What a lovely hat

The fast-paced development and deployment of private messaging applications demands mechanisms to protect against the concomitant potential for abuse. While widely used end-to-end encrypted (E2EE) messaging systems have deployed mechanisms for users to verifiably report abusive messages without compromising the privacy of unreported messages, abuse reporting schemes for systems that additionally protect message metadata are still in their infancy. Existing solutions either focus on a...

We propose a dynamic, low-latency encrypted multi-map (EMM) that operates in two modes: low-leakage mode, which reveals minimal information such as data size, expected response length, and query arrival rate; and subliminal mode, which reveals only the data size while hiding metadata including query and update times, the number of operations executed, and even whether an operation was executed at all---albeit at the cost of full correctness. We achieve this by exploiting a tradeoff...

We propose waterlogs, a new direction to detect and trace synthetic text outputs from large language models based on transparency logs. Waterlogs offer major categorical advantages over watermarking: it (1) allows for the inclusion of arbitrary metadata to facilitate tracing, (2) is publicly verifiable by third parties, and (3) operates in a distributed manner while remaining robust and efficient. Waterlogs rely on a verifiable Hamming distance index, a novel data structure that we...

This paper presents a new architecture for metadata-private messaging that counters scalability challenges by offloading most computations to the clients. At the core of our design is a distributed private information retrieval (PIR) protocol, where the responder delegates its work to alleviate PIR's computational bottleneck and catches misbehaving delegates by efficiently verifying their results. We introduce DPIR, a messaging system that uses distributed PIR to let a server storing...

Arma is a Byzantine Fault Tolerant (BFT) consensus system designed to achieve horizontal scalability across all hardware resources: network bandwidth, CPU, and disk I/O. As opposed to preceding BFT protocols, Arma separates the dissemination and validation of client transactions from the consensus process, restricting the latter to totally ordering only metadata of batches of transactions. This separation enables each party to distribute compute and storage resources for transaction...

In this paper, we introduce a new approach to secure computing by implementing a platform that utilizes an NVMe-based system with an FPGA-based Torus FHE accelerator, SSD, and middleware on the host-side. Our platform is the first of its kind to offer complete secure computing capabilities for TFHE using an FPGA-based accelerator. We have defined secure computing instructions to evaluate 14-bit to 14-bit functions using TFHE, and our middleware allows for communication of ciphertexts, keys,...

Secure Multi-party Computation (MPC) allows two or more parties to compute any public function over their privately-held inputs, without revealing any information beyond the result of the computation. Modern protocols for MPC generate a large amount of input-independent preprocessing material called multiplication triples, in an offline phase. This preprocessing can later be used by the parties to efficiently instantiate an input-dependent online phase computing the function. To date, the...

Anonymous message delivery, as in privacy-preserving blockchain and private messaging applications, needs to protect recipient metadata: eavesdroppers should not be able to link messages to their recipients. This raises the question: how can untrusted servers assist in delivering the pertinent messages to each recipient, without learning which messages are addressed to whom? Recent work constructed Oblivious Message Retrieval (OMR) protocols that outsource the message detection and...

Privacy enhancing technologies must not only protect sensitive data in-transit, but also locally at-rest. For example, anonymity networks hide the sender and/or recipient of a message from network adversaries. However, if a participating device is physically captured, its owner can be pressured to give access to the stored conversations. Therefore, client software should allow the user to plausibly deny the existence of meaningful data. Since biometrics can be collected without consent and...

This study presents an efficient recoverable memory encryption mechanism, named Crystalor. Existing memory encryption mechanisms, such as Intel SGX integrity tree, offer neither crash consistency nor recoverability, which results in attack surfaces and causes a non-trivial limitation of practical availability. Although the crash consistency of encrypted memory has been studied in the research field of microarchitecture, existing mechanisms lack formal security analysis and cannot incorporate...

