---
OA_place: publisher
OA_type: diamond
_id: '20668'
abstract:
- lang: eng
text: "The Message Layer Security (MLS) protocol has recently been standardized
by the IETF. MLS is a scalable secure group messaging protocol expected to run
more efficiently compared to the Signal protocol at scale, while offering a similar
level of strong security. Even though MLS has undergone extensive examination
by researchers, the majority of the works have focused on confidentiality.\r\n\r\nIn
this work, we focus on the authenticity of the application messages exchanged
in MLS. Currently, MLS authenticates every application message with an EdDSA signature
and while manageable, the overhead is greatly amplified in the post-quantum setting
as the NIST-recommended Dilithium signature results in a 40x increase in size.
We view this as an invitation to explore new authentication modes that can be
used instead. We start by taking a systematic view on how application messages
are authenticated in MLS and categorize authenticity into four different security
notions. We then propose several authentication modes, offering a range of different
efficiency and security profiles. For instance, in one of our modes, COSMOS++,
we replace signatures with one-time tokens and a MAC tag, offering roughly a 75x
savings in the post-quantum communication overhead. While this comes at the cost
of weakening security compared to the authentication mode used by MLS, the lower
communication overhead seems to make it a worthwhile trade-off with security."
acknowledgement: This research was partially supported by JST CREST JPMJCR22M1, Japan
and funded by the European Union’s Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie Grant Agreement No.665385.
article_processing_charge: No
author:
- first_name: Keitaro
full_name: Hashimoto, Keitaro
last_name: Hashimoto
- first_name: Shuichi
full_name: Katsumata, Shuichi
last_name: Katsumata
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
citation:
ama: 'Hashimoto K, Katsumata S, Pascual Perez G. Exploring how to authenticate application
messages in MLS: More efficient, post-quantum, and anonymous blocklistable. In:
34th Usenix Security Symposium. Usenix Association; 2025:6699-6716.'
apa: 'Hashimoto, K., Katsumata, S., & Pascual Perez, G. (2025). Exploring how
to authenticate application messages in MLS: More efficient, post-quantum, and
anonymous blocklistable. In 34th Usenix Security Symposium (pp. 6699–6716).
Seattle, WA, USA: Usenix Association.'
chicago: 'Hashimoto, Keitaro, Shuichi Katsumata, and Guillermo Pascual Perez. “Exploring
How to Authenticate Application Messages in MLS: More Efficient, Post-Quantum,
and Anonymous Blocklistable.” In 34th Usenix Security Symposium, 6699–6716.
Usenix Association, 2025.'
ieee: 'K. Hashimoto, S. Katsumata, and G. Pascual Perez, “Exploring how to authenticate
application messages in MLS: More efficient, post-quantum, and anonymous blocklistable,”
in 34th Usenix Security Symposium, Seattle, WA, USA, 2025, pp. 6699–6716.'
ista: 'Hashimoto K, Katsumata S, Pascual Perez G. 2025. Exploring how to authenticate
application messages in MLS: More efficient, post-quantum, and anonymous blocklistable.
34th Usenix Security Symposium. USENIX: Security Symposium, 6699–6716.'
mla: 'Hashimoto, Keitaro, et al. “Exploring How to Authenticate Application Messages
in MLS: More Efficient, Post-Quantum, and Anonymous Blocklistable.” 34th Usenix
Security Symposium, Usenix Association, 2025, pp. 6699–716.'
short: K. Hashimoto, S. Katsumata, G. Pascual Perez, in:, 34th Usenix Security Symposium,
Usenix Association, 2025, pp. 6699–6716.
conference:
end_date: 2025-08-15
location: Seattle, WA, USA
name: 'USENIX: Security Symposium'
start_date: 2025-08-13
corr_author: '1'
cryptoeprintid: 1
das_tickbox: '1'
date_created: 2025-11-23T23:01:40Z
date_published: 2025-01-01T00:00:00Z
date_updated: 2026-07-07T06:02:03Z
day: '01'
ddc:
- '000'
ec_funded: 1
external_id:
cryptoeprintid:
- 2025/426
file:
- access_level: open_access
checksum: fcfe8851aeb751af98c0b1335a0ef149
content_type: application/pdf
creator: dernst
date_created: 2025-11-24T07:44:08Z
date_updated: 2025-11-24T07:44:08Z
file_id: '20671'
file_name: 2025_Usenix_Hashimoto.pdf
file_size: 710733
relation: main_file
success: 1
file_date_updated: 2025-11-24T07:44:08Z
has_accepted_license: '1'
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2025/426
month: '01'
oa: 1
oa_version: Published Version
page: 6699-6716
project:
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
publication: 34th Usenix Security Symposium
publication_identifier:
isbn:
- '9781939133526'
publication_status: published
publisher: Usenix Association
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'Exploring how to authenticate application messages in MLS: More efficient,
post-quantum, and anonymous blocklistable'
type: conference
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
year: '2025'
...
---
OA_place: repository
OA_type: green
_id: '18702'
abstract:
- lang: eng
text: 'In this work we prove lower bounds on the (communication) cost of maintaining
a shared key among a dynamic group of users. Being “dynamic” means one can add
and remove users from the group. This captures important protocols like multicast
encryption (ME) and continuous group-key agreement (CGKA), which is the primitive
underlying many group messaging applications. We prove our bounds in a combinatorial
setting where the state of the protocol progresses in rounds. The state of the
protocol in each round is captured by a set system, with each of its elements
specifying a set of users who share a secret key. We show this combinatorial model
implies bounds in symbolic models for ME and CGKA that capture, as building blocks,
PRGs, PRFs, dual PRFs, secret sharing, and symmetric encryption in the setting
of ME, and PRGs, PRFs, dual PRFs, secret sharing, public-key encryption, and key-updatable
public-key encryption in the setting of CGKA. The models are related to the ones
used by Micciancio and Panjwani (Eurocrypt’04) and Bienstock et al. (TCC’20) to
analyze ME and CGKA, respectively. We prove – using the Bollobás’ Set Pairs Inequality
– that the cost (number of uploaded ciphertexts) for replacing a set of d users
in a group of size n is Ω(dln(n/d)). Our lower bound is asymptotically tight and
both improves on a bound of Ω(d) by Bienstock et al. (TCC’20), and generalizes
a result by Micciancio and Panjwani (Eurocrypt’04), who proved a lower bound of
Ω(log(n)) for d=1. '
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Michael
full_name: Anastos, Michael
id: 0b2a4358-bb35-11ec-b7b9-e3279b593dbb
last_name: Anastos
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Mirza Ahad
full_name: Baig, Mirza Ahad
id: 3EDE6DE4-AA5A-11E9-986D-341CE6697425
last_name: Baig
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Matthew Alan
full_name: Kwan, Matthew Alan
id: 5fca0887-a1db-11eb-95d1-ca9d5e0453b3
last_name: Kwan
orcid: 0000-0002-4003-7567
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
citation:
ama: 'Anastos M, Auerbach B, Baig MA, et al. The cost of maintaining keys in dynamic
groups with applications to multicast encryption and group messaging. In: 22nd
International Conference on Theory of Cryptography. Vol 15364. Springer Nature;
2024:413-443. doi:10.1007/978-3-031-78011-0_14'
apa: 'Anastos, M., Auerbach, B., Baig, M. A., Cueto Noval, M., Kwan, M. A., Pascual
Perez, G., & Pietrzak, K. Z. (2024). The cost of maintaining keys in dynamic
groups with applications to multicast encryption and group messaging. In 22nd
International Conference on Theory of Cryptography (Vol. 15364, pp. 413–443).
