Messagerie instantanée | EPFL Graph Search

La messagerie instantanée (MI) permet l’échange instantané de messages textuels et de fichiers entre plusieurs personnes par l'intermédiaire d’ordinateurs, de tablettes ou de téléphones mobiles connectés à un même réseau informatique, plus communément à Internet. Contrairement au courrier électronique, ce moyen de communication permet de conduire un dialogue interactif. Synonymes et francisation La messagerie instantanée est souvent désignée sous le nom « chat » (/tʃæt/ du verbe anglais to chat , le plus souvent prononcé /tʃat/ en France). Le nom « tchat » et le verbe « tchatter » sont souvent utilisés en français pour désigner la messagerie instantanée. Une écriture souvent utilisée pour cette prononciation est « tchat », qui est utilisé de façon à ne pas confondre la prononciation avec celle du chat (l’animal). On peut aussi trouver « tchatte ». L’Office québécois de la langue française a proposé, en , le mot-valise « clavardage » formé de « clavier » et de « bavardage

Time in cryptography

Gwangbae Choi

Time travel has always been a fascinating topic in literature and physics. In cryptography, one may wonder how to keep data confidential for some time. In this dissertation, we will study how to make private information travel to the future. This dissertation consists of three parts: Timed-release encryption, witness encryption and self-encryption. With timed-release encryption, one can send to a counterpart a message which cannot be read before some time has elapsed. One possible solution is to use a third party. At every time period, the third party releases a time bound key, and a ciphertext which is encrypted for a time period requires the time bound key of that time period for decryption. Then, a problem occurs when the counterpart somehow loses the release time of ciphertext. We propose a solution by introducing a master time bound key which can be considered as a valid time bound key of all time periods. We propose a provably secure construction and show the experimental results. In 2018, Liu, Jager, Kakvi and Warinschi introduced a timed-release encryption scheme based on a blockchain with proof-of-work and witness encryption. Current proposals of witness encryption are based on multilinear maps. In this part, we propose a new construction without. We propose the notion of hidden group with hashing and make a witness encryption from it. We show that the construction is secure in a generic model. We propose a concrete construction based on RSA-related problems. Namely, we use an extension of the knowledge-of-exponent assumption and the order problem. We finally estimate the cost of the bitcoin blockchain implementation. Although our estimates are still high (for a release time of one hour / one year, we respectively use ciphertexts of 567 MB / 4.5 TB and a decryption time of 27 min / 5.2 months on a single core), there is room for improvement by a factor 20 000 by adapting the blockchain structure and by adopting a hash function which is better adapted to this type of programming than SHA256. In self-encryption, a device encrypts some piece of information for itself to decrypt in the future. We are interested in security of self-encryption when the state occasionally leaks. Applications where self-encryption operates are cloud storage, when a client encrypts files to be stored, and in 0-RTT session resumptions, when a server encrypts a resumption key to be stored by the client. Previous works focused on forward secrecy and resistance to replay attacks. In our work, we study post-compromise security. Post-compromise security was already solved in ratcheted instant messaging schemes, at the price of having an inflating state size. However, it was not known whether state inflation was necessary. We prove here that this is the case. In our results, we prove that post-compromise security implies a super-linear state size in terms of the number of ciphertexts which can still be decrypted by the state. We apply our result to self-encryption for cloud storage and 0-RTT session resumption, and also to secure messaging. We further show how to construct a secure scheme matching our bound on the state size.

EPFL2020

Analyzing and Protecting Communication Metadata

Ludovic Barman

Most communication systems (e.g., e-mails, instant messengers, VPNs) use encryption to prevent third parties from learning sensitive information.However, encrypted communications protect the contents but often leak metadata: the amount of data sent and the time it was sent, the way the data should be decrypted, the identity of the sender and the recipient.These metadata are a pervasive threat to privacy: They enable a variety of attacks that range from recovering plaintext contents from encrypted communications to inferring communicating parties.Our goal in this thesis is two-fold: First, to raise awareness about this problem by demonstrating attacks that threaten user privacy; and second, to propose novel solutions that reduce the metadata leakage and maintain acceptable usability and costs.To achieve the first goal, we present the first work that performs an in-depth analysis of the communications of wearable devices under the lens of traffic analysis.We demonstrate that the metadata of Bluetooth communications of wearable devices (smartwatches, fitness trackers, and blood-pressure monitors) leak sensitive information to a passive observer, despite the use of encryption.By design, these devices handle fine-grained and long-term, personal, medical and lifestyle-related data from their users.Our work highlights the need to rethink how sensitive data is exchanged in this setting.More generally, we confirm that metadata can pose a threat to user privacy, even in settings where traffic-analysis attacks are perhaps not an immediate threat.For the second goal, we begin by presenting a theoretical contribution that concerns ciphertext formats:Padmé, a padding function designed to reduce the metadata leakage of files and messages through their length.Padmé efficiently hides the size of objects even when they have very different sizes.Padmé is a part of PURBs, a ciphertext format that does not leak metadata, except for a small amount about the size.Then, we design systems that enable communicating while reducing metadata leakage: Anonymous Communication Networks (ACNs).ACNs protect against some metadata leakage (e.g., who is communicating and when).Traffic-analysis resistant ACNs have not seen widespread adoption yet, possibly due to their technical shortcomings.We focus on two aspects that remain unsolved: small-scale traffic-agnostic communication that achieves low latency, and large-scale asynchronous messaging that handles millions of users.We present PriFi, an ACN that provides provable traffic-analysis resistance and low-latency in the context of a local-area network.The protocol hides the source and recipient of a message by ensuring that the communication patterns of all participants are equal, even in the presence of active attacks.PriFi ensures low-latency and traffic-agnostic communication for a small set of users.We then present Rubato, an ACN for anonymous messaging that handles millions of users.Other large-scale ACNs have an important limitation: A sender and a recipient need to be online at the same time to resist intersection attacks.As participants are uncoordinated, they need to be online and to send cover traffic at all times, which is infeasible for many users.Rubato is a circuit-based mixnet that enables its users to be asynchronous; they can participate in the network according to their own schedule.Unlike previous ACNs, this enables users to participate using their mobile devices.

EPFL2021

Toward Mobile Distributed Ledgers

Dimitrios Chatzopoulos, Boi Faltings, Sujit Prakash Gujar, Anurag Jain

Advances in mobile computing have paved the way for new types of distributed applications that can be executed solely by mobile devices on Device-to-Device (D2D) ecosystems (e.g., crowdsensing). Sophisticated applications, like cryptocurrencies, need distributed ledgers (DLs) to function. DLs, such as blockchains and directed acyclic graphs (DAGs), employ consensus protocols to add data in the form of blocks. However, such protocols are designed for resourceful devices that are interconnected via the Internet. Moreover, existing DLs are not deployable to D2D ecosystems since their storage needs are continuously increasing. In this work, we introduce and analyze Mneme, a DAG-based DL that can be maintained solely by mobile devices. Mneme utilizes two novel consensus protocols: 1) Proof of Context (PoC) and 2) Proof of Equivalence (PoE). PoC employs users' context to add data on Mneme. PoE is executed periodically to summarize data and produce equivalent blocks that require less storage. We analyze Mneme's security and justify the ability of PoC and PoE to guarantee the characteristics of DLs: persistence and liveness. Furthermore, we analyze potential attacks from malicious users and prove that the probability of a successful attack is inversely proportional to the square of the number of mobile users who maintain Mneme.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022