Kenny, Kien and Matteo from the ETH Zurich Applied Cryptography Group present seven attacks against the cryptographic protocols used by Threema, in three distinct threat models. All the attacks are accompanied by proof-of-concept implementations that demonstrate their feasibility in practice.

Links and papers discussed in the show:

• Three Lessons from Threema

Special Guests: Kenny Paterson, Kien Tuong Truong, and Matteo Scarlata.

Sponsored By:

• Symbolic Software: This episode is sponsored by Symbolic Software. Symbolic Software helps you bring in the experience and knowledge necessary to design, or prove secure, state-of-the-art cryptographic systems for new solutions. We've helped design and formally verify some of the world's most widely used cryptographic protocols.

Links:

• Three Lessons from Threema: Analysis of a Secure Messenger

Do we really need to rely on sharing every phone number on our phone in order for mobile messengers to be usable? What are the privacy risks, and do better cryptographic alternatives exist for managing that data? Joining us are researchers looking exactly into this problem, who will tell us more about their interesting results.

Links and papers discussed in the show:
All the Numbers are US: Large-scale Abuse of Contact Discovery in Mobile Messengers

Music composed by Toby Fox and performed by Sean Schafianski.

Special Guests: Alexandra Dmitrienko, Christian Weinert, and Christoph Hagen.

Sponsored By:

• Symbolic Software: This episode is sponsored by Symbolic Software. Symbolic Software helps you bring in the experience and knowledge necessary to design, or prove secure, state-of-the-art cryptographic systems for new solutions. We've helped design and formally verify some of the world's most widely used cryptographic protocols.

Links:

• All the Numbers are US: Large-scale Abuse of Contact Discovery in Mobile Messengers — Contact discovery allows users of mobile messen- gers 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. Our study of three popular mobile messengers (WhatsApp, Signal, and Telegram) shows that, contrary to expectations, large- scale crawling attacks are (still) possible. Using an accurate database of mobile phone number prefixes and very few resources, we have 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. Regarding mitigations, we propose novel techniques to significantly limit the feasibility of our crawling attacks, especially a new incremental contact discovery scheme that strictly improves over Signal’s current approach. Furthermore, we show that currently deployed hashing-based contact discovery protocols are severely broken by comparing three methods for efficient hash reversal of mobile phone numbers. For this, we also propose a significantly improved rainbow table construction for non-uniformly distributed inputs that is of independent interest.

Now, more than a decade later, Off-the-Record messaging, or OTR, has been largely sidelined by Signal variants. But a small team of cryptography engineers is still working on pushing Off-the-Record messaging forward by focusing on use cases that they argue aren’t sufficiently covered by Signal. But what even is deniability, and how much does it matter in the real-world context of secure messaging? Sofía Celi joins us in today’s episode to talk about this and more.

Links and papers discussed in the show:

• OTRv4

Music composed by Toby Fox and performed by Sean Schafianski.

Special Guest: Sofía Celi.

Sponsored By:

• Symbolic Software: This episode is sponsored by Symbolic Software. Symbolic Software helps you bring in the experience and knowledge necessary to design, or prove secure, state-of-the-art cryptographic systems for new solutions. We've helped design and formally verify some of the world's most widely used cryptographic protocols.

One area in which current secure messaging protocols have stalled, however, is the ability to scale secure conversations to groups of dozens, hundreds and even thousands of people. But the IETF’s Messaging Layer Security, or MLS, effort aims to make that happen. Bringing together a collaboration between Wire, Mozilla, Cisco, Facebook, as well as academia, MLS wants to become the TLS of secure messaging, and make it possible to hold secure conversations scaling to thousands of participants.

But what are the real-world implementation risks involved? Are conversations even worth securing when you’ve got hundreds of potential leakers?

Links and papers discussed in the show:

• MLS Website

Music composed by Toby Fox and performed by Sean Schafianski.

Special Guest: Raphael Robert.

Sponsored By:

• Symbolic Software: This episode is sponsored by Symbolic Software. Symbolic Software helps you bring in the experience and knowledge necessary to design, or prove secure, state-of-the-art cryptographic systems for new solutions. We've helped design and formally verify some of the world's most widely used cryptographic protocols.