The Security of Lazy Users in Out-of-Band Authentication

Moni Naor*, Lior Rotem, Gil Segev

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

4 Scopus citations

Abstract

Faced with the threats posed by man-in-the-middle attacks, messaging platforms rely on “out-of-band” authentication, assuming that users have access to an external channel for authenticating one short value. For example, assuming that users recognizing each other’s voice can authenticate a short value, Telegram and WhatApp ask their users to compare 288-bit and 200-bit values, respectively. The existing protocols, however, do not take into account the plausible behavior of users who may be “lazy” and only compare parts of these values (rather than their entirety). Motivated by such a security-critical user behavior, we study the security of lazy users in out-of-band authentication. We start by showing that both the protocol implemented by WhatsApp and the statistically-optimal protocol of Naor, Segev and Smith (CRYPTO ’06) are completely vulnerable to man-in-the-middle attacks when the users consider only a half of the out-of-band authenticated value. In this light, we put forward a framework that captures the behavior and security of lazy users. Our notions of security consider both statistical security and computational security, and for each flavor we derive a lower bound on the tradeoff between the number of positions that are considered by the lazy users and the adversary’s forgery probability. Within our framework we then provide two authentication protocols. First, in the statistical setting, we present a transformation that converts any out-of-band authentication protocol into one that is secure even when executed by lazy users. Instantiating our transformation with a new refinement of the protocol of Naor et al. results in a protocol whose tradeoff essentially matches our lower bound in the statistical setting. Then, in the computational setting, we show that the computationally-optimal protocol of Vaudenay (CRYPTO ’05) is secure even when executed by lazy users – and its tradeoff matches our lower bound in the computational setting.

Original languageAmerican English
Title of host publicationTheory of Cryptography - 16th International Conference, TCC 2018, Proceedings
EditorsAmos Beimel, Stefan Dziembowski
PublisherSpringer Science and Business Media Deutschland GmbH
Pages575-599
Number of pages25
ISBN (Print)9783030038090
DOIs
StatePublished - 2018
Event16th International Conference on Theory of Cryptography, TCC 2018 - Panaji, India
Duration: 11 Nov 201814 Nov 2018

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume11240 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349

Conference

Conference16th International Conference on Theory of Cryptography, TCC 2018
Country/TerritoryIndia
CityPanaji
Period11/11/1814/11/18

Bibliographical note

Publisher Copyright:
© 2018, International Association for Cryptologic Research.

Keywords

  • Authentic Values
  • Forgery Probability
  • Lazy User
  • Messaging Platform
  • WhatsApp

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