The Signal Protocol for non-Cryptographers: An Explanation of the Signal Protocol and its Security Properties

Examensarbete för masterexamen
Yagublu, Lamiya
People tend to socialize and today many people use messaging applications to communicate. While people communicate, they share personal information between each other and they do not want others to observe or access their information and use it against them. Therefore, it is important to keep this information private. The Signal protocol is a communication protocol used to provide security guarantees and keep the users’ information private while they communicate. Since many messaging applications, including WhatsApp and Facebook messenger, uses the Signal protocol and a lot of people use these applications, any flaw in the Signal protocol might affect a large number of users’ private communication. This master thesis aims to study the Signal protocol and explains, in an easy way, its functionality and security properties. The thesis contains a detailed explanation of the core parts of the Signal protocol and its security. A literature review was conducted to investigate how the Signal protocol works and what kind of security properties it has. Due to complexity of the problem in the few academic papers, the Signal protocol is defined and explained in an easier way. The thesis focuses mainly on the academic paper titled "A Formal Security Analysis of the Signal Messaging Protocol" by K. Cohn-Gordon, C. Cremers, B. Dowling, L. Garratt and D. Stebila. As a technical contribution of this thesis, the existing notation from the above mentioned paper is revisited and simplified. Thereafter, the new and simplified notation is used to explain how the Signal protocol works in an easy way. The explanation with the simplified notations helps non-technical people to understand the protocol better. Then the security properties of the Signal protocol are investigated and explained.
Signal, instant messaging, key distribution system, double ratchet, forward Secrecy, post-compromised security, end-to-end encryption, authenticated encryption with associated data
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