Milan, Italy: Springer Nature. https://doi.org/10.1007/978-3-031-78011-0_14'
chicago: Anastos, Michael, Benedikt Auerbach, Mirza Ahad Baig, Miguel Cueto Noval,
Matthew Alan Kwan, Guillermo Pascual Perez, and Krzysztof Z Pietrzak. “The Cost
of Maintaining Keys in Dynamic Groups with Applications to Multicast Encryption
and Group Messaging.” In 22nd International Conference on Theory of Cryptography,
15364:413–43. Springer Nature, 2024. https://doi.org/10.1007/978-3-031-78011-0_14.
ieee: M. Anastos et al., “The cost of maintaining keys in dynamic groups
with applications to multicast encryption and group messaging,” in 22nd International
Conference on Theory of Cryptography, Milan, Italy, 2024, vol. 15364, pp.
413–443.
ista: 'Anastos M, Auerbach B, Baig MA, Cueto Noval M, Kwan MA, Pascual Perez G,
Pietrzak KZ. 2024. The cost of maintaining keys in dynamic groups with applications
to multicast encryption and group messaging. 22nd International Conference on
Theory of Cryptography. TCC: Theory of Cryptography, LNCS, vol. 15364, 413–443.'
mla: Anastos, Michael, et al. “The Cost of Maintaining Keys in Dynamic Groups with Applications
to Multicast Encryption and Group Messaging.” 22nd International Conference
on Theory of Cryptography, vol. 15364, Springer Nature, 2024, pp. 413–43,
doi:10.1007/978-3-031-78011-0_14.
short: M. Anastos, B. Auerbach, M.A. Baig, M. Cueto Noval, M.A. Kwan, G. Pascual
Perez, K.Z. Pietrzak, in:, 22nd International Conference on Theory of Cryptography,
Springer Nature, 2024, pp. 413–443.
conference:
end_date: 2024-12-06
location: Milan, Italy
name: 'TCC: Theory of Cryptography'
start_date: 2024-12-02
corr_author: '1'
date_created: 2024-12-22T23:01:47Z
date_published: 2024-12-02T00:00:00Z
date_updated: 2025-12-02T13:55:46Z
day: '02'
department:
- _id: MaKw
- _id: KrPi
doi: 10.1007/978-3-031-78011-0_14
external_id:
isi:
- '001545628900014'
intvolume: ' 15364'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2024/1097
month: '12'
oa: 1
oa_version: Preprint
page: 413-443
publication: 22nd International Conference on Theory of Cryptography
publication_identifier:
eissn:
- 1611-3349
isbn:
- '9783031780103'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
scopus_import: '1'
status: public
title: The cost of maintaining keys in dynamic groups with applications to multicast
encryption and group messaging
type: conference
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
volume: 15364
year: '2024'
...
---
_id: '18086'
abstract:
- lang: eng
text: "Abstract. Continuous group key agreement (CGKA) allows a group of\r\nusers
to maintain a continuously updated shared key in an asynchronous\r\nsetting where
parties only come online sporadically and their messages\r\nare relayed by an
untrusted server. CGKA captures the basic primitive\r\nunderlying group messaging
schemes.\r\nCurrent solutions including TreeKEM (“Messaging Layer Security”\r\n(MLS)
IETF RFC 9420) cannot handle concurrent requests while retaining low communication
complexity. The exception being CoCoA, which\r\nis concurrent while having extremely
low communication complexity (in\r\ngroups of size n and for m concurrent updates
the communication per\r\nuser is log(n), i.e., independent of m). The main downside
of CoCoA\r\nis that in groups of size n, users might have to do up to log(n) update\r\nrequests
to the server to ensure their (potentially corrupted) key material has been refreshed.\r\nIn
this work we present a “fast healing” concurrent CGKA protocol,\r\nnamed DeCAF,
where users will heal after at most log(t) requests, with\r\nt being the number
of corrupted users. While also suitable for the standard central-server setting,
our protocol is particularly interesting for\r\nrealizing decentralized group
messaging, where protocol messages (add,\r\nremove, update) are being posted on
some append-only data structure\r\nrather than sent to a server. In this setting,
concurrency is crucial once\r\nthe rate of requests exceeds, say, the rate at
which new blocks are added\r\nto a blockchain.\r\nIn the central-server setting,
CoCoA (the only alternative with concurrency, sub-linear communication and basic
post-compromise security)\r\nenjoys much lower download communication. However,
in the decentralized setting – where there is no server which can craft specific
messages\r\nfor different users to reduce their download communication – our protocol\r\nsignificantly
outperforms CoCoA. DeCAF heals in fewer epochs (log(t)\r\nvs. log(n)) while incurring
a similar per epoch per user communication\r\ncost."
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Joel F
full_name: Alwen, Joel F
id: 2A8DFA8C-F248-11E8-B48F-1D18A9856A87
last_name: Alwen
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Karen
full_name: Klein, Karen
id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87
last_name: Klein
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
citation:
ama: 'Alwen JF, Auerbach B, Cueto Noval M, Klein K, Pascual Perez G, Pietrzak KZ.
DeCAF: Decentralizable CGKA with fast healing. In: Galdi C, Phan DH, eds. Security
and Cryptography for Networks: 14th International Conference. Vol 14974. Cham:
Springer Nature; 2024:294–313. doi:10.1007/978-3-031-71073-5_14'
apa: 'Alwen, J. F., Auerbach, B., Cueto Noval, M., Klein, K., Pascual Perez, G.,
& Pietrzak, K. Z. (2024). DeCAF: Decentralizable CGKA with fast healing. In
C. Galdi & D. H. Phan (Eds.), Security and Cryptography for Networks: 14th
International Conference (Vol. 14974, pp. 294–313). Cham: Springer Nature.
https://doi.org/10.1007/978-3-031-71073-5_14'
chicago: 'Alwen, Joel F, Benedikt Auerbach, Miguel Cueto Noval, Karen Klein, Guillermo
Pascual Perez, and Krzysztof Z Pietrzak. “DeCAF: Decentralizable CGKA with Fast
Healing.” In Security and Cryptography for Networks: 14th International Conference,
edited by Clemente Galdi and Duong Hieu Phan, 14974:294–313. Cham: Springer Nature,
2024. https://doi.org/10.1007/978-3-031-71073-5_14.'
ieee: 'J. F. Alwen, B. Auerbach, M. Cueto Noval, K. Klein, G. Pascual Perez, and
K. Z. Pietrzak, “DeCAF: Decentralizable CGKA with fast healing,” in Security
and Cryptography for Networks: 14th International Conference, Amalfi, Italy,
2024, vol. 14974, pp. 294–313.'
ista: 'Alwen JF, Auerbach B, Cueto Noval M, Klein K, Pascual Perez G, Pietrzak KZ.
