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Bytes to Schlep? Use a FEP: Hiding Protocol Metadata with Fully Encrypted Protocols

Ellis Fenske, Aaron Johnson

TL;DR

Novel security definitions are provided that capture the metadata-protection goals of Fully Encrypted Protocols, and novel ways in which these protocols are identifiable are identified, including their responses to the introduction of data errors and the sizes of their smallest protocol messages.

Abstract

Fully Encrypted Protocols (FEPs) have arisen in practice as a technique to avoid network censorship. Such protocols are designed to produce messages that appear completely random. This design hides communications metadata, such as version and length fields, and makes it difficult to even determine what protocol is being used. Moreover, these protocols frequently support padding to hide the length of protocol fields and the contained message. These techniques have relevance well beyond censorship circumvention, as protecting protocol metadata has security and privacy benefits for all Internet communications. The security of FEP designs depends on cryptographic assumptions, but neither security definitions nor proofs exist for them. We provide novel security definitions that capture the metadata-protection goals of FEPs. Our definitions are given in both the datastream and datagram settings, which model the ubiquitous TCP and UDP interfaces available to protocol designers. We prove relations among these new notions and existing security definitions. We further present new FEP constructions and prove their security. Finally, we survey existing FEP candidates and characterize the extent to which they satisfy FEP security. We identify novel ways in which these protocols are identifiable, including their responses to the introduction of data errors and the sizes of their smallest protocol messages.

Bytes to Schlep? Use a FEP: Hiding Protocol Metadata with Fully Encrypted Protocols

TL;DR

Novel security definitions are provided that capture the metadata-protection goals of Fully Encrypted Protocols, and novel ways in which these protocols are identifiable are identified, including their responses to the introduction of data errors and the sizes of their smallest protocol messages.

Abstract

Fully Encrypted Protocols (FEPs) have arisen in practice as a technique to avoid network censorship. Such protocols are designed to produce messages that appear completely random. This design hides communications metadata, such as version and length fields, and makes it difficult to even determine what protocol is being used. Moreover, these protocols frequently support padding to hide the length of protocol fields and the contained message. These techniques have relevance well beyond censorship circumvention, as protecting protocol metadata has security and privacy benefits for all Internet communications. The security of FEP designs depends on cryptographic assumptions, but neither security definitions nor proofs exist for them. We provide novel security definitions that capture the metadata-protection goals of FEPs. Our definitions are given in both the datastream and datagram settings, which model the ubiquitous TCP and UDP interfaces available to protocol designers. We prove relations among these new notions and existing security definitions. We further present new FEP constructions and prove their security. Finally, we survey existing FEP candidates and characterize the extent to which they satisfy FEP security. We identify novel ways in which these protocols are identifiable, including their responses to the introduction of data errors and the sizes of their smallest protocol messages.
Paper Structure (45 sections, 22 theorems, 3 figures, 1 table, 15 algorithms)

This paper contains 45 sections, 22 theorems, 3 figures, 1 table, 15 algorithms.

Table of Contents

  1. Introduction
  2. Background and Preliminaries
  3. Data Stream Channels
  4. Channel Model
  5. Correctness
  6. Secure Close Functions
  7. Confidentiality and Integrity Definitions
  8. Fully Encrypted Datastream Protocols
  9. Traffic Shaping
  10. Relations Between Datastream Notions
  11. A Datastream Fully Encrypted Protocol
  12. The Construction
  13. Properties
  14. Fully Encrypted Datagram Transport Protocols
  15. Channel Model
  16. ...and 30 more sections

Key Result

Theorem 1

Suppose that a channel satisfies FEP-CPFA and further that the Send function is length regular in the sense of Definition def:length-regular-channel. Then that channel satisfies IND-CPFA.

Figures (3)

  • Figure 1: Relations between notions for correct datastream channels that realize a secure close function $\mathcal{C}$. Rectangles and solid arrows are results from Fischlin et al.DIAS; ellipses and dashed arrows are novel notions and relations
  • Figure 2: A Datastream Fully Encrypted Protocol
  • Figure 3: Our datagram channel construction

Theorems & Definitions (33)

  • Definition 1
  • Definition 2
  • Definition 3
  • Definition 4
  • Theorem 1
  • Theorem 2
  • Theorem 3
  • Theorem 4
  • Theorem 5
  • Definition 5
  • ...and 23 more