Paper 2022/046

Membership Privacy for Asynchronous Group Messaging

Keita Emura, Kaisei Kajita, Ryo Nojima, Kazuto Ogawa, and Go Ohtake


The Signal protocol is a secure messaging protocol providing end-to-end encrypted asynchronous communication. In this paper, we focus on a method capable of hiding membership information from the viewpoint of non group members in a secure group messaging (SGM) protocol, which we call "membership privacy''. Although Chase et al. (ACM CCS 2020) have considered the same notion, their proposal is an extension of Signal so called "Pairwise Signal'' where a group message is repeatedly sent over individual Signal channels. Thus their protocol is not scalable. In this work, we extend the Cohn-Gordon et al. SGM protocol (ACM CCS 2018), which we call the Asynchronous Ratcheting Trees (ART) protocol, to add membership privacy. We employ a key-private and robust public-key encryption (Abdalla et al., TCC2010/JoC2018) for hiding membership-related values in the setup phase. Furthermore, we concentrate on the fact that a group common key provides anonymity. This fact is used to encrypt membership information in the key update phase. Our extension does not affect the forward secrecy and post-compromise security of the original ART protocol. Our modification achieves asymptotically the same efficiency of the ART protocol in the setup phase. Any additional cost for key update does not depend on the number of group members (specifically, one encryption and decryption of a symmetric key-encryption scheme and one execution of a key-derivation function for each key update are employed). Therefore, the proposed protocol can add membership privacy to the ART protocol with a quite small overhead.

Available format(s)
Cryptographic protocols
Publication info
Preprint. MINOR revision.
Secure Group MessagingMembership Privacy
Contact author(s)
k-emura @ nict go jp
kajita k-bu @ nhk or jp
kaz_ogawa @ nict go jp
ryo-no @ nict go jp
ohtake g-fw @ nhk or jp
2022-03-09: revised
2022-01-14: received
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Creative Commons Attribution


      author = {Keita Emura and Kaisei Kajita and Ryo Nojima and Kazuto Ogawa and Go Ohtake},
      title = {Membership Privacy for Asynchronous Group Messaging},
      howpublished = {Cryptology ePrint Archive, Paper 2022/046},
      year = {2022},
      note = {\url{}},
      url = {}
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