2024. DeCAF: Decentralizable CGKA with fast healing. Security and Cryptography
for Networks: 14th International Conference. SCN: Security and Cryptography for
Networks, LNCS, vol. 14974, 294–313.'
mla: 'Alwen, Joel F., et al. “DeCAF: Decentralizable CGKA with Fast Healing.” Security
and Cryptography for Networks: 14th International Conference, edited by Clemente
Galdi and Duong Hieu Phan, vol. 14974, Springer Nature, 2024, pp. 294–313, doi:10.1007/978-3-031-71073-5_14.'
short: 'J.F. Alwen, B. Auerbach, M. Cueto Noval, K. Klein, G. Pascual Perez, K.Z.
Pietrzak, in:, C. Galdi, D.H. Phan (Eds.), Security and Cryptography for Networks:
14th International Conference, Springer Nature, Cham, 2024, pp. 294–313.'
conference:
end_date: 2024-09-13
location: Amalfi, Italy
name: 'SCN: Security and Cryptography for Networks'
start_date: 2024-09-11
corr_author: '1'
date_created: 2024-09-18T11:35:14Z
date_published: 2024-09-10T00:00:00Z
date_updated: 2026-04-07T13:01:26Z
day: '10'
department:
- _id: GradSch
- _id: KrPi
doi: 10.1007/978-3-031-71073-5_14
editor:
- first_name: Clemente
full_name: Galdi, Clemente
last_name: Galdi
- first_name: Duong Hieu
full_name: Phan, Duong Hieu
last_name: Phan
external_id:
isi:
- '001330408000014'
intvolume: ' 14974'
isi: 1
language:
- iso: eng
month: '09'
oa_version: None
page: 294–313
place: Cham
publication: 'Security and Cryptography for Networks: 14th International Conference'
publication_identifier:
eisbn:
- '9783031710735'
eissn:
- 1611-3349
isbn:
- '9783031710728'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
related_material:
record:
- id: '18088'
relation: dissertation_contains
status: public
status: public
title: 'DeCAF: Decentralizable CGKA with fast healing'
type: conference
user_id: 317138e5-6ab7-11ef-aa6d-ffef3953e345
volume: 14974
year: '2024'
...
---
OA_place: publisher
_id: '18088'
abstract:
- lang: eng
text: "Instant messaging applications like Whatsapp, Signal or Telegram have become
ubiquitous in today's society.\r\nMany of them provide not only end-to-end encryption,
but also security guarantees even when the key material gets compromised.\r\nThese
are achieved through frequent key update performed by users.\r\nIn particular,
the compromise of a group key should preserve confidentiality of previously exchanged
messages (forward secrecy), and a subsequent key update will ensure security for
future ones (post-compromise security).\r\nThough great protocols for one-on-one
communication have been known for some time, the design of ones that scale efficiently
for larger groups while achieving akin security guarantees is a hard problem.\r\nA
great deal of research has been aimed at this topic, much of it under the umbrella
of the Messaging Layer Security (MLS) working group at the IETF. \r\nStarted in
2018, this joint effort by academics and industry culminated in 2023 with the
publication of the first standard for secure group messaging [IETF, RFC9420].\r\n\r\nAt
the core of secure group messaging is a cryptographic primitive termed Continuous
Group Key Agreement, or CGKA [Alwen et al. 2021], that essentially allows a changing
group of users to agree on a common key with the added functionality security
against compromises is achieved by users asynchronously issuing a key update.
In this thesis we contribute to the understanding of CGKA across different angles.\r\nFirst,
we present a new technique to effect dynamic operations in groups, i.e., add or
remove members, that can be more efficient that the one employed by MLS in certain
settings.\r\nConsidering the setting of users belonging to multiple overlapping
groups, we then show lowerbounds on the communication cost of constructions that
leverage said overlap, at the same time showing protocols that are asymptotically
optimal and efficient for practical settings, respectively. Along the way, we
show that the communication cost of key updates in MLS is average-cost optimal.\r\nAn
important feature in CGKA protocols, particularly for big groups, is the possibility
of executing several group operations concurrently. While later versions of MLS
support this, they do at the cost of worsening the communication efficiency of
future group operations.\r\nIn this thesis we introduce two new protocols that
permit concurrency without any negative effect on efficiency. Our protocols circumvent
previously existing lower bounds by satisfying a new notion of post-compromise
security that only asks for security to be re-established after a certain number
of key updates have taken place. While this can be slower than MLS in terms of
rounds of communication, we show that it leads to more efficient overall communication.
\r\nAdditionally, we introduce a new technique that allows group members to decrease
the information they need to store and download, which makes one of our protocols
enjoy much lower download cost than any other existing CGKA constructions. "
alternative_title:
- ISTA Thesis
article_processing_charge: No
author:
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
citation:
ama: Pascual Perez G. On the efficiency and security of secure group messaging.
2024. doi:10.15479/at:ista:18088
apa: Pascual Perez, G. (2024). On the efficiency and security of secure group
messaging. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:18088
chicago: Pascual Perez, Guillermo. “On the Efficiency and Security of Secure Group
Messaging.” Institute of Science and Technology Austria, 2024. https://doi.org/10.15479/at:ista:18088.
ieee: G. Pascual Perez, “On the efficiency and security of secure group messaging,”
Institute of Science and Technology Austria, 2024.
ista: Pascual Perez G. 2024. On the efficiency and security of secure group messaging.
Institute of Science and Technology Austria.
mla: Pascual Perez, Guillermo. On the Efficiency and Security of Secure Group
Messaging. Institute of Science and Technology Austria, 2024, doi:10.15479/at:ista:18088.
short: G. Pascual Perez, On the Efficiency and Security of Secure Group Messaging,
Institute of Science and Technology Austria, 2024.
corr_author: '1'
date_created: 2024-09-18T12:59:49Z
date_published: 2024-09-18T00:00:00Z
date_updated: 2026-04-07T13:01:26Z
day: '18'
ddc:
- '000'
degree_awarded: PhD
department:
- _id: KrPi
- _id: GradSch
doi: 10.15479/at:ista:18088
ec_funded: 1
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language:
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oa: 1
oa_version: Published Version
page: '239'
project:
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
publication_identifier:
issn:
- 2663-337X
publication_status: published
publisher: Institute of Science and Technology Austria
related_material:
record:
- id: '10408'
relation: part_of_dissertation
status: public
- id: '11476'
relation: part_of_dissertation
status: public
- id: '18086'
relation: part_of_dissertation
status: public
- id: '10049'
relation: part_of_dissertation
status: public
status: public
supervisor:
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
title: On the efficiency and security of secure group messaging
tmp:
image: /images/cc_by_nc_sa.png
legal_code_url: https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode
name: Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC
BY-NC-SA 4.0)
short: CC BY-NC-SA (4.0)
type: dissertation
user_id: ba8df636-2132-11f1-aed0-ed93e2281fdd
year: '2024'
...
