We present $\textit{Rhombus}$, a new secure matrix-vector multiplication (MVM) protocol in the semi-honest two-party setting, which is able to be seamlessly integrated into existing privacy-preserving machine learning (PPML) frameworks and serve as the basis of secure computation in linear layers. $\textit{Rhombus}$ adopts RLWE-based homomorphic encryption (HE) with coefficient encoding, which allows messages to be chosen from not only a field $\mathbb{F}_p$ but also a ring...

Many messaging platforms have integrated end-to-end (E2E) encryption into their services. This widespread adoption of E2E encryption has triggered a technical tension between user privacy and illegal content moderation. The existing solutions either support only unframeability or deniability, or they are prone to abuse (the moderator can perform content moderation for all messages, whether illegal or not), or they lack mechanisms for retrospective content moderation. To address the above...

Nowadays, the problem of point-to-point encryption is solved by the wide adaptation of protocols like TLS. However, challenges persist for End-to-End Encryption (E2EE). Current E2EE solutions, such as PGP and secure messengers like Signal, suffer from issues like 1) low usability, 2) small user base, 3) dependence on central service providers, and 4) susceptibility to backdoors. Concerns over legally mandated backdoors are rising as the US and EU are proposing new surveillance regulations...

Users increasingly store their data in the cloud, thereby benefiting from easy access, sharing, and redundancy. To additionally guarantee security of the outsourced data even against a server compromise, some service providers have started to offer end-to-end encrypted (E2EE) cloud storage. With this cryptographic protection, only legitimate owners can read or modify the data. However, recent attacks on the largest E2EE providers have highlighted the lack of solid foundations for this...

We analyse the cryptographic protocols underlying Delta Chat, a decentralised messaging application which uses e-mail infrastructure for message delivery. It provides end-to-end encryption by implementing the Autocrypt standard and the SecureJoin protocols, both making use of the OpenPGP standard. Delta Chat's adoption by categories of high-risk users such as journalists and activists, but also more generally users in regions affected by Internet censorship, makes it a target for powerful...

End-to-End (E2E) encrypted messaging, which prevents even the service provider from learning communication contents, is gaining popularity. Since users care about maintaining access to their data even if their devices are lost or broken or just replaced, these systems are often paired with cloud backup solutions: Typically, the user will encrypt their messages with a fixed key, and upload the ciphertexts to the server. Unfortunately, this naive solution has many drawbacks. First, it often...

End-to-end encrypted messaging applications ensure that an attacker cannot read a user's message history without their decryption keys. While this provides strong privacy, it creates a usability problem: if a user loses their devices and cannot access their decryption keys, they can no longer access their account. To solve this usability problem, users should be able to back up their account information with the messaging provider. For privacy, this backup should be encrypted and the...

Causal reasoning plays an important role in the comprehension of communication, but it has been elusive so far how causality should be properly preserved by instant messaging services. To the best of our knowledge, causality preservation is not even treated as a desired security property by most (if not all) existing secure messaging protocols like Signal. This is probably due to the intuition that causality seems already preserved when all received messages are intact and displayed...

The iMessage PQ3 protocol is an end-to-end encrypted messaging protocol designed for exchanging data in long-lived sessions between two devices. It aims to provide classical and post-quantum confidentiality for forward secrecy and post-compromise secrecy, as well as classical authentication. Its initial authenticated key exchange is constructed from digital signatures plus elliptic curve Diffie–Hellman and post-quantum key exchanges; to derive per-message keys on an ongoing basis, it employs...

This research extends Abadi and Andersen's exploration of neural networks using secret keys for information protection in multiagent systems. Focusing on enhancing confidentiality properties, we employ end-to-end adversarial training with neural networks Alice, Bob, and Eve. Unlike prior work limited to 64-bit messages, our study spans message sizes from 4 to 1024 bits, varying batch sizes and training steps. An innovative aspect involves training model Bob to approach a minimal error value...

Recent constructions of private information retrieval (PIR) have seen significant improvements in computational performance. However, these improvements rely on heavy offline preprocessing that is typically difficult in real-world applications. Motivated by the question of PIR with no offline processing, we introduce WhisPIR, a fully stateless PIR protocol with low per-query communication. WhisPIR clients are all ephemeral, meaning that they appear with only the protocol public parameters...

We argue that there are some scenarios in which plausible deniability might be desired for a digital signature scheme. For instance, the non-repudiation property of conventional signature schemes is problematic in designing an Instant Messaging system (WPES 2004). In this paper, we formally define a non-binding signature scheme in which the Signer is able to disavow her own signature if she wants, but, the Verifier is not able to dispute a signature generated by the Signer. That is,...

