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Session: End-To-End Encrypted Conversations With Minimal Metadata Leakage

Kee Jefferys, Maxim Shishmarev, Simon Harman

TL;DR

Session addresses metadata leakage in secure messaging by combining end-to-end encryption with onion routing and decentralised storage. It replaces phone-number-based identities with pseudonymous Ed25519 public-private keys and uses a three-hop onion request path to obscure IP addresses, backed by a stake-based Session Node network on Arbitrum. The paper introduces the Session Protocol, a bespoke encryption protocol optimized for decentralised networks, supports one-on-one and group communications with mechanisms for key rotation and blinded identities in communities. The combination offers robust privacy with scalable group sizes while preserving essential features like multi-device sync, attachments, and offline delivery, though limitations such as $PFS$ in one-on-one and traffic analysis remain challenges.

Abstract

Session is an open-source, public-key-based secure messaging application which uses a set of decentralised storage servers and an onion routing protocol to send end-to-end encrypted messages with minimal exposure of user metadata. It does this while providing the common features expected of mainstream messaging applications, such as multi-device syncing, offline inboxes, and voice/video calling.

Session: End-To-End Encrypted Conversations With Minimal Metadata Leakage

TL;DR

Session addresses metadata leakage in secure messaging by combining end-to-end encryption with onion routing and decentralised storage. It replaces phone-number-based identities with pseudonymous Ed25519 public-private keys and uses a three-hop onion request path to obscure IP addresses, backed by a stake-based Session Node network on Arbitrum. The paper introduces the Session Protocol, a bespoke encryption protocol optimized for decentralised networks, supports one-on-one and group communications with mechanisms for key rotation and blinded identities in communities. The combination offers robust privacy with scalable group sizes while preserving essential features like multi-device sync, attachments, and offline delivery, though limitations such as in one-on-one and traffic analysis remain challenges.

Abstract

Session is an open-source, public-key-based secure messaging application which uses a set of decentralised storage servers and an onion routing protocol to send end-to-end encrypted messages with minimal exposure of user metadata. It does this while providing the common features expected of mainstream messaging applications, such as multi-device syncing, offline inboxes, and voice/video calling.

Paper Structure

This paper contains 69 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Threat Model
  3. In Scope
  4. Session Node Operators --- Passive/Active Attacks
  5. Network Adversary --- Passive Attacks
  6. Protections
  7. Out of Scope
  8. Network Adversary --- Active Attacks
  9. Global Passive Adversary
  10. Out of Band Key Discovery
  11. Foundations
  12. Session Nodes
  13. Onion Requests
  14. Building on Foundations
  15. Storage
  16. ...and 54 more sections

Figures (7)

  • Figure 1: A client sends an onion request which hops via 3 Session Nodes to a destination, in this case, another Session Node
  • Figure 2: Alice uses an onion request to send a message to a Session Node within Bob’s swarm, the message is then propagated within the swarm. Bob later sends an onion request to retrieve said message.
  • Figure 3: Client sends message using client-side fanout: The sender (green computer) sends messages to other clients (solid red lines) but is unable to send to an offline device (yellow phone)
  • Figure 4: Client sends message using server-side fanout: The sender client (green computer) pushes the message to the server (solid red line) and the server distributes the messages to recipient clients (dotted red lines)
  • Figure 5: Example of a Session Community Server URL
  • ...and 2 more figures