This paper introduces Lasso, a new family of lookup arguments, which allow an untrusted prover to commit to a vector $a \in \mathbb{F}^m$ and prove that all entries of a reside in some predetermined table $t \in \mathbb{F}^n$. Lasso’s performance characteristics unlock the so-called "lookup singularity". Lasso works with any multilinear polynomial commitment scheme, and provides the following efficiency properties. For $m$ lookups into a table of size $n$, Lasso’s prover commits to just...

Anonymous tokens are digital signature schemes that enable an issuer to provider users with signatures without learning the input message or the resulting signature received by the user. These primitives allow applications to propagate trust while simultaneously protecting the identity of the user. Anonymous tokens have become a core component for improving the privacy of several real-world applications including ad measurements, authorization protocols, spam detection and VPNs. In...

As interest in metadata-hiding communication grows in both research and practice, a need exists for stronger abuse reporting features on metadata-hiding platforms. While message franking has been deployed on major end-to-end encrypted platforms as a lightweight and effective abuse reporting feature, there is no comparable technique for metadata-hiding platforms. Existing efforts to support abuse reporting in this setting, such as asymmetric message franking or the Hecate scheme, require...

Commodity encrypted storage platforms (e.g., IceDrive, pCloud) permit data store and sharing across multiple users while preserving data confidentiality. However, end-to-end encryption may not be sufficient since it only offers confidentiality when the data is at rest or in transit. Meanwhile, sensitive information can be leaked from metadata representing activities during data operations (e.g., query, processing). Recent encrypted search platforms such as DORY (OSDI’20) or DURASIFT...

Anonymous message delivery, as in private communication and privacy-preserving blockchain applications, ought to protect recipient metadata: a message should not be inadvertently linkable to its destination. But how can messages then be delivered to each recipient, without each recipient scanning all messages? Recent work constructed Oblivious Message Retrieval (OMR) protocols that outsource this job to untrusted servers in a privacy-preserving manner. We consider the case of group...

From hashing and commitment schemes to Fiat-Shamir and encryption, hash functions are everywhere in zero-knowledge proofsystems (ZKPs), and minor performance changes in ``vanilla'' implementations can translate in major discrepancies when the hash is processed as a circuit within the proofsystem. Protocol designers have resorted to a number of techniques and custom modes to optimize hash functions for ZKPs settings, but so far without a single established, well-studied construction. To...

Protecting metadata of communications has been an area of active research since the dining cryptographers problem was introduced by David Chaum in 1988. The Snowden revelations from 2013 resparked research in this direction. Consequently over the last decade we have witnessed a flurry of novel systems designed to protect metadata of users' communications online. However, such systems leverage different assumptions and design choices to achieve their goal; resulting in a scattered view of...

Traffic correlation attacks have illustrated challenges with protecting communication meta-data, yet short flows as in messaging applications like Signal have been protected by practical Mixnets such as Loopix from prior traffic correlation attacks. This paper introduces a novel traffic correlation attack against short-flow applications like Signal that are tunneled through practical Mixnets like Loopix. We propose the MixFlow model, an approach for analyzing the unlinkability of...

On the one hand, the web needs to be secured from malicious activities such as bots or DoS attacks; on the other hand, such needs ideally should not justify services tracking people's activities on the web. Anonymous tokens provide a nice tradeoff between allowing an issuer to ensure that a user has been vetted and protecting the users' privacy. However, in some cases, whether or not a token is issued reveals a lot of information to an adversary about the strategies used to distinguish...

Trellis is a mix-net based anonymous broadcast system with cryptographic security guarantees. Trellis can be used to anonymously publish documents or communicate with other users, all while assuming full network surveillance. In Trellis, users send messages through a set of servers in successive rounds. The servers mix and post the messages to a public bulletin board, hiding which users sent which messages. Trellis hides all network metadata, remains robust to changing network conditions,...

Secure group messaging (SGM) protocols allow large groups of users to communicate in a secure and asynchronous manner. In recent years, continuous group key agreements (CGKAs) have provided a powerful abstraction to reason on the security properties we expect from SGM protocols. While robust techniques have been developed to protect the contents of conversations in this context, it is in general more challenging to protect metadata (e.g. the identity and social relationships of group...