In May 2020, Zoom Video Communications, Inc. (Zoom) announced a multi-step plan to comprehensively support end-to-end encrypted (E2EE) group video calls and subsequently rolled out basic E2EE support to customers in October 2020. In this work we provide the first formal security analysis of Zoom's E2EE protocol, and also lay foundation to the general problem of E2EE group video communication. We observe that the vast security literature analyzing asynchronous messaging does not translate...

Post-compromise security (PCS) has been a core goal of end-to-end encrypted messaging applications for many years, both in one-to-one continuous key agreement (CKA) and for groups (CGKA). At its essence, PCS relies on a compromised party to perform a key update in order to `self-heal'. However, due to bandwidth constraints, receive-only mode, and various other environmental demands of the growing number of use cases for such CGKA protocols, a group member may not be able to issue such...

Developing end-to-end encrypted instant messaging solutions for group conversations is an ongoing challenge that has garnered significant attention from practitioners and the cryptographic community alike. Notably, industry-leading messaging apps such as WhatsApp and Signal Messenger have adopted the Sender Keys protocol, where each group member shares their own symmetric encryption key with others. Despite its widespread adoption, Sender Keys has never been formally modelled in the...

Key-policy attribute-based encryption scheme (KP-ABE) uses a set of attributes as public keys for encryption. It allows homomorphic evaluation of ciphertext into another ciphertext of the same message, which can be decrypted if a certain access policy based on the attributes is satisfied. A lattice-based KP-ABE scheme is reported in several works in the literature, and its software implementation is available in an open-source library called PALISADE. However, as the cryptographic primitives...

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...

WhatsApp is an end-to-end encrypted (E2EE) messaging service used by billions of people. In late 2021, WhatsApp rolled out a new protocol for backing up chat histories. The E2EE WhatsApp backup protocol (WBP) allows users to recover their chat history from passwords, leaving WhatsApp oblivious of the actual encryption keys. The WBP builds upon the OPAQUE framework for password-based key exchange, which is currently undergoing standardization. While considerable efforts have gone into the...

A Key Derivation Function (KDF) generates a uniform and highly random key-stream from weakly random key material. KDFs are broadly used in various security protocols such as digital signatures and key exchange protocols. HKDF, the most deployed KDF in practice, is based on the extract-then-expand paradigm. It is presently used, among others, in the Signal Protocol for end-to-end encrypted messaging. HKDF is a generic KDF for general input sources and thus is not optimized for...

We propose a new cryptographic primitive called "verifiably encrypted threshold key derivation" (vetKD) that extends identity-based encryption with a decentralized way of deriving decryption keys. We show how vetKD can be leveraged on modern blockchains to build scalable decentralized applications (or "dapps") for a variety of purposes, including preventing front-running attacks on decentralized finance (DeFi) platforms, end-to-end encryption for decentralized messaging and social networks...

We report several practically-exploitable cryptographic vulnerabilities in the Matrix standard for federated real-time communication and its flagship client and prototype implementation, Element. These, together, invalidate the confidentiality and authentication guarantees claimed by Matrix against a malicious server. This is despite Matrix’ cryptographic routines being constructed from well-known and -studied cryptographic building blocks. The vulnerabilities we exploit differ in their...

Continuous Group Key Agreement (CGKA) lets a evolving group of clients agree on a sequence of group keys. An important application of CGKA is scalable asynchronous end-to-end (E2E) encrypted group messaging. A major problem preventing the use of CGKA over unreliable infrastructure are so-called forks. A fork occurs when group members have diverging views of the group's history (and thus its current state); e.g. due to network or server failures. Once communication channels are restored,...

The Digital Markets Act (DMA) is a nascent European Union regulation adopted in May 2022. One of its most controversial provisions is a requirement that so-called “gatekeepers” offering end-to-end encrypted messaging apps, such as WhatsApp, implement “interoperability” with other messaging apps: in essence, encrypted messaging across service providers. This requirement represents a fundamental shift in the design assumptions of existing encrypted messaging systems, most of which are designed...

The goal of this research is to raise technical doubts regarding the usefulness of the repeated attempts by governments to curb Cryptography (aka the ``Crypto Wars''), and argue that they, in fact, cause more damage than adding effective control. The notion of Anamorphic Encryption was presented in Eurocrypt '22 for a similar aim. There, despite the presence of a Dictator who possesses all keys and knows all messages, parties can arrange a hidden ``anamorphic'' message in an otherwise...

