--- _id: '14506' abstract: - lang: eng text: "Payment channel networks are a promising approach to improve the scalability bottleneck\r\nof cryptocurrencies. Two design principles behind payment channel networks are\r\nefficiency and privacy. Payment channel networks improve efficiency by allowing users\r\nto transact in a peer-to-peer fashion along multi-hop routes in the network, avoiding\r\nthe lengthy process of consensus on the blockchain. Transacting over payment channel\r\nnetworks also improves privacy as these transactions are not broadcast to the blockchain.\r\nDespite the influx of recent protocols built on top of payment channel networks and\r\ntheir analysis, a common shortcoming of many of these protocols is that they typically\r\nfocus only on either improving efficiency or privacy, but not both. Another limitation\r\non the efficiency front is that the models used to model actions, costs and utilities of\r\nusers are limited or come with unrealistic assumptions.\r\nThis thesis aims to address some of the shortcomings of recent protocols and algorithms\r\non payment channel networks, particularly in their privacy and efficiency aspects. We\r\nfirst present a payment route discovery protocol based on hub labelling and private\r\ninformation retrieval that hides the route query and is also efficient. We then present\r\na rebalancing protocol that formulates the rebalancing problem as a linear program\r\nand solves the linear program using multiparty computation so as to hide the channel\r\nbalances. The rebalancing solution as output by our protocol is also globally optimal.\r\nWe go on to develop more realistic models of the action space, costs, and utilities of\r\nboth existing and new users that want to join the network. In each of these settings,\r\nwe also develop algorithms to optimise the utility of these users with good guarantees\r\non the approximation and competitive ratios." alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Michelle X full_name: Yeo, Michelle X id: 2D82B818-F248-11E8-B48F-1D18A9856A87 last_name: Yeo citation: ama: Yeo MX. Advances in efficiency and privacy in payment channel network analysis. 2023. doi:10.15479/14506 apa: Yeo, M. X. (2023). Advances in efficiency and privacy in payment channel network analysis. Institute of Science and Technology Austria. https://doi.org/10.15479/14506 chicago: Yeo, Michelle X. “Advances in Efficiency and Privacy in Payment Channel Network Analysis.” Institute of Science and Technology Austria, 2023. https://doi.org/10.15479/14506. ieee: M. X. Yeo, “Advances in efficiency and privacy in payment channel network analysis,” Institute of Science and Technology Austria, 2023. ista: Yeo MX. 2023. Advances in efficiency and privacy in payment channel network analysis. Institute of Science and Technology Austria. mla: Yeo, Michelle X. Advances in Efficiency and Privacy in Payment Channel Network Analysis. Institute of Science and Technology Austria, 2023, doi:10.15479/14506. short: M.X. Yeo, Advances in Efficiency and Privacy in Payment Channel Network Analysis, Institute of Science and Technology Austria, 2023. date_created: 2023-11-10T08:10:43Z date_published: 2023-11-10T00:00:00Z date_updated: 2023-11-30T10:54:51Z day: '10' ddc: - '000' degree_awarded: PhD department: - _id: GradSch - _id: KrPi doi: 10.15479/14506 ec_funded: 1 file: - access_level: closed checksum: 521c72818d720a52b377207b2ee87b6a content_type: application/x-zip-compressed creator: cchlebak date_created: 2023-11-23T10:29:55Z date_updated: 2023-11-23T10:29:55Z file_id: '14598' file_name: thesis_yeo.zip file_size: 3037720 relation: source_file - access_level: open_access checksum: 0ed5d16899687aecf13d843c9878c9f2 content_type: application/pdf creator: cchlebak date_created: 2023-11-23T10:30:08Z date_updated: 2023-11-23T10:30:08Z file_id: '14599' file_name: thesis_yeo.pdf file_size: 2717256 relation: main_file success: 1 file_date_updated: 2023-11-23T10:30:08Z has_accepted_license: '1' language: - iso: eng month: '11' oa: 1 oa_version: Published Version page: '162' 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: '9969' relation: part_of_dissertation status: public - id: '13238' relation: part_of_dissertation status: public - id: '14490' 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: Advances in efficiency and privacy in payment channel network analysis type: dissertation user_id: 8b945eb4-e2f2-11eb-945a-df72226e66a9 year: '2023' ... --- _id: '10035' abstract: - lang: eng text: 'Many security definitions come in two flavors: a stronger “adaptive” flavor, where the adversary can arbitrarily make various choices during the course of the attack, and a weaker “selective” flavor where the adversary must commit to some or all of their choices a-priori. For example, in the context of identity-based encryption, selective security requires the adversary to decide on the identity of the attacked party at the very beginning of the game whereas adaptive security allows