Private Message Franking with After Opening Privacy

Recently Grubbs et al. [GLR17] initiated the formal study of message franking protocols. This new type of service launched by Facebook, allows the receiver in a secure messaging application to verifiably report to a third party an abusive message some sender has sent. A novel cryptographic primitive: committing AEAD has been initiated, whose functionality apart from confidentiality and authenticity asks for a compact commitment over the message, which is delivered to the receiver as part of the ciphertext. A new construction CEP (Committing Encrypt and PRF) has then been proposed, which is multi-opening secure and reduces the computational costs for the sender and the receiver. Despite the merits of the message franking protocols [GLR17], our observation which launched this work, is that all the designs be it compositional or the CEP construction, leak too much when the receiver needs to open the abusive message to the third party. Namely, the receiver opens the entire message along with the opening key to the third party, thus confidentiality of the message is entirely broken. Moreover, the opening of the entire message increases the communication cost of the protocol and in cases of big messages being exchanged (attachments, videos, multimedia files, etc.) it might be unnecessary. We provide to the best of our knowledge the first formal treatment of message franking protocols with minimum leakage whereby only the abusive blocks are opened, while the rest non-abusive blocks of the message remain private. First we give a new definition for multi-opening indistinguishability with partial opening (MO-IND-PO), which forces an adversary to distinguish encryptions of abusive blocks. We then design and analyze two protocols CEP-AOP1 (Committing Encrypt and PRF with After Opening Privacy) and CEP-AOP2, which adhere to the new privacy definition. As a side contribution we show a multi-opening secure CEP-AOP2 construction using only one PRF evaluation over the message, in a weaker but meaningful security model, relying only on standard assumptions of the underlying symmetric primitives

People Who Live in Glass Houses Should not Throw Stones: Targeted Opening Message Franking Schemes

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 session in order to report one abuse. This is highly undesirable in many settings where revealing other non-abusive part of the communication leaks too much information; what is worse, a foxy adversary may intentionally mixing private information of the receiver with the abusive message so that the receiver will be reluctant to report. This essentially renders the abuse reporting mechanism ineffective. To tackle this problem, we propose a new primitive called targeted opening compactly committing AEAD (TOCE for short). In a TOCE, the receiver can select arbitrary subset of bits from the plaintext to reveal during opening, while keep all the rest still secure as in an authenticated encryption. We gave a careful formulation, together with a generic construction which allowing a bit level targeted opening. While the generic construction may require a substantial number of passes of symmetric key ciphers when encrypting a large message such as a picture, we thus further set forth and give a more efficient non-black-box construction allowing a block-level (e.g., 256 bit) opening. We also propose a privacy-efficiency trade off if we can relax the security of non-opened messages to be one way secure after the abusive reporting (they are still semantically secure if no opening)

Hecate: abuse reporting in secure messengers with sealed sender

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 work, we provide a new construction for asymmetric message franking called Hecate that is faster, more secure, and introduces additional functionality compared to Tyagi et al. First, our construction uses fewer invocations of standardized crypto primitives and operates in the plain model. Second, on top of AMF’s accountability and deniability requirements, we also add forward and backward secrecy. Third, we combine AMF with source tracing, another approach to content moderation that has previously been considered only in the setting of non-anonymous networks. Source tracing allows for messages to be forwarded, and a report only identifies the original source who created a message. To provide anonymity for senders and forwarders, we introduce a model of "AMF with preprocessing" whereby every client authenticates with the moderator out-of-band to receive a token that they later consume when sending a message anonymously.CNS-1718135 - National Science Foundation; CNS-1801564 - National Science Foundation; OAC-1739000 - National Science Foundation; CNS-1931714 - National Science Foundation; CNS-1915763 - National Science Foundation; HR00112020021 - Department of Defense/DARPA; 000000000000000000000000000000000000000000000000000000037211 - SRI Internationalhttps://www.usenix.org/system/files/sec22-issa.pdfPublished versio

Abuse Reporting for Metadata-Hiding Communication Based on Secret Sharing

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 order of magnitude increases in client and server computation or fundamental changes to the architecture of messaging systems. As a result, while metadata-hiding communication inches closer to practice, critical content moderation concerns remain unaddressed. This paper demonstrates that, for broad classes of metadata-hiding schemes, lightweight abuse reporting can be deployed with minimal changes to the overall architecture of the system. Our insight is that much of the structure needed to support abuse reporting already exists in these schemes. By taking a non-generic approach, we can reuse this structure to achieve abuse reporting with minimal overhead. In particular, we show how to modify schemes based on secret sharing user inputs to support a message franking-style protocol. Compared to prior work, our shared franking technique results in a $50\%$ reduction in the time to prepare a franked message and order of magnitude reductions in server-side message processing times, as well as the time to decrypt a message and verify a report