Encryption alone is not enough for secure end-to-end encrypted messaging: a server must also honestly serve public keys to users. Key transparency has been presented as an efficient solution for detecting (and hence deterring) a server that attempts to dishonestly serve keys. Key transparency involves two major components: (1) a username to public key mapping, stored and cryptographically committed to by the server, and, (2) an out-of-band consistency protocol for serving short commitments...

Facebook introduced message franking to enable users to report abusive content verifiably in end-to-end encrypted messaging. Grubbs et al. formalized the underlying primitive called compactly committing authenticated encryption with associated data (ccAEAD) and presented schemes with provable security. Dodis et al. proposed a core building block called encryptment and presented a generic construction of ccAEAD with encryptment and standard AEAD. This paper first proposes to use a...

As end-to-end encrypted messaging services become widely adopted, law enforcement agencies have increasingly expressed concern that such services interfere with their ability to maintain public safety. Indeed, there is a direct tension between preserving user privacy and enabling content moderation on these platforms. Recent research has begun to address this tension, proposing systems that purport to strike a balance between the privacy of ''honest'' users and traceability of ''malicious''...

Key Transparency (KT) systems allow end-to-end encrypted service providers (messaging, calls, etc.) to maintain an auditable directory of their users’ public keys, producing proofs that all participants have a consistent view of those keys, and allowing each user to check updates to their own keys. KT has lately received a lot of attention, in particular its privacy preserving variants, which also ensure that users and auditors do not learn anything beyond what is necessary to use the...

We investigate how users of instant messaging (IM) services can acquire strong encryption keys to back up their messages and media with strong cryptographic guarantees. Many IM users regularly change their devices and use multiple devices simultaneously, ruling out any long-term secret storage. Extending the end-to-end encryption guarantees from just message communication to also incorporate backups has so far required either some trust in an IM or outsourced storage provider, or use of...

Secure messaging applications are deployed on devices that can be compromised, lost, stolen, or corrupted in many ways. Thus, recovering from attacks to get back to a clean state is essential and known as healing. Signal is a widely-known, privacy-friendly messaging application, that uses key-ratcheting mechanism updates keys at each stage to provide end-to-end channel security, forward secrecy, and post-compromise security. We strengthen this last property, by providing a faster healing. ...

End-to-end encryption (E2EE) is vitally important to security and privacy online, yet currently under-defined. In this note, we map intuitive notions of end-to-end encryption to existing notions of encryption. In particular, we introduce the notion of endness as an notion which end-to-end systems must achieve in addition to traditional security notions associated with encryption, and provide formalizations to capture practical requirements. We demonstrate how the notion of encryption plus...

We model and analyze the Signal end-to-end secure messaging protocol within the Universal Composability (UC) framework. Specifically: (1) We formulate an ideal functionality that captures end-to-end secure messaging in a setting with Public Key Infrastructure (PKI) and an untrusted server, against an adversary that has full control over the network and can adaptively and momentarily compromise parties at any time, obtaining their entire internal states. Our analysis captures the forward...

Recently, there has been an increase in the popularity of messaging applications that use end-to-end encryption. Among them were Telegram (in October 2021 it has 550 million active users), Signal (in January 2022 it has over 50 million downloads in the Google Play Store), WhatsApp (according to Statista, in 2021 it has over 2 billion active users), Wire (until January 2022 it has been downloaded for over 1 million times on Android devices). Two distinct protocols underlying these...

The Signal protocol is a secure messaging protocol providing end-to-end encrypted asynchronous communication. In this paper, we focus on a method capable of hiding membership information from the viewpoint of non group members in a secure group messaging (SGM) protocol, which we call "membership privacy''. Although Chase et al. (ACM CCS 2020) have considered the same notion, their proposal is an extension of Signal so called "Pairwise Signal'' where a group message is repeatedly sent over...

End-to-end encryption provides strong privacy protections to billions of people, but it also complicates efforts to moderate content that can seriously harm people. To address this concern, Tyagi et al. [CRYPTO 2019] introduced the concept of asymmetric message franking (AMF), which allows people to report abusive content to a moderator, while otherwise retaining end-to-end privacy by default and even compatibility with anonymous communication systems like Signal’s sealed sender. In this...

