@inproceedings{18086,
  abstract     = {Abstract. Continuous group key agreement (CGKA) allows a group of
users to maintain a continuously updated shared key in an asynchronous
setting where parties only come online sporadically and their messages
are relayed by an untrusted server. CGKA captures the basic primitive
underlying group messaging schemes.
Current solutions including TreeKEM (“Messaging Layer Security”
(MLS) IETF RFC 9420) cannot handle concurrent requests while retaining low communication complexity. The exception being CoCoA, which
is concurrent while having extremely low communication complexity (in
groups of size n and for m concurrent updates the communication per
user is log(n), i.e., independent of m). The main downside of CoCoA
is that in groups of size n, users might have to do up to log(n) update
requests to the server to ensure their (potentially corrupted) key material has been refreshed.
In this work we present a “fast healing” concurrent CGKA protocol,
named DeCAF, where users will heal after at most log(t) requests, with
t being the number of corrupted users. While also suitable for the standard central-server setting, our protocol is particularly interesting for
realizing decentralized group messaging, where protocol messages (add,
remove, update) are being posted on some append-only data structure
rather than sent to a server. In this setting, concurrency is crucial once
the rate of requests exceeds, say, the rate at which new blocks are added
to a blockchain.
In the central-server setting, CoCoA (the only alternative with concurrency, sub-linear communication and basic post-compromise security)
enjoys much lower download communication. However, in the decentralized setting – where there is no server which can craft specific messages
for different users to reduce their download communication – our protocol
significantly outperforms CoCoA. DeCAF heals in fewer epochs (log(t)
vs. log(n)) while incurring a similar per epoch per user communication
cost.},
  author       = {Alwen, Joel F and Auerbach, Benedikt and Cueto Noval, Miguel and Klein, Karen and Pascual Perez, Guillermo and Pietrzak, Krzysztof Z},
  booktitle    = {Security and Cryptography for Networks: 14th International Conference},
  editor       = {Galdi, Clemente and Phan, Duong Hieu},
  isbn         = {9783031710728},
  issn         = {1611-3349},
  location     = {Amalfi, Italy},
  pages        = {294–313},
  publisher    = {Springer Nature},
  title        = {{DeCAF: Decentralizable CGKA with fast healing}},
  doi          = {10.1007/978-3-031-71073-5_14},
  volume       = {14974},
  year         = {2024},
}