---
_id: '14691'
abstract:
- lang: eng
text: "Continuous Group-Key Agreement (CGKA) allows a group of users to maintain
a shared key. It is the fundamental cryptographic primitive underlying group messaging
schemes and related protocols, most notably TreeKEM, the underlying key agreement
protocol of the Messaging Layer Security (MLS) protocol, a standard for group
messaging by the IETF. CKGA works in an asynchronous setting where parties only
occasionally must come online, and their messages are relayed by an untrusted
server. The most expensive operation provided by CKGA is that which allows for
a user to refresh their key material in order to achieve forward secrecy (old
messages are secure when a user is compromised) and post-compromise security (users
can heal from compromise). One caveat of early CGKA protocols is that these update
operations had to be performed sequentially, with any user wanting to update their
key material having had to receive and process all previous updates. Late versions
of TreeKEM do allow for concurrent updates at the cost of a communication overhead
per update message that is linear in the number of updating parties. This was
shown to be indeed necessary when achieving PCS in just two rounds of communication
by [Bienstock et al. TCC’20].\r\nThe recently proposed protocol CoCoA [Alwen et
al. Eurocrypt’22], however, shows that this overhead can be reduced if PCS requirements
are relaxed, and only a logarithmic number of rounds is required. The natural
question, thus, is whether CoCoA is optimal in this setting.\r\nIn this work we
answer this question, providing a lower bound on the cost (concretely, the amount
of data to be uploaded to the server) for CGKA protocols that heal in an arbitrary
k number of rounds, that shows that CoCoA is very close to optimal. Additionally,
we extend CoCoA to heal in an arbitrary number of rounds, and propose a modification
of it, with a reduced communication cost for certain k.\r\nWe prove our bound
in a combinatorial setting where the state of the protocol progresses in rounds,
and the state of the protocol in each round is captured by a set system, each
set specifying a set of users who share a secret key. We show this combinatorial
model is equivalent to a symbolic model capturing building blocks including PRFs
and public-key encryption, related to the one used by Bienstock et al.\r\nOur
lower bound is of order k•n1+1/(k-1)/log(k), where 2≤k≤log(n) is the number of
updates per user the protocol requires to heal. This generalizes the n2 bound
for k=2 from Bienstock et al.. This bound almost matches the k⋅n1+2/(k-1) or k2⋅n1+1/(k-1)
efficiency we get for the variants of the CoCoA protocol also introduced in this
paper."
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
citation:
ama: 'Auerbach B, Cueto Noval M, Pascual Perez G, Pietrzak KZ. On the cost of post-compromise
security in concurrent Continuous Group-Key Agreement. In: 21st International
Conference on Theory of Cryptography. Vol 14371. Springer Nature; 2023:271-300.
doi:10.1007/978-3-031-48621-0_10'
apa: 'Auerbach, B., Cueto Noval, M., Pascual Perez, G., & Pietrzak, K. Z. (2023).
On the cost of post-compromise security in concurrent Continuous Group-Key Agreement.
In 21st International Conference on Theory of Cryptography (Vol. 14371,
pp. 271–300). Taipei, Taiwan: Springer Nature. https://doi.org/10.1007/978-3-031-48621-0_10'
chicago: Auerbach, Benedikt, Miguel Cueto Noval, Guillermo Pascual Perez, and Krzysztof
Z Pietrzak. “On the Cost of Post-Compromise Security in Concurrent Continuous
Group-Key Agreement.” In 21st International Conference on Theory of Cryptography,
14371:271–300. Springer Nature, 2023. https://doi.org/10.1007/978-3-031-48621-0_10.
ieee: B. Auerbach, M. Cueto Noval, G. Pascual Perez, and K. Z. Pietrzak, “On the cost
of post-compromise security in concurrent Continuous Group-Key Agreement,” in
21st International Conference on Theory of Cryptography, Taipei, Taiwan,
2023, vol. 14371, pp. 271–300.
ista: 'Auerbach B, Cueto Noval M, Pascual Perez G, Pietrzak KZ. 2023. On the cost
of post-compromise security in concurrent Continuous Group-Key Agreement. 21st
International Conference on Theory of Cryptography. TCC: Theory of Cryptography,
LNCS, vol. 14371, 271–300.'
mla: Auerbach, Benedikt, et al. “On the Cost of Post-Compromise Security in Concurrent
Continuous Group-Key Agreement.” 21st International Conference on Theory of
Cryptography, vol. 14371, Springer Nature, 2023, pp. 271–300, doi:10.1007/978-3-031-48621-0_10.
short: B. Auerbach, M. Cueto Noval, G. Pascual Perez, K.Z. Pietrzak, in:, 21st International
Conference on Theory of Cryptography, Springer Nature, 2023, pp. 271–300.
conference:
end_date: 2023-12-02
location: Taipei, Taiwan
name: 'TCC: Theory of Cryptography'
start_date: 2023-11-29
corr_author: '1'
date_created: 2023-12-17T23:00:53Z
date_published: 2023-11-27T00:00:00Z
date_updated: 2025-09-09T13:42:16Z
day: '27'
department:
- _id: KrPi
doi: 10.1007/978-3-031-48621-0_10
external_id:
isi:
- '001160724400010'
intvolume: ' 14371'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2023/1123
month: '11'
oa: 1
oa_version: Preprint
page: 271-300
publication: 21st International Conference on Theory of Cryptography
publication_identifier:
eissn:
- 1611-3349
isbn:
- '9783031486203'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
scopus_import: '1'
status: public
title: On the cost of post-compromise security in concurrent Continuous Group-Key
Agreement
type: conference
user_id: 317138e5-6ab7-11ef-aa6d-ffef3953e345
volume: 14371
year: '2023'
...
---
_id: '14692'
abstract:
- lang: eng
text: "The generic-group model (GGM) aims to capture algorithms working over groups
of prime order that only rely on the group operation, but do not exploit any additional
structure given by the concrete implementation of the group. In it, it is possible
to prove information-theoretic lower bounds on the hardness of problems like the
discrete logarithm (DL) or computational Diffie-Hellman (CDH). Thus, since its
introduction, it has served as a valuable tool to assess the concrete security
provided by cryptographic schemes based on such problems. A work on the related
algebraic-group model (AGM) introduced a method, used by many subsequent works,
to adapt GGM lower bounds for one problem to another, by means of conceptually
simple reductions.\r\nIn this work, we propose an alternative approach to extend
GGM bounds from one problem to another. Following an idea by Yun [EC15], we show
that, in the GGM, the security of a large class of problems can be reduced to
that of geometric search-problems. By reducing the security of the resulting geometric-search
problems to variants of the search-by-hypersurface problem, for which information
theoretic lower bounds exist, we give alternative proofs of several results that
used the AGM approach.\r\nThe main advantage of our approach is that our reduction
from geometric search-problems works, as well, for the GGM with preprocessing
(more precisely the bit-fixing GGM introduced by Coretti, Dodis and Guo [Crypto18]).
As a consequence, this opens up the possibility of transferring preprocessing
GGM bounds from one problem to another, also by means of simple reductions. Concretely,
we prove novel preprocessing bounds on the hardness of the d-strong discrete logarithm,
the d-strong Diffie-Hellman inversion, and multi-instance CDH problems, as well
as a large class of Uber assumptions. Additionally, our approach applies to Shoup’s
GGM without additional restrictions on the query behavior of the adversary, while
the recent works of Zhang, Zhou, and Katz [AC22] and Zhandry [Crypto22] highlight
that this is not the case for the AGM approach."