Storage-as-a-service (STaaS) permits the client to outsource her data to the cloud thereby, reducing data management and maintenance costs. However, STaaS also brings significant data integrity and soundness concerns since the storage provider might not keep the client data intact and retrievable all the time (e.g., cost saving via deletions). Proof of Retrievability (PoR) can validate the integrity and retrievability of remote data effectively. This technique can be useful for regular...

Anonymity is an (abstract) security goal that is especially important to threatened user groups. Therefore, widely deployed communication protocols implement various measures to hide different types of information (i.e., metadata) about their users. Before actually defining anonymity, we consider an attack vector about which targeted user groups can feel concerned: continuous, temporary exposure of their secrets. Examples for this attack vector include intentionally planted viruses on...

Multi-input functional encryption, MIFE, is a powerful generalization of functional encryption that allows computation on encrypted data coming from multiple different data sources. In a recent work, Agrawal, Goyal, and Tomida (CRYPTO 2021) constructed MIFE for the class of quadratic functions. This was the first MIFE construction from bilinear maps that went beyond inner product computation. We advance the state-of-the-art in MIFE, and propose new constructions with stronger security and...

We present a novel, complete definition of metadata-private messaging (MPM) and show that our definition is achievable and non-trivially more general than previous attempts that we are aware of. Our main contributions are: 1) We describe a vulnerability in existing MPM implementations through a variation of the compromised-friend (CF) attack proposed by Angel et al. Our attack can compromise the exact metadata of any conversations between honest users. 2) We present a security...

Owner-centric control is a widely adopted method for easing owners' concerns over data abuses and motivating them to share their data out to gain collective knowledge. However, while many control enforcement techniques have been proposed, privacy threats due to the metadata leakage therein are largely neglected in existing works. Unfortunately, a sophisticated attacker can infer very sensitive information based on either owners' data control policies or their analytic task participation...

Modern-day mobile communications allow users to connect from any place, at any time. However, this ubiquitous access comes at the expense of their privacy. Currently, the operators providing mobile service to users learn call-and SMS-metadata, and even the contents of those exchanges. A main reason behind this is the Lawful-Interception (LI) requirement, by which serving networks must provide this (meta-)data to authorities, given a warrant. At ESORICS 2021, Arfaoui et al. pioneered a...

Many online applications, such as online file backup services, support the sharing of indexed data between a set of devices. These systems may offer client-side encryption of the data, so that the stored data is inaccessible to the online host. A potentially desirable goal in this setting would be to protect not just the contents of the backed-up files, but also their identifiers. However, as these identifiers are typically used for indexing, a deterministic consistent mapping across devices...

Onion services enable bidirectional anonymity for parties that communicate over the Tor network, thus providing improved privacy properties compared to standard TLS connections. Since these services are designed to support server-side anonymity, the entry points for these services shuffle across the Tor network periodically. In order to connect to an onion service at a given time, the client has to resolve the .onion address for the service, which requires querying volunteer Tor nodes called...

The Deuring correspondence defines a bijection between isogenies of supersingular elliptic curves and ideals of maximal orders in a quaternion algebra. We present a new algorithm to translate ideals of prime-power norm to their corresponding isogenies --- a central task of the effective Deuring correspondence. The new method improves upon the algorithm introduced in 2021 by De Feo, Kohel, Leroux, Petit and Wesolowski as a building-block of the SQISign signature scheme. SQISign is the...

Given a private string q and a remote server that holds a set of public documents D, how can one of the K most relevant documents to q in D be selected and viewed without anyone (not even the server) learning anything about q or the document? This is the oblivious document ranking and retrieval problem. In this paper, we describe Coeus, a system that solves this problem. At a high level, Coeus composes two cryptographic primitives: secure matrix-vector product for scoring document relevance...

End-to-end encrypted file-sharing systems enable users to share files without revealing the file contents to the storage servers. However, the servers still learn metadata, including user identities and access patterns. Prior work tried to remove such leakage but relied on strong assumptions. Metal (NDSS '20) is not secure against malicious servers. MCORAM (ASIACRYPT '20) provides confidentiality against malicious servers, but not integrity. Titanium is a metadata-hiding file-sharing system...

