Security Analysis of the MLS Key Derivation

Chris Brzuska, Eric Cornelissen, Konrad Kohbrok

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

8 Citations (Scopus)
39 Downloads (Pure)


Cryptographic communication protocols provide confidentiality, integrity and authentication properties for end-to-end communication under strong corruption attacks, including, notably, post-compromise security (PCS). Most protocols are designed for one-to-one communication. Protocols for group communication are less common, less efficient, and tend to provide weaker security guarantees. This is because group communication poses unique challenges, such as coordinated key updates, changes to group membership and complex post-compromise recovery procedures. We need to tackle this complex challenge as a community. Thus, the Internet Engineering Task Force (IETF) has created a working group with the goal of developing a sound standard for a continuous asynchronous key-exchange protocol for dynamic groups that is secure and remains efficient for large group sizes. The current version of the Messaging Layer Security (MLS) security protocol is in a feature freeze, i.e., no changes are made in order to provide a stable basis for cryptographic analysis. The key schedule and TreeKEM design are of particular concern since they are crucial to distribute and combine several keys to achieve PCS. In this work, we study the MLS continuous group key derivation (CGKD) which comprises the MLS key schedule, TreeKEM and their composition, as specified in Draft 11 of the MLS RFC, while abstracting away signatures, message flow and authentication guarantees. We establish the uniqueness and key indistinguishability properties of the MLS CGKD as computational security properties.

Original languageEnglish
Title of host publicationProceedings - 43rd IEEE Symposium on Security and Privacy, SP 2022
Number of pages19
ISBN (Electronic)978-1-6654-1316-9
Publication statusPublished - 2022
MoE publication typeA4 Conference publication
EventIEEE Symposium on Security and Privacy - San Francisco, United States
Duration: 23 May 202225 May 2022
Conference number: 43

Publication series

NameProceedings - IEEE Symposium on Security and Privacy
ISSN (Print)1081-6011
ISSN (Electronic)2357-1207


ConferenceIEEE Symposium on Security and Privacy
Abbreviated titleSP
Country/TerritoryUnited States
CitySan Francisco


  • key-derivation
  • key-schedule
  • messaging-layer-security
  • mls
  • protocol-analysis
  • reduction-proof
  • state-separating-proof


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