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Charlotte
full_name: Hoffmann, Charlotte
id: 0f78d746-dc7d-11ea-9b2f-83f92091afe7
last_name: Hoffmann
orcid: 0000-0003-2027-5549
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
citation:
ama: 'Auerbach B, Hoffmann C, Pascual Perez G. Generic-group lower bounds via reductions
between geometric-search problems: With and without preprocessing. In: 21st
International Conference on Theory of Cryptography. Vol 14371. Springer Nature;
2023:301-330. doi:10.1007/978-3-031-48621-0_11'
apa: 'Auerbach, B., Hoffmann, C., & Pascual Perez, G. (2023). Generic-group
lower bounds via reductions between geometric-search problems: With and without
preprocessing. In 21st International Conference on Theory of Cryptography
(Vol. 14371, pp. 301–330). Springer Nature. https://doi.org/10.1007/978-3-031-48621-0_11'
chicago: 'Auerbach, Benedikt, Charlotte Hoffmann, and Guillermo Pascual Perez. “Generic-Group
Lower Bounds via Reductions between Geometric-Search Problems: With and without
Preprocessing.” In 21st International Conference on Theory of Cryptography,
14371:301–30. Springer Nature, 2023. https://doi.org/10.1007/978-3-031-48621-0_11.'
ieee: 'B. Auerbach, C. Hoffmann, and G. Pascual Perez, “Generic-group lower bounds
via reductions between geometric-search problems: With and without preprocessing,”
in 21st International Conference on Theory of Cryptography, 2023, vol.
14371, pp. 301–330.'
ista: 'Auerbach B, Hoffmann C, Pascual Perez G. 2023. Generic-group lower bounds
via reductions between geometric-search problems: With and without preprocessing.
21st International Conference on Theory of Cryptography. , LNCS, vol. 14371, 301–330.'
mla: 'Auerbach, Benedikt, et al. “Generic-Group Lower Bounds via Reductions between
Geometric-Search Problems: With and without Preprocessing.” 21st International
Conference on Theory of Cryptography, vol. 14371, Springer Nature, 2023, pp.
301–30, doi:10.1007/978-3-031-48621-0_11.'
short: B. Auerbach, C. Hoffmann, G. Pascual Perez, in:, 21st International Conference
on Theory of Cryptography, Springer Nature, 2023, pp. 301–330.
corr_author: '1'
date_created: 2023-12-17T23:00:54Z
date_published: 2023-11-27T00:00:00Z
date_updated: 2025-09-09T14:02:04Z
day: '27'
department:
- _id: KrPi
doi: 10.1007/978-3-031-48621-0_11
external_id:
isi:
- '001160724400011'
intvolume: ' 14371'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2023/808
month: '11'
oa: 1
oa_version: Preprint
page: 301-330
publication: 21st International Conference on Theory of Cryptography
publication_identifier:
eissn:
- 1611-3349
isbn:
- '9783031486203'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'Generic-group lower bounds via reductions between geometric-search problems:
With and without preprocessing'
type: conference
user_id: 317138e5-6ab7-11ef-aa6d-ffef3953e345
volume: 14371
year: '2023'
...
---
_id: '11476'
abstract:
- lang: eng
text: "Messaging platforms like Signal are widely deployed and provide strong security
in an asynchronous setting. It is a challenging problem to construct a protocol
with similar security guarantees that can efficiently scale to large groups. A
major bottleneck are the frequent key rotations users need to perform to achieve
post compromise forward security.\r\n\r\nIn current proposals – most notably in
TreeKEM (which is part of the IETF’s Messaging Layer Security (MLS) protocol draft)
– for users in a group of size n to rotate their keys, they must each craft a
message of size log(n) to be broadcast to the group using an (untrusted) delivery
server.\r\n\r\nIn larger groups, having users sequentially rotate their keys requires
too much bandwidth (or takes too long), so variants allowing any T≤n users to
simultaneously rotate their keys in just 2 communication rounds have been suggested
(e.g. “Propose and Commit” by MLS). Unfortunately, 2-round concurrent updates
are either damaging or expensive (or both); i.e. they either result in future
operations being more costly (e.g. via “blanking” or “tainting”) or are costly
themselves requiring Ω(T) communication for each user [Bienstock et al., TCC’20].\r\n\r\nIn
this paper we propose CoCoA; a new scheme that allows for T concurrent updates
that are neither damaging nor costly. That is, they add no cost to future operations
yet they only require Ω(log2(n)) communication per user. To circumvent the [Bienstock
et al.] lower bound, CoCoA increases the number of rounds needed to complete all
updates from 2 up to (at most) log(n); though typically fewer rounds are needed.\r\n\r\nThe
key insight of our protocol is the following: in the (non-concurrent version of)
TreeKEM, a delivery server which gets T concurrent update requests will approve
one and reject the remaining T−1. In contrast, our server attempts to apply all
of them. If more than one user requests to rotate the same key during a round,
the server arbitrarily picks a winner. Surprisingly, we prove that regardless
of how the server chooses the winners, all previously compromised users will recover
after at most log(n) such update rounds.\r\n\r\nTo keep the communication complexity
low, CoCoA is a server-aided CGKA. That is, the delivery server no longer blindly
forwards packets, but instead actively computes individualized packets tailored
to each user. As the server is untrusted, this change requires us to develop new
mechanisms ensuring robustness of the protocol."
acknowledgement: We thank Marta Mularczyk and Yiannis Tselekounis for their very helpful
feedback on an earlier draft of this paper.
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Joël
full_name: Alwen, Joël
last_name: Alwen
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Karen
full_name: Klein, Karen
id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87
last_name: Klein
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
- first_name: Michael
full_name: Walter, Michael
last_name: Walter
citation:
ama: 'Alwen J, Auerbach B, Cueto Noval M, et al. CoCoA: Concurrent continuous group
key agreement. In: Advances in Cryptology – EUROCRYPT 2022. Vol 13276.
Cham: Springer Nature; 2022:815–844. doi:10.1007/978-3-031-07085-3_28'
apa: 'Alwen, J., Auerbach, B., Cueto Noval, M., Klein, K., Pascual Perez, G., Pietrzak,
K. Z., & Walter, M. (2022). CoCoA: Concurrent continuous group key agreement.
In Advances in Cryptology – EUROCRYPT 2022 (Vol. 13276, pp. 815–844). Cham:
Springer Nature. https://doi.org/10.1007/978-3-031-07085-3_28'
chicago: 'Alwen, Joël, Benedikt Auerbach, Miguel Cueto Noval, Karen Klein, Guillermo
Pascual Perez, Krzysztof Z Pietrzak, and Michael Walter. “CoCoA: Concurrent Continuous
Group Key Agreement.” In Advances in Cryptology – EUROCRYPT 2022, 13276:815–844.
Cham: Springer Nature, 2022. https://doi.org/10.1007/978-3-031-07085-3_28.'
ieee: 'J. Alwen et al., “CoCoA: Concurrent continuous group key agreement,”
in Advances in Cryptology – EUROCRYPT 2022, Trondheim, Norway, 2022, vol.
