Paper 2022/875

Contact Discovery in Mobile Messengers: Low-cost Attacks, Quantitative Analyses, and Efficient Mitigations

Christoph Hagen, University of Würzburg
Christian Weinert, Royal Holloway, University of London
Christoph Sendner, University of Würzburg
Alexandra Dmitrienko, University of Würzburg
Thomas Schneider, Technical University of Darmstadt

Contact discovery allows users of mobile messengers to conveniently connect with people in their address book. In this work, we demonstrate that severe privacy issues exist in currently deployed contact discovery methods and propose suitable mitigations. Our study of three popular messengers (WhatsApp, Signal, and Telegram) shows that large-scale crawling attacks are (still) possible. Using an accurate database of mobile phone number prefixes and very few resources, we queried 10% of US mobile phone numbers for WhatsApp and 100% for Signal. For Telegram we find that its API exposes a wide range of sensitive information, even about numbers not registered with the service. We present interesting (cross-messenger) usage statistics, which also reveal that very few users change the default privacy settings. Furthermore, we demonstrate that currently deployed hashing-based contact discovery protocols are severely broken by comparing three methods for efficient hash reversal. Most notably, we show that with the password cracking tool "JTR" we can iterate through the entire world-wide mobile phone number space in <150s on a consumer-grade GPU. We also propose a significantly improved rainbow table construction for non-uniformly distributed input domains that is of independent interest. Regarding mitigations, we most notably propose two novel rate-limiting schemes: our incremental contact discovery for services without server-side contact storage strictly improves over Signal's current approach while being compatible with private set intersection, whereas our differential scheme allows even stricter rate limits at the overhead for service providers to store a small constant-size state that does not reveal any contact information.

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Publication info
Published elsewhere. ACM TOPS'22
Mobile Contact Discovery Hash Reversal Rainbow Table Crawling Private Set Intersection Signal
Contact author(s)
christoph hagen @ uni-wuerzburg de
christian weinert @ rhul ac uk
christoph sendner @ uni-wuerzburg de
alexandra dmitrienko @ uni-wuerzburg de
schneider @ encrypto cs tu-darmstadt de
2022-07-07: approved
2022-07-04: received
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      author = {Christoph Hagen and Christian Weinert and Christoph Sendner and Alexandra Dmitrienko and Thomas Schneider},
      title = {Contact Discovery in Mobile Messengers: Low-cost Attacks, Quantitative Analyses, and Efficient Mitigations},
      howpublished = {Cryptology ePrint Archive, Paper 2022/875},
      year = {2022},
      note = {\url{}},
      url = {}
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