the attacker to first see the master public key and some secret keys before making this choice. Often, it appears to be much easier to achieve selective security than it is to achieve adaptive security. A series of several recent works shows how to cleverly achieve adaptive security in several such scenarios including generalized selective decryption [Pan07][FJP15], constrained PRFs [FKPR14], and Yao’s garbled circuits [JW16]. Although the above works expressed vague intuition that they share a common technique, the connection was never made precise. In this work we present a new framework (published at Crypto ’17 [JKK+17a]) that connects all of these works and allows us to present them in a unified and simplified fashion. Having the framework in place, we show how to achieve adaptive security for proxy re-encryption schemes (published at PKC ’19 [FKKP19]) and provide the first adaptive security proofs for continuous group key agreement protocols (published at S&P ’21 [KPW+21]). Questioning optimality of our framework, we then show that currently used proof techniques cannot lead to significantly better security guarantees for "graph-building" games (published at TCC ’21 [KKPW21a]). These games cover generalized selective decryption, as well as the security of prominent constructions for constrained PRFs, continuous group key agreement, and proxy re-encryption. Finally, we revisit the adaptive security of Yao’s garbled circuits and extend the analysis of Jafargholi and Wichs in two directions: While they prove adaptive security only for a modified construction with increased online complexity, we provide the first positive results for the original construction by Yao (published at TCC ’21 [KKP21a]). On the negative side, we prove that the results of Jafargholi and Wichs are essentially optimal by showing that no black-box reduction can provide a significantly better security bound (published at Crypto ’21 [KKPW21c]).' acknowledgement: "I want to acknowledge the funding by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (682815 - TOCNeT).\r\n" alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Karen full_name: Klein, Karen id: 3E83A2F8-F248-11E8-B48F-1D18A9856A87 last_name: Klein citation: ama: Klein K. On the adaptive security of graph-based games. 2021. doi:10.15479/at:ista:10035 apa: Klein, K. (2021). On the adaptive security of graph-based games. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:10035 chicago: Klein, Karen. “On the Adaptive Security of Graph-Based Games.” Institute of Science and Technology Austria, 2021. https://doi.org/10.15479/at:ista:10035. ieee: K. Klein, “On the adaptive security of graph-based games,” Institute of Science and Technology Austria, 2021. ista: Klein K. 2021. On the adaptive security of graph-based games. Institute of Science and Technology Austria. mla: Klein, Karen. On the Adaptive Security of Graph-Based Games. Institute of Science and Technology Austria, 2021, doi:10.15479/at:ista:10035. short: K. Klein, On the Adaptive Security of Graph-Based Games, Institute of Science and Technology Austria, 2021. date_created: 2021-09-23T07:31:44Z date_published: 2021-09-23T00:00:00Z date_updated: 2023-10-17T09:24:07Z day: '23' ddc: - '519' degree_awarded: PhD department: - _id: GradSch - _id: KrPi doi: 10.15479/at:ista:10035 ec_funded: 1 file: - access_level: open_access checksum: 73a44345c683e81f3e765efbf86fdcc5 content_type: application/pdf creator: cchlebak date_created: 2021-10-04T12:22:33Z date_updated: 2021-10-04T12:22:33Z file_id: '10082' file_name: thesis_pdfa.pdf file_size: 2104726 relation: main_file success: 1 - access_level: closed checksum: 7b80df30a0e686c3ef6a56d4e1c59e29 content_type: application/x-zip-compressed creator: cchlebak date_created: 2021-10-05T07:04:37Z date_updated: 2022-03-10T12:15:18Z file_id: '10085' file_name: thesis_final (1).zip file_size: 9538359 relation: source_file file_date_updated: 2022-03-10T12:15:18Z has_accepted_license: '1' language: - iso: eng month: '09' oa: 1 oa_version: Published Version page: '276' project: - _id: 258AA5B2-B435-11E9-9278-68D0E5697425 call_identifier: H2020 grant_number: '682815' name: Teaching Old Crypto New Tricks publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '10044' relation: part_of_dissertation status: public - id: '10049' relation: part_of_dissertation status: public - id: '637' relation: part_of_dissertation status: public - id: '10041' relation: part_of_dissertation status: public - id: '6430' relation: part_of_dissertation status: public - id: '10048' 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 adaptive security of graph-based games tmp: image: /images/cc_by.png legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0) short: CC BY (4.0) type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2021' ... --- _id: '7896' abstract: - lang: eng text: "A search problem lies in the complexity class FNP if a solution to the given instance of the problem can be verified efficiently. The