We present a new construction for publicly verifiable anonymous tokens with private metadata. This primitive enables an issuer to generate an anonymous authentication token for a user while embedding a single private metadata bit. The token can be publicly verified, while the value of the private metadata is only accessible to the party holding the secret issuing key and remains hidden to any other party, even to the user. The security properties of this primitive also...

This paper studies the role of multiparty shuffling protocols in enabling more efficient metadata-hiding communication. We show that the process of shuffling messages can be expedited by having servers collaboratively shuffle and verify secret-shares of messages instead of using a conventional mixnet approach where servers take turns performing independent verifiable shuffles of user messages. We apply this technique to achieve both practical and asymptotic improvements in anonymous...

Improving the reliability of account protection in the blockchain is one of the most important goals of the entire cryptographic arsenal used in the blockchain and cryptocurrency exchange. We propose a new threshold multisignature scheme with a double boundary condition. Access to funds stored on a multisig wallet is possible only when two or more signatures are provided at the same time.

Anonymous message delivery systems, such as private messaging services and privacy-preserving payment systems, need a mechanism for recipients to retrieve the messages addressed to them, without leaking metadata or letting their messages be linked. Recipients could download all posted messages and scan for those addressed to them, but communication and computation costs are excessive at scale. We show how untrusted servers can detect messages on behalf of recipients, and summarize these...

As the Internet of Things (IoT) rolls out today to devices whose lifetime may well exceed a decade, conservative threat models should consider attackers with access to quantum computing power. The SUIT standard (specified by the IETF) defines a security architecture for IoT software updates, standardizing the metadata and the cryptographic tools---namely, digital signatures and hash functions---that guarantee the legitimacy of software updates. While the performance of SUIT has...

Updatable encryption (UE) is symmetric encryption which additionally supports key rotation. UE was introduced for scenarios where a user stores encrypted data on a cloud and, in order to mitigate secret key leakage, periodically sends a short update token, which the cloud uses to re-encrypt stored data to a fresh key. A long line of research resulted in a wide variety of security properties UE schemes can provide, including confidentiality, integrity protection, and hiding...

We present Spectrum, a high-bandwidth, metadata-private file broadcasting system. In Spectrum, a small number of broadcasters share a file with many subscribers via two or more non-colluding broadcast servers. Subscribers generate cover traffic by sending dummy files, hiding which users are broadcasters and which users are only consumers. Spectrum optimizes for a setting with few broadcasters and many subscribers—as is common to many real-world applications—to drastically improve throughput...

Anonymous single-use tokens have seen recent applications in private Internet browsing and anonymous statistics collection. We develop new schemes in order to include public metadata such as expiration dates for tokens. This inclusion enables planned mass revocation of tokens without distributing new keys, which for natural instantiations can give 77 % and 90 % amortized traffic savings compared to Privacy Pass (Davidson et al., 2018) and DIT: De-Identified Authenticated Telemetry at Scale...

Metadata from voice calls, such as the knowledge of who is communicating with whom, contains rich information about people’s lives. Indeed, it is a prime target for powerful adversaries such as nation states. Existing systems that hide voice call metadata either require trusted intermediaries in the network or scale to only tens of users. This paper describes the design, implementation, and evaluation of Addra, the first system for voice communication that hides metadata over fully untrusted...

Cloud computing and cloud storage are among the most efficient technologies for storing and processing metadata. But there are many privacy concerns within this domain. Most of the challenges are coming from trusted or semi trusted cloud servers where some computations must be applied to high confidential data. Data encryption can solve some confidentiality issues on the cloud but it is not easy to provide privacy preserving data processing services such as searching a query over encrypted...

Blockchain protocols based on variations of the longest-chain rule—whether following the proof-of-work paradigm or one of its alternatives—suffer from a fundamental latency barrier. This arises from the need to collect a sufficient number of blocks on top of a transaction-bearing block to guarantee the transaction’s stability while limiting the rate at which blocks can be created in order to prevent security-threatening forks. Our main result is a black-box security-amplifying combiner based...