13276, pp. 815–844.'
ista: 'Alwen J, Auerbach B, Cueto Noval M, Klein K, Pascual Perez G, Pietrzak KZ,
Walter M. 2022. CoCoA: Concurrent continuous group key agreement. Advances in
Cryptology – EUROCRYPT 2022. EUROCRYPT: Theory and Applications of Cryptology
and Information Security, LNCS, vol. 13276, 815–844.'
mla: 'Alwen, Joël, et al. “CoCoA: Concurrent Continuous Group Key Agreement.” Advances
in Cryptology – EUROCRYPT 2022, vol. 13276, Springer Nature, 2022, pp. 815–844,
doi:10.1007/978-3-031-07085-3_28.'
short: J. Alwen, B. Auerbach, M. Cueto Noval, K. Klein, G. Pascual Perez, K.Z. Pietrzak,
M. Walter, in:, Advances in Cryptology – EUROCRYPT 2022, Springer Nature, Cham,
2022, pp. 815–844.
conference:
end_date: 2022-06-03
location: Trondheim, Norway
name: 'EUROCRYPT: Theory and Applications of Cryptology and Information Security'
start_date: 2022-05-30
corr_author: '1'
date_created: 2022-06-30T16:48:00Z
date_published: 2022-05-25T00:00:00Z
date_updated: 2026-04-07T13:01:26Z
day: '25'
department:
- _id: GradSch
- _id: KrPi
doi: 10.1007/978-3-031-07085-3_28
ec_funded: 1
external_id:
isi:
- '000832305300028'
intvolume: ' 13276'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2022/251
month: '05'
oa: 1
oa_version: Preprint
page: 815–844
place: Cham
project:
- _id: 258AA5B2-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '682815'
name: Teaching Old Crypto New Tricks
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
publication: Advances in Cryptology – EUROCRYPT 2022
publication_identifier:
eisbn:
- '9783031070853'
eissn:
- 1611-3349
isbn:
- '9783031070846'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
related_material:
record:
- id: '18088'
relation: dissertation_contains
status: public
scopus_import: '1'
status: public
title: 'CoCoA: Concurrent continuous group key agreement'
type: conference
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
volume: 13276
year: '2022'
...
---
_id: '10049'
abstract:
- lang: eng
text: While messaging systems with strong security guarantees are widely used in
practice, designing a protocol that scales efficiently to large groups and enjoys
similar security guarantees remains largely open. The two existing proposals to
date are ART (Cohn-Gordon et al., CCS18) and TreeKEM (IETF, The Messaging Layer
Security Protocol, draft). TreeKEM is the currently considered candidate by the
IETF MLS working group, but dynamic group operations (i.e. adding and removing
users) can cause efficiency issues. In this paper we formalize and analyze a variant
of TreeKEM which we term Tainted TreeKEM (TTKEM for short). The basic idea underlying
TTKEM was suggested by Millican (MLS mailing list, February 2018). This version
is more efficient than TreeKEM for some natural distributions of group operations,
we quantify this through simulations.Our second contribution is two security proofs
for TTKEM which establish post compromise and forward secrecy even against adaptive
attackers. The security loss (to the underlying PKE) in the Random Oracle Model
is a polynomial factor, and a quasipolynomial one in the Standard Model. Our proofs
can be adapted to TreeKEM as well. Before our work no security proof for any TreeKEM-like
protocol establishing tight security against an adversary who can adaptively choose
the sequence of operations was known. We also are the first to prove (or even
formalize) active security where the server can arbitrarily deviate from the protocol
specification. Proving fully active security – where also the users can arbitrarily
deviate – remains open.
acknowledgement: The first three authors contributed equally to this work. Funded
by the European Research Council (ERC) under the European Union’s Horizon2020 research
and innovation programme (682815-TOCNeT). Funded by the European Union’s Horizon
2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement
No.665385.
article_processing_charge: No
author:
- first_name: Karen
full_name: Klein, Karen
id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87
last_name: Klein
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Michael
full_name: Walter, Michael
id: 488F98B0-F248-11E8-B48F-1D18A9856A87
last_name: Walter
orcid: 0000-0003-3186-2482
- first_name: Chethan
full_name: Kamath Hosdurg, Chethan
id: 4BD3F30E-F248-11E8-B48F-1D18A9856A87
last_name: Kamath Hosdurg
orcid: 0009-0006-6812-7317
- first_name: Margarita
full_name: Capretto, Margarita
last_name: Capretto
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Ilia
full_name: Markov, Ilia
id: D0CF4148-C985-11E9-8066-0BDEE5697425
last_name: Markov
- first_name: Michelle X
full_name: Yeo, Michelle X
id: 2D82B818-F248-11E8-B48F-1D18A9856A87
last_name: Yeo
orcid: 0009-0001-3676-4809
- first_name: Joel F
full_name: Alwen, Joel F
id: 2A8DFA8C-F248-11E8-B48F-1D18A9856A87
last_name: Alwen
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
citation:
ama: 'Klein K, Pascual Perez G, Walter M, et al. Keep the dirt: tainted TreeKEM,
adaptively and actively secure continuous group key agreement. In: 2021 IEEE
Symposium on Security and Privacy . IEEE; 2021:268-284. doi:10.1109/sp40001.2021.00035'
apa: 'Klein, K., Pascual Perez, G., Walter, M., Kamath Hosdurg, C., Capretto, M.,
Cueto Noval, M., … Pietrzak, K. Z. (2021). Keep the dirt: tainted TreeKEM, adaptively
and actively secure continuous group key agreement. In 2021 IEEE Symposium
on Security and Privacy (pp. 268–284). San Francisco, CA, United States:
IEEE. https://doi.org/10.1109/sp40001.2021.00035'
chicago: 'Klein, Karen, Guillermo Pascual Perez, Michael Walter, Chethan Kamath
Hosdurg, Margarita Capretto, Miguel Cueto Noval, Ilia Markov, Michelle X Yeo,
Joel F Alwen, and Krzysztof Z Pietrzak. “Keep the Dirt: Tainted TreeKEM, Adaptively
and Actively Secure Continuous Group Key Agreement.” In 2021 IEEE Symposium
on Security and Privacy , 268–84. IEEE, 2021. https://doi.org/10.1109/sp40001.2021.00035.'
ieee: 'K. Klein et al., “Keep the dirt: tainted TreeKEM, adaptively and actively
secure continuous group key agreement,” in 2021 IEEE Symposium on Security
and Privacy , San Francisco, CA, United States, 2021, pp. 268–284.'
ista: 'Klein K, Pascual Perez G, Walter M, Kamath Hosdurg C, Capretto M, Cueto Noval
M, Markov I, Yeo MX, Alwen JF, Pietrzak KZ. 2021. Keep the dirt: tainted TreeKEM,
adaptively and actively secure continuous group key agreement. 2021 IEEE Symposium
on Security and Privacy . SP: Symposium on Security and Privacy, 268–284.'
mla: 'Klein, Karen, et al. “Keep the Dirt: Tainted TreeKEM, Adaptively and Actively
Secure Continuous Group Key Agreement.” 2021 IEEE Symposium on Security and
Privacy , IEEE, 2021, pp. 268–84, doi:10.1109/sp40001.2021.00035.'
short: K. Klein, G. Pascual Perez, M. Walter, C. Kamath Hosdurg, M. Capretto, M.