complexity class TFNP consists of all search problems in FNP that are total in the sense that a solution is guaranteed to exist. TFNP contains a host of interesting problems from fields such as algorithmic game theory, computational topology, number theory and combinatorics. Since TFNP is a semantic class, it is unlikely to have a complete problem. Instead, one studies its syntactic subclasses which are defined based on the combinatorial principle used to argue totality. Of particular interest is the subclass PPAD, which contains important problems\r\nlike computing Nash equilibrium for bimatrix games and computational counterparts of several fixed-point theorems as complete. In the thesis, we undertake the study of averagecase hardness of TFNP, and in particular its subclass PPAD.\r\nAlmost nothing was known about average-case hardness of PPAD before a series of recent results showed how to achieve it using a cryptographic primitive called program obfuscation.\r\nHowever, it is currently not known how to construct program obfuscation from standard cryptographic assumptions. Therefore, it is desirable to relax the assumption under which average-case hardness of PPAD can be shown. In the thesis we take a step in this direction. First, we show that assuming the (average-case) hardness of a numbertheoretic\r\nproblem related to factoring of integers, which we call Iterated-Squaring, PPAD is hard-on-average in the random-oracle model. Then we strengthen this result to show that the average-case hardness of PPAD reduces to the (adaptive) soundness of the Fiat-Shamir Transform, a well-known technique used to compile a public-coin interactive protocol into a non-interactive one. As a corollary, we obtain average-case hardness for PPAD in the random-oracle model assuming the worst-case hardness of #SAT. Moreover, the above results can all be strengthened to obtain average-case hardness for the class CLS ⊆ PPAD.\r\nOur main technical contribution is constructing incrementally-verifiable procedures for computing Iterated-Squaring and #SAT. By incrementally-verifiable, we mean that every intermediate state of the computation includes a proof of its correctness, and the proof can be updated and verified in polynomial time. Previous constructions of such procedures relied on strong, non-standard assumptions. Instead, we introduce a technique called recursive proof-merging to obtain the same from weaker assumptions. " alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Chethan full_name: Kamath Hosdurg, Chethan id: 4BD3F30E-F248-11E8-B48F-1D18A9856A87 last_name: Kamath Hosdurg citation: ama: Kamath Hosdurg C. On the average-case hardness of total search problems. 2020. doi:10.15479/AT:ISTA:7896 apa: Kamath Hosdurg, C. (2020). On the average-case hardness of total search problems. Institute of Science and Technology Austria. https://doi.org/10.15479/AT:ISTA:7896 chicago: Kamath Hosdurg, Chethan. “On the Average-Case Hardness of Total Search Problems.” Institute of Science and Technology Austria, 2020. https://doi.org/10.15479/AT:ISTA:7896. ieee: C. Kamath Hosdurg, “On the average-case hardness of total search problems,” Institute of Science and Technology Austria, 2020. ista: Kamath Hosdurg C. 2020. On the average-case hardness of total search problems. Institute of Science and Technology Austria. mla: Kamath Hosdurg, Chethan. On the Average-Case Hardness of Total Search Problems. Institute of Science and Technology Austria, 2020, doi:10.15479/AT:ISTA:7896. short: C. Kamath Hosdurg, On the Average-Case Hardness of Total Search Problems, Institute of Science and Technology Austria, 2020. date_created: 2020-05-26T14:08:55Z date_published: 2020-05-25T00:00:00Z date_updated: 2023-09-07T13:15:55Z day: '25' ddc: - '000' degree_awarded: PhD department: - _id: KrPi doi: 10.15479/AT:ISTA:7896 ec_funded: 1 file: - access_level: open_access checksum: b39e2e1c376f5819b823fb7077491c64 content_type: application/pdf creator: dernst date_created: 2020-05-26T14:08:13Z date_updated: 2020-07-14T12:48:04Z file_id: '7897' file_name: 2020_Thesis_Kamath.pdf file_size: 1622742 relation: main_file - access_level: closed checksum: 8b26ba729c1a85ac6bea775f5d73cdc7 content_type: application/x-zip-compressed creator: dernst date_created: 2020-05-26T14:08:23Z date_updated: 2020-07-14T12:48:04Z file_id: '7898' file_name: Thesis_Kamath.zip file_size: 15301529 relation: source_file file_date_updated: 2020-07-14T12:48:04Z has_accepted_license: '1' language: - iso: eng month: '05' oa: 1 oa_version: Published Version page: '126' project: - _id: 258C570E-B435-11E9-9278-68D0E5697425 call_identifier: FP7 grant_number: '259668' name: Provable Security for Physical Cryptography - _id: 258AA5B2-B435-11E9-9278-68D0E5697425 call_identifier: H2020 grant_number: '682815' name: Teaching Old Crypto New Tricks publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '6677' 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 average-case hardness of total search problems tmp: image: /images/cc_by.png legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0) short: CC BY (4.0) type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2020' ... --- _id: '83' abstract: - lang: eng text: "A proof system is a protocol between a prover and a verifier over a common input in which an honest prover convinces the verifier of the validity of true statements. Motivated by the success of decentralized cryptocurrencies, exemplified by Bitcoin, the focus of this thesis will be on proof systems which found applications in some sustainable alternatives to Bitcoin, such as the Spacemint and Chia cryptocurrencies. In particular, we focus on proofs of space and proofs of sequential work.