This paper will outline, for the first time, exactly how the ITM Attack (a linkability attack against shielded transactions) works against Zcash Protocol and how Hush is the first cryptocoin with a defensive mitigation against it, called ”Sietch ”. Sietch is already running live in production and undergoing rounds of improvement from expert feedback. This is not an academic paper about pipedreams. It describes production code and networks. We begin with a literature review of all known...

File-sharing systems like Dropbox offer insufficient privacy because a compromised server can see the file contents in the clear. Although encryption can hide such contents from the servers, metadata leakage remains significant. The goal of our work is to develop a file-sharing system that hides metadata---including user identities and file access patterns. Metal is the first file-sharing system that hides such metadata from malicious users and that has a latency of only a few seconds. The...

We present a cryptographic construction for anonymous tokens with private metadata bit, called PMBTokens. This primitive enables an issuer to provide a user with a lightweight, single-use anonymous trust token that can embed a single private bit, which is accessible only to the party who holds the secret authority key and is private with respect to anyone else. Our construction generalizes and extends the functionality of Privacy Pass (PETS’18) with this private metadata bit capability. It...

Proof-of-burn has been used as a mechanism to destroy cryptocurrency in a verifiable manner. Despite its well known use, the mechanism has not been previously formally studied as a primitive. In this paper, we put forth the first cryptographic definition of what a proof-of-burn protocol is. It consists of two functions: First, a function which generates a cryptocurrency address. When a user sends money to this address, the money is irrevocably destroyed. Second, a verification function...

Anonymous communication, that is secure end-to-end and unlinkable, plays a critical role in protecting user privacy by preventing service providers from using message metadata to discover communication links between any two users. Techniques, such as Mix-net, DC-net, time delay, cover traffic, Secure Multiparty Computation and Private Information Retrieval techniques, can be used to achieve anonymous communication. However, the existing solutions are very complex and difficult to implement...

Content moderation is crucial for stopping abuse and harassment via messaging on online platforms. Existing moderation mechanisms, such as message franking, require platform providers to see user identifiers on encrypted traffic. These mechanisms cannot be used in messaging systems in which users can hide their identities, such as Signal. The key technical challenge preventing moderation is in simultaneously achieving cryptographic accountability while preserving deniability. In this work,...

A large number of studies on passwords make use of passwords leaked by attackers who compromised online services. Frequently, these leaks contain only the passwords themselves, or basic information such as usernames or email addresses. While metadata-rich leaks exist, they are often limited in the variety of demographics they cover. In this work, we analyze a meta-data rich data leak from a Middle Eastern bank with a demographically-diverse user base. We provide an analysis of...

Email communication is amongst the most prominent online activities, and as such, can put sensitive information at risk. It is thus of high importance that internet email applications are designed in a privacy-aware manner and analyzed under a rigorous threat model. The Snowden revelations (2013) suggest that such a model should feature a global adversary, in light of the observational tools available. Furthermore, the fact that protecting metadata can be of equal importance as protecting...

The US federal court system is exploring ways to improve the accountability of electronic surveillance, an opaque process often involving cases sealed from public view and tech companies subject to gag orders against informing surveilled users. One judge has proposed publicly releasing some metadata about each case on a paper cover sheet as a way to balance the competing goals of (1) secrecy, so the target of an investigation does not discover and sabotage it, and (2) accountability, to...

Software side channel attacks have become a serious concern with the recent rash of attacks on speculative processor architectures. Most attacks that have been demonstrated exploit the cache tag state as their exfiltration channel. While many existing defense mechanisms that can be implemented solely in software have been proposed, these mechanisms appear to patch specific attacks, and can be circumvented. In this paper, we propose minimal modifications to hardware to defend against a broad...

We present ‘MCMix’, an anonymous messaging system that completely hides communication metadata and can scale in the order of hundreds of thousands of users. Our approach is to isolate two suitable functionalities, called dialing and conversation, that when used in succession realize anonymous messaging. With this as a starting point, we apply secure multiparty computation (``MC'' or MPC) and proceed to realize them. We present an implementation using a prevalent MPC system (Sharemind) that...

