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Documents authored by Tschudi, Daniel


Document
Secure Communication in Dynamic Incomplete Networks

Authors: Ivan Damgård, Divya Ravi, Daniel Tschudi, and Sophia Yakoubov

Published in: LIPIcs, Volume 267, 4th Conference on Information-Theoretic Cryptography (ITC 2023)


Abstract
In this paper, we explore the feasibility of reliable and private communication in dynamic networks, where in each round the adversary can choose which direct peer-to-peer links are available in the network graph, under the sole condition that the graph is k-connected at each round (for some k). We show that reliable communication is possible in such a dynamic network if and only if k > 2t. We also show that if k = cn > 2 t for a constant c, we can achieve reliable communication with polynomial round and communication complexity. For unconditionally private communication, we show that for a passive adversary, k > t is sufficient (and clearly necessary). For an active adversary, we show that k > 2t is sufficient for statistical security (and clearly necessary), while k > 3t is sufficient for perfect security. We conjecture that, in contrast to the static case, k > 2t is not enough for perfect security, and we give evidence that the conjecture is true. Once we have reliable and private communication between each pair of parties, we can emulate a complete network with secure channels, and we can use known protocols to do secure computation.

Cite as

Ivan Damgård, Divya Ravi, Daniel Tschudi, and Sophia Yakoubov. Secure Communication in Dynamic Incomplete Networks. In 4th Conference on Information-Theoretic Cryptography (ITC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 267, pp. 13:1-13:21, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2023)


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@InProceedings{damgard_et_al:LIPIcs.ITC.2023.13,
  author =	{Damg\r{a}rd, Ivan and Ravi, Divya and Tschudi, Daniel and Yakoubov, Sophia},
  title =	{{Secure Communication in Dynamic Incomplete Networks}},
  booktitle =	{4th Conference on Information-Theoretic Cryptography (ITC 2023)},
  pages =	{13:1--13:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-271-6},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{267},
  editor =	{Chung, Kai-Min},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2023.13},
  URN =		{urn:nbn:de:0030-drops-183419},
  doi =		{10.4230/LIPIcs.ITC.2023.13},
  annote =	{Keywords: Secure Communication, Dynamic Incomplete Network, Information-theoretic}
}
Document
Strong Separations Between Broadcast and Authenticated Channels

Authors: Julian Loss, Ueli Maurer, and Daniel Tschudi

Published in: LIPIcs, Volume 121, 32nd International Symposium on Distributed Computing (DISC 2018)


Abstract
In the theory of distributed systems and cryptography one considers a setting with n parties, (often) connected via authenticated bilateral channels, who want to achieve a certain goal even if some fraction of the parties is dishonest. A classical goal of this type is to construct a broadcast channel. A broadcast channel guarantees that all honest recipients get the same value v (consistency) and, if the sender is honest, that v is the sender's input (validity). Lamport et al. showed that it is possible to construct broadcast if and only if the fraction of cheaters is less than a third. A natural question, first raised by Lamport, is whether there are weaker, still useful primitives achievable from authenticated channels. He proposed weak broadcast, where the validity condition must hold only if all parties are honest, and showed that it can be achieved with an unbounded number of protocol rounds, while broadcast cannot, suggesting that weak broadcast is in a certain sense weaker than broadcast. The purpose of this paper is to deepen the investigation of the separation between broadcast and authenticated channels. This is achieved by proving the following results. First, we prove a stronger impossibility result for 3-party broadcast. Even if two of the parties can broadcast, one can not achieve broadcast for the third party. Second, we prove a strong separation between authenticated channels and broadcast by exhibiting a new primitive, called XOR-cast, which satisfies two conditions: (1) XOR-cast is strongly unachievable (even with small error probability) from authenticated channels (which is not true for weak broadcast), and (2) broadcast is strongly unachievable from XOR-cast (and authenticated channels). This demonstrates that the hierarchy of primitives has a more complex structure than previously known. Third, we prove a strong separation between weak broadcast and broadcast which is not implied by Lamport's results. The proofs of these results requires the generalization of known techniques for impossibility proofs.

Cite as

Julian Loss, Ueli Maurer, and Daniel Tschudi. Strong Separations Between Broadcast and Authenticated Channels. In 32nd International Symposium on Distributed Computing (DISC 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 121, pp. 36:1-36:17, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)


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@InProceedings{loss_et_al:LIPIcs.DISC.2018.36,
  author =	{Loss, Julian and Maurer, Ueli and Tschudi, Daniel},
  title =	{{Strong Separations Between Broadcast and Authenticated Channels}},
  booktitle =	{32nd International Symposium on Distributed Computing (DISC 2018)},
  pages =	{36:1--36:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-092-7},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{121},
  editor =	{Schmid, Ulrich and Widder, Josef},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2018.36},
  URN =		{urn:nbn:de:0030-drops-98252},
  doi =		{10.4230/LIPIcs.DISC.2018.36},
  annote =	{Keywords: cryptography, multi-party computation, broadcast, impossibility}
}
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