Threshold Moderation for End-to-End Encrypted Messaging

Encrypted messaging is used by billions of people daily to ensure private communications. This privacy enables malicious uses, including spam and other abusive or illegal content. Meta’s Messenger service uses a system called message franking to moderate end-to-end encrypted (E2EE) messages. This system gives Meta total authority in moderation decisions as well as full responsibility for moderating many millions of messages a day. This paper modifies message franking to allow many users to act as moderators, while maintaining the system’s original security properties. The threshold moderation system uses small groups of moderators responsible for each message, who vote on how to act on each report. The protocol uses threshold secret sharing to condition the undeniability of message reports on a majority vote of the moderators. This decentralized protocol prevents the platform from enforcing unpopular moderation policies while alleviating the burden of moderating every message themselves.Bachelor of Scienc

Hecate: Abuse Reporting in Secure Messengers with Sealed Sender

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 work, we provide a new construction for asymmetric message franking called Hecate that is faster, more secure, and introduces additional functionality compared to Tyagi et al. First, our construction uses fewer invocations of standardized crypto primitives and operates in the plain model. Second, on top of AMF’s accountability and deniability requirements, we also add forward and backward secrecy. Third, we combine AMF with source tracing, another approach to content moderation that has previously been considered only in the setting of non-anonymous networks. Source tracing allows for messages to be forwarded, and a report only identifies the original source who created a message. To provide anonymity for senders and forwarders, we introduce a model of AMF with preprocessing whereby every client authenticates with the moderator out-of-band to receive a token that they later consume when sending a message anonymously

One Protocol to Rule Them All? On Securing Interoperable Messaging

European lawmakers have ruled that users on different platforms should be able to exchange messages with each other. Yet messaging interoperability opens up a Pandora's box of security and privacy challenges. While championed not just as an anti-trust measure but as a means of providing a better experience for the end user, interoperability runs the risk of making the user experience worse if poorly executed. There are two fundamental questions: how to enable the actual message exchange, and how to handle the numerous residual challenges arising from encrypted messages passing from one service provider to another -- including but certainly not limited to content moderation, user authentication, key management, and metadata sharing between providers. In this work, we identify specific open questions and challenges around interoperable communication in end-to-end encrypted messaging, and present high-level suggestions for tackling these challenges

Towards secure computation for people

My research investigates three questions: How do we customize protocols and implementations to account for the unique requirement of each setting and its target community, what are necessary steps that we can take to transition secure computation tools into practice, and how can we promote their adoption for users at large? In this dissertation I present several of my works that address these three questions with a particular focus on one of them. First my work on "Hecate: Abuse Reporting in Secure Messengers with Sealed Sender" designs a customized protocol to protect people from abuse and surveillance in online end to end encrypted messaging. Our key insight is to add pre-processing to asymmetric message franking, where the moderating entity can generate batches of tokens per user during off-peak hours that can later be deposited when reporting abuse. This thesis then demonstrates that by carefully tailoring our cryptographic protocols for real world use cases, we can achieve orders of magnitude improvements over prior works with minimal assumptions over the resources available to people. Second, my work on "Batched Differentially Private Information Retrieval" contributes a novel Private Information Retrieval (PIR) protocol called DP-PIR that is designed to provide high throughput at high query rates. It does so by pushing all public key operations into an offline stage, batching queries from multiple clients via techniques similar to mixnets, and maintain differential privacy guarantees over the access patterns of the database. Finally, I provide three case studies showing that we cannot hope to further the adoption of cryptographic tools in practice without collaborating with the very people we are trying to protect. I discuss a pilot deployment of secure multi-party computation (MPC) that I have done with the Department of Education, deployments of MPC I have done for the Boston Women’s Workforce Council and the Greater Boston Chamber of Commerce, and ongoing work in developing tool chain support for MPC via an automated resource estimation tool called Carousels

Integrating Causality in Messaging Channels

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 according to their sending order. Our starting point is to notice that this intuition is wrong. Until now, for messaging channels (where conversations take place), both the proper causality model and the provably secure constructions have been left open. Our work fills this gap, with the goal to facilitate the formal understanding of causality preservation in messaging. First, we focus on the common two-user secure messaging channels and model the desired causality preservation property. We take the popular Signal protocol as an example and analyze the causality security of its cryptographic core (the double-ratchet mechanism). We show its inadequacy with a simple causality attack, then fix it such that the resulting Signal channel is causality-preserving, even in a strong sense that guarantees post-compromise security. Our fix is actually generic: it can be applied to any bidirectional channel to gain strong causality security. Then, we model causality security for the so-called message franking channels. Such a channel additionally enables end users to report individual abusive messages to a server (e.g., the service provider), where this server relays the end-to-end-encrypted communication between users. Causality security in this setting further allows the server to retrieve the necessary causal dependencies of each reported message, essentially extending isolated reported messages to message flows. This has great security merit for dispute resolution, because a benign message may be deemed abusive when isolated from the context. As an example, we apply our model to analyze Facebook’s message franking scheme. We show that a malicious user can easily trick Facebook (i.e., the server) to accuse an innocent user. Then we fix this issue by amending the underlying message franking channel to preserve the desired causality