Continuous Group Key Agreement (CGKA) -- or Group Ratcheting -- lies at the heart of a new generation of scalable End-to-End secure (E2E) cryptographic multi-party applications. One of the most important (and first deployed) CGKAs is ITK which underpins the IETF's upcoming Messaging Layer Security E2E secure group messaging standard. To scale beyond the group sizes possible with earlier E2E protocols, a central focus of CGKA protocol design is to minimize bandwidth requirements (i.e....

Sender-anonymous end-to-end encrypted messaging allows sending messages to a recipient without revealing the sender’s identity to the messaging platform. Signal recently introduced a sender anonymity feature that includes an abuse mitigation mechanism meant to allow the platform to block malicious senders on behalf of a recipient. We explore the tension between sender anonymity and abuse mitigation. We start by showing limitations of Signal’s deployed mechanism, observing that it results in...

Recent years have seen a strong uptick in both the prevalence and real-world consequences of false information spread through online platforms. At the same time, encrypted messaging systems such as WhatsApp, Signal, and Telegram, are rapidly gaining popularity as users seek increased privacy in their digital lives. The challenge we address is how to combat the viral spread of misinformation without compromising privacy. Our FACTS system tracks user complaints on messages obliviously, only...

The Signal protocol is used in many messaging applications today. While it is an active research topic to design a post-quantum variant of the protocol, no such variant is currently realized in the real world. In the following document we describe a hybrid version of the Signal protocol, that will be implemented to achieve post-quantum security for Tutanota’s end-to-end encrypted e-mails.

The key exchange protocol that establishes initial shared secrets in the handshake of the Signal end-to-end encrypted messaging protocol has several important characteristics: (1) it runs asynchronously (without both parties needing to be simultaneously online), (2) it provides implicit mutual authentication while retaining deniability (transcripts cannot be used to prove either party participated in the protocol), and (3) it retains security even if some keys are compromised (forward...

Symmetric key cryptography is applied in almost all secure communications to protect all sensitive information from attackers, for instance, banking, and thus, it requires extra attention due to diverse applications. Moreover, it is vulnerable to various attacks, for example, cryptanalysis attacks. Cryptanalysis attacks are possible due to a single-keyed encryption system. The state-of-the-art symmetric communication protocol uses a single secret key to encrypt/decrypt the entire...

Zoom Meeting is an enterprise online video conferencing solution with real-time messaging and content sharing. However, they are lack of privacy protection since centralized Zoom servers are capable of monitoring user’s messages. Thereby, to solve the privacy problem, in May 2020, Zoom acquired Keybase so that Keybase’s team can help it to build end-to-end encryption meeting while remain Zoom’s current scalability and high-performance. Nonetheless, according to the latest released Zoom’s...

Secure group messaging protocols, providing end-to-end encryption for group communication, need to handle mobile devices frequently being offline, group members being added or removed, and the possibility of device compromises during long-lived chat sessions. Existing work targets a centralized network model in which all messages are routed through a single server, which is trusted to provide a consistent total order on updates to the group state. In this paper we adapt secure group...

Messaging systems are used to spread misinformation and other malicious content, often with dire consequences. End-to-end encryption improves privacy but hinders content-based moderation and, in particular, obfuscates the original source of malicious content. We introduce the idea of message traceback, a new cryptographic approach that enables platforms to simultaneously provide end-to-end encryption while also being able to track down the source of malicious content reported by users. We...

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,...

Message franking enables cryptographically verifiable reporting of abusive content in end-to-end encrypted messaging. Grubbs, Lu, and Ristenpart recently formalized the needed underlying primitive, what they call compactly committing authenticated encryption (AE), and analyzed the security of a number of approaches. But all known secure schemes are still slow compared to the fastest standard AE schemes. For this reason Facebook Messenger uses AES-GCM for franking of attachments such as...

Message franking enables a receiver to report a potential abuse in a secure messaging system which employs an end to end encryption. Such mechanism is crucial for accountability and is already widely adopted in real world products such as the Facebook messenger. Grubs et al initiated a systematic study of such a new primitive, and Dodis et al gave a more efficient construction. We observe that in all existing message franking schemes, the receiver has to reveal the whole communication for a...

In this paper, we analyze the security of an end-to-end encryption scheme (E2EE) of LINE, a.k.a Letter Sealing. LINE is one of the most widely-deployed instant messaging applications, especially in East Asia. By a close inspection of their protocols, we give several attacks against the message integrity of Letter Sealing. Specifically, we propose forgery and impersonation attacks on the one-to-one message encryption and the group message encryption. All of our attacks are feasible with...