Cueto Noval, I. Markov, M.X. Yeo, J.F. Alwen, K.Z. Pietrzak, in:, 2021 IEEE Symposium
on Security and Privacy , IEEE, 2021, pp. 268–284.
conference:
end_date: 2021-05-27
location: San Francisco, CA, United States
name: 'SP: Symposium on Security and Privacy'
start_date: 2021-05-24
corr_author: '1'
date_created: 2021-09-27T13:46:27Z
date_published: 2021-08-26T00:00:00Z
date_updated: 2026-04-08T07:01:44Z
day: '26'
department:
- _id: KrPi
- _id: DaAl
doi: 10.1109/sp40001.2021.00035
ec_funded: 1
external_id:
isi:
- '001316065000016'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2019/1489
month: '08'
oa: 1
oa_version: Preprint
page: 268-284
project:
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
- _id: 258AA5B2-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '682815'
name: Teaching Old Crypto New Tricks
publication: '2021 IEEE Symposium on Security and Privacy '
publication_status: published
publisher: IEEE
quality_controlled: '1'
related_material:
record:
- id: '18088'
relation: dissertation_contains
status: public
- id: '10035'
relation: dissertation_contains
status: public
scopus_import: '1'
status: public
title: 'Keep the dirt: tainted TreeKEM, adaptively and actively secure continuous
group key agreement'
type: conference
user_id: 317138e5-6ab7-11ef-aa6d-ffef3953e345
year: '2021'
...
---
_id: '10408'
abstract:
- lang: eng
text: 'Key trees are often the best solution in terms of transmission cost and storage
requirements for managing keys in a setting where a group needs to share a secret
key, while being able to efficiently rotate the key material of users (in order
to recover from a potential compromise, or to add or remove users). Applications
include multicast encryption protocols like LKH (Logical Key Hierarchies) or group
messaging like the current IETF proposal TreeKEM. A key tree is a (typically balanced)
binary tree, where each node is identified with a key: leaf nodes hold users’
secret keys while the root is the shared group key. For a group of size N, each
user just holds log(N) keys (the keys on the path from its leaf to the root)
and its entire key material can be rotated by broadcasting 2log(N) ciphertexts
(encrypting each fresh key on the path under the keys of its parents). In this
work we consider the natural setting where we have many groups with partially
overlapping sets of users, and ask if we can find solutions where the cost of
rotating a key is better than in the trivial one where we have a separate key
tree for each group. We show that in an asymptotic setting (where the number m
of groups is fixed while the number N of users grows) there exist more general
key graphs whose cost converges to the cost of a single group, thus saving a factor
linear in the number of groups over the trivial solution. As our asymptotic “solution”
converges very slowly and performs poorly on concrete examples, we propose an
algorithm that uses a natural heuristic to compute a key graph for any given group
structure. Our algorithm combines two greedy algorithms, and is thus very efficient:
it first converts the group structure into a “lattice graph”, which is then turned
into a key graph by repeatedly applying the algorithm for constructing a Huffman
code. To better understand how far our proposal is from an optimal solution, we
prove lower bounds on the update cost of continuous group-key agreement and multicast
encryption in a symbolic model admitting (asymmetric) encryption, pseudorandom
generators, and secret sharing as building blocks.'
acknowledgement: B. Auerbach, M.A. Baig and K. Pietrzak—received funding from the
European Research Council (ERC) under the European Union’s Horizon 2020 research
and innovation programme (682815 - TOCNeT); Karen Klein was supported in part by
ERC CoG grant 724307 and conducted part of this work at IST Austria, funded by the
ERC under the European Union’s Horizon 2020 research and innovation programme (682815
- TOCNeT); Guillermo Pascual-Perez was funded by the European Union’s Horizon 2020
research and innovation programme under the Marie Skłodowska-Curie Grant Agreement
No. 665385; Michael Walter conducted part of this work at IST Austria, funded by
the ERC under the European Union’s Horizon 2020 research and innovation programme
(682815 - TOCNeT).
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Joel F
full_name: Alwen, Joel F
id: 2A8DFA8C-F248-11E8-B48F-1D18A9856A87
last_name: Alwen
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Mirza Ahad
full_name: Baig, Mirza Ahad
id: 3EDE6DE4-AA5A-11E9-986D-341CE6697425
last_name: Baig
- first_name: Miguel
full_name: Cueto Noval, Miguel
id: ffc563a3-f6e0-11ea-865d-e3cce03d17cc
last_name: Cueto Noval
orcid: 0000-0002-2505-4246
- first_name: Karen
full_name: Klein, Karen
id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87
last_name: Klein
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
- first_name: Michael
full_name: Walter, Michael
id: 488F98B0-F248-11E8-B48F-1D18A9856A87
last_name: Walter
orcid: 0000-0003-3186-2482
citation:
ama: 'Alwen JF, Auerbach B, Baig MA, et al. Grafting key trees: Efficient key management
for overlapping groups. In: 19th International Conference. Vol 13044. Springer
Nature; 2021:222-253. doi:10.1007/978-3-030-90456-2_8'
apa: 'Alwen, J. F., Auerbach, B., Baig, M. A., Cueto Noval, M., Klein, K., Pascual
Perez, G., … Walter, M. (2021). Grafting key trees: Efficient key management for
overlapping groups. In 19th International Conference (Vol. 13044, pp. 222–253).
Raleigh, NC, United States: Springer Nature. https://doi.org/10.1007/978-3-030-90456-2_8'
chicago: 'Alwen, Joel F, Benedikt Auerbach, Mirza Ahad Baig, Miguel Cueto Noval,
Karen Klein, Guillermo Pascual Perez, Krzysztof Z Pietrzak, and Michael Walter.
“Grafting Key Trees: Efficient Key Management for Overlapping Groups.” In 19th
International Conference, 13044:222–53. Springer Nature, 2021. https://doi.org/10.1007/978-3-030-90456-2_8.'
ieee: 'J. F. Alwen et al., “Grafting key trees: Efficient key management
for overlapping groups,” in 19th International Conference, Raleigh, NC,
United States, 2021, vol. 13044, pp. 222–253.'
ista: 'Alwen JF, Auerbach B, Baig MA, Cueto Noval M, Klein K, Pascual Perez G, Pietrzak
KZ, Walter M. 2021. Grafting key trees: Efficient key management for overlapping
groups. 19th International Conference. TCC: Theory of Cryptography, LNCS, vol.
13044, 222–253.'
mla: 'Alwen, Joel F., et al. “Grafting Key Trees: Efficient Key Management for Overlapping
Groups.” 19th International Conference, vol. 13044, Springer Nature, 2021,
pp. 222–53, doi:10.1007/978-3-030-90456-2_8.'
short: J.F. Alwen, B. Auerbach, M.A. Baig, M. Cueto Noval, K. Klein, G. Pascual
Perez, K.Z. Pietrzak, M. Walter, in:, 19th International Conference, Springer
Nature, 2021, pp. 222–253.
conference:
end_date: 2021-11-11
location: Raleigh, NC, United States
name: 'TCC: Theory of Cryptography'
start_date: 2021-11-08
date_created: 2021-12-05T23:01:42Z
date_published: 2021-11-04T00:00:00Z
date_updated: 2026-04-07T13:01:26Z
day: '04'
department:
- _id: KrPi
doi: 10.1007/978-3-030-90456-2_8
ec_funded: 1
external_id:
isi:
- '000728363700008'
intvolume: ' 13044'
isi: 1
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2021/1158
month: '11'
oa: 1
oa_version: Preprint
page: 222-253
project:
- _id: 258AA5B2-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '682815'
name: Teaching Old Crypto New Tricks
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
publication: 19th International Conference
publication_identifier:
eisbn:
- 978-3-030-90456-2
eissn:
- 1611-3349
isbn:
- 9-783-0309-0455-5
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
related_material:
record:
- id: '18088'
relation: dissertation_contains
status: public
scopus_import: '1'
status: public
title: 'Grafting key trees: Efficient key management for overlapping groups'
type: conference
user_id: 4359f0d1-fa6c-11eb-b949-802e58b17ae8
volume: 13044
year: '2021'
...
