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FoSAM: Forward Secret Messaging in Ad-Hoc Networks

Daniel Schadt, Christoph Coijanovic, Thorsten Strufe

Abstract

Apps such as Firechat and Bridgefy have been used during recent protests in Hong Kong and Iran, as they allow communication over ad-hoc wireless networks even when internet access is restricted. However, these apps do not provide sufficient protection as they do not achieve forward secrecy in unreliable networks. Without forward secrecy, caught protesters' devices will disclose all previous messages to the authorities, putting them and others at great risk. In this paper, we introduce FoSAM, the first protocol to provide proven anonymous and forward secret messaging in unreliable ad-hoc networks. Communication in FoSAM requires only the receiver's public key, rather than an interactive handshake. We evaluate the performance of FoSAM using a large-scale simulation with different user movement patterns, showing that it achieves between 92% and 99% successful message delivery. We additionally implement a FoSAM prototype for Android.

FoSAM: Forward Secret Messaging in Ad-Hoc Networks

Abstract

Apps such as Firechat and Bridgefy have been used during recent protests in Hong Kong and Iran, as they allow communication over ad-hoc wireless networks even when internet access is restricted. However, these apps do not provide sufficient protection as they do not achieve forward secrecy in unreliable networks. Without forward secrecy, caught protesters' devices will disclose all previous messages to the authorities, putting them and others at great risk. In this paper, we introduce FoSAM, the first protocol to provide proven anonymous and forward secret messaging in unreliable ad-hoc networks. Communication in FoSAM requires only the receiver's public key, rather than an interactive handshake. We evaluate the performance of FoSAM using a large-scale simulation with different user movement patterns, showing that it achieves between 92% and 99% successful message delivery. We additionally implement a FoSAM prototype for Android.
Paper Structure (28 sections, 3 theorems, 1 equation, 8 figures, 2 tables, 5 algorithms)

This paper contains 28 sections, 3 theorems, 1 equation, 8 figures, 2 tables, 5 algorithms.

Table of Contents

  1. Introduction
  2. Related Work
  3. Background
  4. Forward Secrecy
  5. Models & Goals
  6. System Model
  7. Threat Model
  8. Goals
  9. Anonymous Communication
  10. Forward Secrecy
  11. Unidirectionality
  12. Design
  13. Specification
  14. Time Synchronization
  15. Asymmetric Ratchet
  16. ...and 13 more sections

Key Result

Lemma 1

The winning advantage of $\mathcal{A}{}$ in game:1 is negligible.

Figures (8)

  • Figure 1: Identities labelled with their epoch number. A key in the tree can be derived from the key of its parent. When switching from epoch 0 to 1, the key for epoch 4 is also generated and stored for future reference so that key 0 can be deleted. When switching from 1 to 2, the key for epoch 3 is stored and key 1 is deleted.
  • Figure 2: Overview of the two protocol components. On the left, the time synchronization is shown, in which device "a" broadcasts its current epoch and device "d" keeps track of the epoch broadcasts it has seen. On the right, the message flooding is shown, in which "a" sends a message that is flooded all the way to "f".
  • Figure 3: Time to network synchronization for a variable user count (left), a variable density (middle) and a variable attacker count (right). The standard parameters are 625 users, 1 user per square meter, and no attackers.
  • Figure 4: Time to network synchronization for a variable user count in the converging scenario.
  • Figure 5: Share of the network in the real-life scenario that shares the same epoch as time progresses.
  • ...and 3 more figures

Theorems & Definitions (11)

  • Definition 1: ke-PKE
  • Definition 2: fs-CPA canetti2003
  • Definition 3: Kuhn's Privacy Game kuhn2019
  • Definition 4: HIBE
  • Definition 5
  • Lemma 1
  • proof
  • Lemma 2
  • proof
  • Theorem 1
  • ...and 1 more