functions.Comment: 52 page

Weighted average problem revisited under hybrid and malicious model

It is getting more common for two or more parties to jointly compute some statistics, say for marketing, by combining information on their private databases without disclosing the private data to the others. The core problem is usually known as secure multi-party computation (SMC). A number of solutions have been proposed. However, almost all of them assume a semi-honest model which is unrealistic. On the other hand, protocols that work under the malicious model (all participating parties can be malicious) are usually complicated and expensive in terms of communication and computation. In this paper, we try to consider a more reasonable model, the hybrid security model, in which at least one party is semi-honest. We want to make sure that the malicious parties will not get the correct final result if they perform malicious behaviors. We propose a scheme to solve the two-party weighted average problem (WAP) under this hybrid security model. We also show that the scheme can be extended to work under the malicious model using any fair exchange scheme. We formally show that our schemes are secure. We also implemented the schemes and showed that our scheme under the hybrid security model is reasonably fast and efficient for practical use. © 2012 AICIT.published_or_final_versio

Exclusion-intersection encryption

Identity-based encryption (IBE) has shown to be a useful cryptographic scheme enabling secure yet flexible role-based access control. We propose a new variant of IBE named as exclusion-intersection encryption: during encryption, the sender can specify the targeted groups that are legitimate and interested in reading the documents; there exists a trusted key generation centre generating the intersection private decryption keys on request. This special private key can only be used to decrypt the ciphertext which is of all the specified groups' interests, its holders are excluded from decrypting when the documents are not targeted to all these groups (e.g., the ciphertext of only a single group's interest). While recent advances in cryptographic techniques (e.g., attribute-based encryption or wicked IBE) can support a more general access control policy, the private key size may be as long as the number of attributes or identifiers that can be specified in a ciphertext, which is undesirable, especially when each user may receive a number of such keys for different decryption power. One of the applications of our notion is to support an ad-hoc joint project of two or more groups which needs extra helpers that are not from any particular group. © 2011 IEEE.published_or_final_versionThe 1st IEEE International Workshop on Security in Computers, Networking and Communications (SCNC 2011) in conjuntion with IEEE INFOCOM 2011, Shanghai, China, 10-15 April 2011. In Conference Proceedings of INFOCOM WKSHPS, 2011, p. 1048-1053The 1st IEEE International Workshop on Security in Computers, Networking and Communications (SCNC 2011) in conjuntion with IEEE INFOCOM 2011, Shanghai, China, 10-15 April 2011. In Conference Proceedings of INFOCOM WKSHPS, 2011, p. 1048-105

Cryptographic Tools for Privacy Preservation

Data permeates every aspect of our daily life and it is the backbone of our digitalized society. Smartphones, smartwatches and many more smart devices measure, collect, modify and share data in what is known as the Internet of Things.Often, these devices don’t have enough computation power/storage space thus out-sourcing some aspects of the data management to the Cloud. Outsourcing computation/storage to a third party poses natural questions regarding the security and privacy of the shared sensitive data.Intuitively, Cryptography is a toolset of primitives/protocols of which security prop- erties are formally proven while Privacy typically captures additional social/legislative requirements that relate more to the concept of “trust” between people, “how” data is used and/or “who” has access to data. This thesis separates the concepts by introducing an abstract model that classifies data leaks into different types of breaches. Each class represents a specific requirement/goal related to cryptography, e.g. confidentiality or integrity, or related to privacy, e.g. liability, sensitive data management and more.The thesis contains cryptographic tools designed to provide privacy guarantees for different application scenarios. In more details, the thesis:(a) defines new encryption schemes that provide formal privacy guarantees such as theoretical privacy definitions like Differential Privacy (DP), or concrete privacy-oriented applications covered by existing regulations such as the European General Data Protection Regulation (GDPR);(b) proposes new tools and procedures for providing verifiable computation’s guarantees in concrete scenarios for post-quantum cryptography or generalisation of signature schemes;(c) proposes a methodology for utilising Machine Learning (ML) for analysing the effective security and privacy of a crypto-tool and, dually, proposes a secure primitive that allows computing specific ML algorithm in a privacy-preserving way;(d) provides an alternative protocol for secure communication between two parties, based on the idea of communicating in a periodically timed fashion