\r\nProofs of space (PoSpace) were suggested as more ecological, economical, and egalitarian alternative to the energy-wasteful proof-of-work mining of Bitcoin. However, the state-of-the-art constructions of PoSpace are based on sophisticated graph pebbling lower bounds, and are therefore complex. Moreover, when these PoSpace are used in cryptocurrencies like Spacemint, miners can only start mining after ensuring that a commitment to their space is already added in a special transaction to the blockchain. Proofs of sequential work (PoSW) are proof systems in which a prover, upon receiving a statement x and a time parameter T, computes a proof which convinces the verifier that T time units had passed since x was received. Whereas Spacemint assumes synchrony to retain some interesting Bitcoin dynamics, Chia requires PoSW with unique proofs, i.e., PoSW in which it is hard to come up with more than one accepting proof for any true statement. In this thesis we construct simple and practically-efficient PoSpace and PoSW. When using our PoSpace in cryptocurrencies, miners can start mining on the fly, like in Bitcoin, and unlike current constructions of PoSW, which either achieve efficient verification of sequential work, or faster-than-recomputing verification of correctness of proofs, but not both at the same time, ours achieve the best of these two worlds." alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Hamza M full_name: Abusalah, Hamza M id: 40297222-F248-11E8-B48F-1D18A9856A87 last_name: Abusalah citation: ama: Abusalah HM. Proof systems for sustainable decentralized cryptocurrencies. 2018. doi:10.15479/AT:ISTA:TH_1046 apa: Abusalah, H. M. (2018). Proof systems for sustainable decentralized cryptocurrencies. Institute of Science and Technology Austria. https://doi.org/10.15479/AT:ISTA:TH_1046 chicago: Abusalah, Hamza M. “Proof Systems for Sustainable Decentralized Cryptocurrencies.” Institute of Science and Technology Austria, 2018. https://doi.org/10.15479/AT:ISTA:TH_1046. ieee: H. M. Abusalah, “Proof systems for sustainable decentralized cryptocurrencies,” Institute of Science and Technology Austria, 2018. ista: Abusalah HM. 2018. Proof systems for sustainable decentralized cryptocurrencies. Institute of Science and Technology Austria. mla: Abusalah, Hamza M. Proof Systems for Sustainable Decentralized Cryptocurrencies. Institute of Science and Technology Austria, 2018, doi:10.15479/AT:ISTA:TH_1046. short: H.M. Abusalah, Proof Systems for Sustainable Decentralized Cryptocurrencies, Institute of Science and Technology Austria, 2018. date_created: 2018-12-11T11:44:32Z date_published: 2018-09-05T00:00:00Z date_updated: 2023-09-07T12:30:23Z day: '05' ddc: - '004' degree_awarded: PhD department: - _id: KrPi doi: 10.15479/AT:ISTA:TH_1046 ec_funded: 1 file: - access_level: open_access checksum: c4b5f7d111755d1396787f41886fc674 content_type: application/pdf creator: dernst date_created: 2019-04-09T06:43:41Z date_updated: 2020-07-14T12:48:11Z file_id: '6245' file_name: 2018_Thesis_Abusalah.pdf file_size: 876241 relation: main_file - access_level: closed checksum: 0f382ac56b471c48fd907d63eb87dafe content_type: application/x-gzip creator: dernst date_created: 2019-04-09T06:43:41Z date_updated: 2020-07-14T12:48:11Z file_id: '6246' file_name: 2018_Thesis_Abusalah_source.tar.gz file_size: 2029190 relation: source_file file_date_updated: 2020-07-14T12:48:11Z has_accepted_license: '1' language: - iso: eng month: '09' oa: 1 oa_version: Published Version page: '59' project: - _id: 258C570E-B435-11E9-9278-68D0E5697425 call_identifier: FP7 grant_number: '259668' name: Provable Security for Physical Cryptography - _id: 258AA5B2-B435-11E9-9278-68D0E5697425 call_identifier: H2020 grant_number: '682815' name: Teaching Old Crypto New Tricks publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria publist_id: '7971' pubrep_id: '1046' related_material: record: - id: '1229' relation: part_of_dissertation status: public - id: '1235' relation: part_of_dissertation status: public - id: '1236' relation: part_of_dissertation status: public - id: '559' 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: Proof systems for sustainable decentralized cryptocurrencies type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2018' ...