End-to-end encrypted messaging (E2E) is only secure if participants have a way to retrieve the correct public key for the desired recipient. However, to make these systems usable, users must be able to replace their keys (e.g. when they lose or reset their devices, or reinstall their app), and we cannot assume any cryptographic means of authenticating the new keys. In the current E2E systems, the service provider manages the directory of public keys of its registered users; this allows a...

A public key infrastructure (PKI) binds public keys to the identities of their respective owners. It employs certificate authorities or a web of trust over social links to transitively build cryptographic trust across parties in the form of chains of certificates. In existing PKIs, Alice cannot send a message to Bob confidentially until a complete chain of trust from Alice to Bob exists. We observe that this temporal restriction---which may be severely limiting in some contexts like...

Secure instant messaging is utilized in two variants: one-to-one communication and group communication. While the first variant has received much attention lately (Frosch et al., EuroS&P16; Cohn-Gordon et al., EuroS&P17; Kobeissi et al., EuroS&P17), little is known about the cryptographic mechanisms and security guarantees of secure group communication in instant messaging. To approach an investigation of group instant messaging protocols, we first provide a comprehensive and realistic...

In the past few years secure messaging has become mainstream, with over a billion active users of end-to-end encryption protocols through apps such as WhatsApp, Signal, Facebook Messenger, Google Allo, Wire and many more. While these users' two-party communications now enjoy very strong security guarantees, it turns out that many of these apps provide, without notifying the users, a weaker property for group messaging: an adversary who compromises a single group member can...

We initiate the study of message franking, recently introduced in Facebook’s end-to-end encrypted message system. It targets verifiable reporting of abusive messages to Facebook without compromising security guarantees. We capture the goals of message franking via a new cryptographic primitive: compactly committing authenticated encryption with associated data (AEAD). This is an AEAD scheme for which a small part of the ciphertext can be used as a cryptographic commitment to the message...

The Signal protocol is a cryptographic messaging protocol that provides end-to-end encryption for instant messaging in WhatsApp, Wire, and Facebook Messenger among many others, serving well over 1 billion active users. Signal includes several uncommon security properties (such as "future secrecy" or "post-compromise security"), enabled by a novel technique called *ratcheting* in which session keys are updated with every message sent. We conduct a formal security analysis of Signal's...

Web applications rely on servers to store and process confidential information. However, anyone who gains access to the server (e.g., an attacker, a curious administrator, or a government) can obtain all of the data stored there. This paper presents Mylar, a platform that provides end-to-end encryption to web applications. Mylar protects the confidentiality of sensitive data fields against attackers that gained access to servers. Mylar stores sensitive data encrypted on the server, and...

Silent Text, the instant messaging application by the company Silent Circle, provides its users with end-to-end encrypted communication on the Blackphone and other smartphones. The underlying protocol, SCimp, has received many extensions during the update to version 2, but has not been subjected to critical review from the cryptographic community. In this paper, we analyze both the design and implementation of SCimp by inspection of the documentation (to the extent it exists) and code. Many...

Telegram is a popular messaging app which supports end-to-end encrypted communication. In Spring 2015 we performed an audit of Telegram's source code. This short paper summarizes our findings. Our main discovery is that the symmetric encryption scheme used in Telegram -- known as MTProto -- is not IND-CCA secure, since it is possible to turn any ciphertext into a different ciphertext that decrypts to the same message. We stress that this is a theoretical attack on the definition of...

Instant messaging services are quickly becoming the most dominant form of communication among consumers around the world. Apple iMessage, for example, handles over 2 billion message each day, while WhatsApp claims 16 billion messages from 400 million international users. To protect user privacy, these services typically implement end-to-end and transport layer encryption, which are meant to make eavesdropping infeasible even for the service providers themselves. In this paper, however, we...

The certificate authority model for authenticating public keys of websites has been attacked in recent years, and several proposals have been made to reinforce it. We develop and extend "certificate transparency}", a proposal in this direction, so that it efficiently handles certificate revocation. We show how this extension can be used to build a secure end-to-end email or messaging system using PKI with no requirement to trust certificate authorities, or to rely on complex peer-to-peer...

Mechanisms for secure mobile communication can be enablers for novel applications in the area of cooperative work. In this context, this article exemplarily investigates an emergency management setting. An efficient support of emergency communication is of high practical importance, but has specific challenges: unpredictable local crisis situations harden the establishment of communication structures, legal requirements dictate the use of end-to-end secure and documentable approaches, while...