---
_id: '9826'
abstract:
- lang: eng
text: "Automated contract tracing aims at supporting manual contact tracing during
pandemics by alerting users of encounters with infected people. There are currently
many proposals for protocols (like the “decentralized” DP-3T and PACT or the “centralized”
ROBERT and DESIRE) to be run on mobile phones, where the basic idea is to regularly
broadcast (using low energy Bluetooth) some values, and at the same time store
(a function of) incoming messages broadcasted by users in their proximity. In
the existing proposals one can trigger false positives on a massive scale by an
“inverse-Sybil” attack, where a large number of devices (malicious users or hacked
phones) pretend to be the same user, such that later, just a single person needs
to be diagnosed (and allowed to upload) to trigger an alert for all users who
were in proximity to any of this large group of devices.\r\n\r\nWe propose the
first protocols that do not succumb to such attacks assuming the devices involved
in the attack do not constantly communicate, which we observe is a necessary assumption.
The high level idea of the protocols is to derive the values to be broadcasted
by a hash chain, so that two (or more) devices who want to launch an inverse-Sybil
attack will not be able to connect their respective chains and thus only one of
them will be able to upload. Our protocols also achieve security against replay,
belated replay, and one of them even against relay attacks."
acknowledgement: Guillermo Pascual-Perez and Michelle Yeo were funded by the European
Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska–Curie
Grant Agreement No. 665385; the remaining contributors to this project have received
funding from the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (682815 - TOCNeT).
alternative_title:
- LNCS
article_processing_charge: No
author:
- first_name: Benedikt
full_name: Auerbach, Benedikt
id: D33D2B18-E445-11E9-ABB7-15F4E5697425
last_name: Auerbach
orcid: 0000-0002-7553-6606
- first_name: Suvradip
full_name: Chakraborty, Suvradip
id: B9CD0494-D033-11E9-B219-A439E6697425
last_name: Chakraborty
- first_name: Karen
full_name: Klein, Karen
id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87
last_name: Klein
- first_name: Guillermo
full_name: Pascual Perez, Guillermo
id: 2D7ABD02-F248-11E8-B48F-1D18A9856A87
last_name: Pascual Perez
orcid: 0000-0001-8630-415X
- first_name: Krzysztof Z
full_name: Pietrzak, Krzysztof Z
id: 3E04A7AA-F248-11E8-B48F-1D18A9856A87
last_name: Pietrzak
orcid: 0000-0002-9139-1654
- first_name: Michael
full_name: Walter, Michael
id: 488F98B0-F248-11E8-B48F-1D18A9856A87
last_name: Walter
orcid: 0000-0003-3186-2482
- first_name: Michelle X
full_name: Yeo, Michelle X
id: 2D82B818-F248-11E8-B48F-1D18A9856A87
last_name: Yeo
orcid: 0009-0001-3676-4809
citation:
ama: 'Auerbach B, Chakraborty S, Klein K, et al. Inverse-Sybil attacks in automated
contact tracing. In: Topics in Cryptology – CT-RSA 2021. Vol 12704. Springer
Nature; 2021:399-421. doi:10.1007/978-3-030-75539-3_17'
apa: 'Auerbach, B., Chakraborty, S., Klein, K., Pascual Perez, G., Pietrzak, K.
Z., Walter, M., & Yeo, M. X. (2021). Inverse-Sybil attacks in automated contact
tracing. In Topics in Cryptology – CT-RSA 2021 (Vol. 12704, pp. 399–421).
Virtual Event: Springer Nature. https://doi.org/10.1007/978-3-030-75539-3_17'
chicago: Auerbach, Benedikt, Suvradip Chakraborty, Karen Klein, Guillermo Pascual
Perez, Krzysztof Z Pietrzak, Michael Walter, and Michelle X Yeo. “Inverse-Sybil
Attacks in Automated Contact Tracing.” In Topics in Cryptology – CT-RSA 2021,
12704:399–421. Springer Nature, 2021. https://doi.org/10.1007/978-3-030-75539-3_17.
ieee: B. Auerbach et al., “Inverse-Sybil attacks in automated contact tracing,”
in Topics in Cryptology – CT-RSA 2021, Virtual Event, 2021, vol. 12704,
pp. 399–421.
ista: 'Auerbach B, Chakraborty S, Klein K, Pascual Perez G, Pietrzak KZ, Walter
M, Yeo MX. 2021. Inverse-Sybil attacks in automated contact tracing. Topics in
Cryptology – CT-RSA 2021. CT-RSA: Cryptographers’ Track at the RSA Conference,
LNCS, vol. 12704, 399–421.'
mla: Auerbach, Benedikt, et al. “Inverse-Sybil Attacks in Automated Contact Tracing.”
Topics in Cryptology – CT-RSA 2021, vol. 12704, Springer Nature, 2021,
pp. 399–421, doi:10.1007/978-3-030-75539-3_17.
short: B. Auerbach, S. Chakraborty, K. Klein, G. Pascual Perez, K.Z. Pietrzak, M.
Walter, M.X. Yeo, in:, Topics in Cryptology – CT-RSA 2021, Springer Nature, 2021,
pp. 399–421.
conference:
end_date: 2021-05-20
location: Virtual Event
name: 'CT-RSA: Cryptographers’ Track at the RSA Conference'
start_date: 2021-05-17
corr_author: '1'
date_created: 2021-08-08T22:01:30Z
date_published: 2021-05-11T00:00:00Z
date_updated: 2026-04-16T09:28:46Z
day: '11'
department:
- _id: KrPi
- _id: GradSch
doi: 10.1007/978-3-030-75539-3_17
ec_funded: 1
intvolume: ' 12704'
language:
- iso: eng
main_file_link:
- open_access: '1'
url: https://eprint.iacr.org/2020/670
month: '05'
oa: 1
oa_version: Submitted Version
page: 399-421
project:
- _id: 2564DBCA-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '665385'
name: International IST Doctoral Program
- _id: 258AA5B2-B435-11E9-9278-68D0E5697425
call_identifier: H2020
grant_number: '682815'
name: Teaching Old Crypto New Tricks
publication: Topics in Cryptology – CT-RSA 2021
publication_identifier:
eissn:
- 1611-3349
isbn:
- '9783030755386'
issn:
- 0302-9743
publication_status: published
publisher: Springer Nature
quality_controlled: '1'
scopus_import: '1'
status: public
title: Inverse-Sybil attacks in automated contact tracing
type: conference
user_id: ba8df636-2132-11f1-aed0-ed93e2281fdd
volume: 12704
year: '2021'
...