Using the cloud to store data offers many advantages for businesses and individuals alike. The cloud storage provider, however, has to be trusted not to inspect or even modify the data they are entrusted with. Encrypting the data offers a remedy, but current solutions have various drawbacks. Providers which offer encrypted storage themselves cannot necessarily be trusted, since they have no open implementation. Existing encrypted file systems are not designed for usage in the cloud and do...

Although the transactions on the Bitcoin blockchain have the main purpose of recording currency transfers, they can also carry a few bytes of metadata. A sequence of transaction metadata forms a subchain of the Bitcoin blockchain, and it can be used to store a tamper-proof execution trace of a smart contract. Except for the trivial case of contracts which admit any trace, in general there may exist inconsistent subchains which represent incorrect contract executions. A crucial issue is how...

Private communication over the Internet remains a challenging problem. Even if messages are encrypted, it is hard to deliver them without revealing metadata about which pairs of users are communicating. Scalable anonymity systems, such as Tor, are susceptible to traffic analysis attacks that leak metadata. In contrast, the largest-scale systems with metadata privacy require passing all messages through a small number of providers, requiring a high operational cost for each provider and...

With the growing popularity of remote storage, the ability to outsource a large private database yet be able to search on this encrypted data is critical. Searchable symmetric encryption (SSE) is a practical method of encrypting data so that natural operations such as searching can be performed on this data. It can be viewed as an efficient private-key alternative to powerful tools such as fully homomorphic encryption, oblivious RAM, or secure multiparty computation. The main drawbacks of...

An efficient Dynamic Provable Data Possession scheme with Public Verifiability and Data Privacy was recently published in ACISP'15. It appears that three attacks menace this scheme. The first one enables the server to store only one block of a file $m$ and still pass the data integrity verification on any number of file blocks. The second attack permits the server to keep the old version of a file block $m_{i}$ and the corresponding verification metadata $T_{m_{i}}$, after the client asked...

Demands for lawful access to encrypted data are a long standing obstacle to integrating cryptographic protections into communication systems. A common approach is to allow a trusted third party (TTP) to gain access to private data. However, there is no way to verify that this trust is well place as the TTP may open all messages indiscriminately. Moreover, existing approaches do not scale well when, in addition to the content of the conversation, one wishes to hide ones identity. Given the...

When outsourcing large sets of data to the cloud, it is desirable for clients to efficiently check, whether all outsourced data is still retrievable at any later point in time without requiring to download all of it. Provable data possession (PDP)/proofs of retrievability (PoR), for which various constructions exist, are concepts to solve this issue. Interestingly, by now, no PDP/PoR scheme leading to an efficient construction supporting both private and public verifiability simultaneously...

Data integrity and storage efficiency are two important requirements for cloud storage. Proof of Retrievability (POR) and Proof of Data Possession (PDP) techniques assure data integrity for cloud storage. Proof of Ownership (POW) improves storage efficiency by securely removing unnecessarily duplicated data on the storage server. However, trivial combination of the two techniques, in order to achieve both data integrity and storage efficiency, results in non-trivial duplication of metadata...

With the launch of Mac OS X 10.7 (Lion), Apple has introduced a volume encryption mechanism known as FileVault 2. Apple only disclosed marketing aspects of the closed-source software, e.g. its use of the AES-XTS tweakable encryption, but a publicly available security evaluation and detailed description was unavailable until now. We have performed an extensive analysis of FileVault 2 and we have been able to find all the algorithms and parameters needed to successfully read an encrypted...

We introduce a model for {\em provable data possession} ($\pdp$) that allows a client that has stored data at an untrusted server to verify that the server possesses the original data without retrieving it. The model generates probabilistic proofs of possession by sampling random sets of blocks from the server, which drastically reduces I/O costs. The client maintains a constant amount of metadata to verify the proof. The challenge/response protocol transmits a small, constant amount of...