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Lecture notes in computer science

Continuous Group Key Agreement with Active Security

Joël Alwen, Sandro Coretti, Daniel Jost, Marta Mularczyk

Published Jan 1, 2020• 48 citations
Publisher
DOI
10.1007/978-3-030-64378-2_10

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Secure Group Messagings are cryptographic protocols that allow safe group conversation within potentially large user groups. They are based on a dedicated type of group key exchange mechanism named Continuous Group Key Agreement (CGKA), which generates a continuously evolving group key in a safe and efficient way. Messaging Layer Security (MLS) is an IETF-standardized secure group messaging protocol which uses a protocol called TreeKEM as its CGKA.We study in this thesis the security and the efficiency of the CGKAs used within secure group messagings. Firstly, we propose a variant of TreeKEM (called Quarantined-TreeKEM) that solves a security issue related to inactive users, whose behavior prevents fundamental security properties to be fulfilled. Moreover, we improve the efficiency of TreeKEM, especially regarding its communication cost, by considering the best way to manage the tree graph used by that CGKA. Finally, we show that in the particular – yet common – use case where only a subset of users is granted the administration rights, we can design a new CGKA whose performances outperform those of regular decentralized CGKAs such as TreeKEM.This thesis thus aims to improve the state-of-the-art in secure group messaging protocols, in a context where strong security and energy sobriety are necessary features.

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Secure messaging (SM) protocols allow users to communicate securely over an untrusted infrastructure. Alwen et al. formally defined the key agreement protocol used in secure group messaging (SGM) as continuous group key agreement (CGKA) at CRYPTO 2020. In their CGKA protocol, all of the group members have the same rights to execute algorithms, and a trusted third party is needed. On the other hand, some applications such as a broadcasting service may have an administrator in the group. When the administrator as the group manager (GM) is distinguished from other group members, i.e., in a one-to-many setting, it would be better for the GM and the other group members to have different authorities. We propose a new CGKA protocol, called CGKA with flexible authorization (CGKA-FA) by incorporating a ratcheting digital signature scheme (Cremers et al. at USENIX Security 2021) into the existing CGKA protocol and demonstrate that such a simple modification allows us to provide flexible authorization. Our proposed protocol does not need an external public key infrastructure (PKI) functionality as a trusted third party. So no third party outside the group handles confidential information of users and thus no one except group members can read messages regardless of key updates. In our CGKA-FA protocol, it is possible to set privileges of users in a group flexibly, which is highly compatible with the field of broadcasting.

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Secure Group Messagings are cryptographic protocols that allow safe group conversation within potentially large user groups. They are based on a dedicated type of group key exchange mechanism named Continuous Group Key Agreement (CGKA), which generates a continuously evolving group key in a safe and efficient way. Messaging Layer Security (MLS) is an IETF-standardized secure group messaging protocol which uses a protocol called TreeKEM as its CGKA.We study in this thesis the security and the efficiency of the CGKAs used within secure group messagings. Firstly, we propose a variant of TreeKEM (called Quarantined-TreeKEM) that solves a security issue related to inactive users, whose behavior prevents fundamental security properties to be fulfilled. Moreover, we improve the efficiency of TreeKEM, especially regarding its communication cost, by considering the best way to manage the tree graph used by that CGKA. Finally, we show that in the particular – yet common – use case where only a subset of users is granted the administration rights, we can design a new CGKA whose performances outperform those of regular decentralized CGKAs such as TreeKEM.This thesis thus aims to improve the state-of-the-art in secure group messaging protocols, in a context where strong security and energy sobriety are necessary features.
Secure messaging (SM) protocols allow users to communicate securely over an untrusted infrastructure. Alwen et al. formally defined the key agreement protocol used in secure group messaging (SGM) as continuous group key agreement (CGKA) at CRYPTO 2020. In their CGKA protocol, all of the group members have the same rights to execute algorithms, and a trusted third party is needed. On the other hand, some applications such as a broadcasting service may have an administrator in the group. When the administrator as the group manager (GM) is distinguished from other group members, i.e., in a one-to-many setting, it would be better for the GM and the other group members to have different authorities. We propose a new CGKA protocol, called CGKA with flexible authorization (CGKA-FA) by incorporating a ratcheting digital signature scheme (Cremers et al. at USENIX Security 2021) into the existing CGKA protocol and demonstrate that such a simple modification allows us to provide flexible authorization. Our proposed protocol does not need an external public key infrastructure (PKI) functionality as a trusted third party. So no third party outside the group handles confidential information of users and thus no one except group members can read messages regardless of key updates. In our CGKA-FA protocol, it is possible to set privileges of users in a group flexibly, which is highly compatible with the field of broadcasting.