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Instead of sharing raw data, the federated learning process cooperatively exchanges the model parameters and aggregates them in a decentralized manner through multiple users. In this study, we designed and implemented a hierarchical blockchain system using a public blockchain for a federated learning process without a trusted curator. This prevents model-poisoning attacks and provides secure updates of a global model. We conducted a comprehensive empirical study to characterize the performance of federated learning in our testbed and identify potential performance bottlenecks, thereby gaining a better understanding of the system.<\/jats:p>","DOI":"10.3390\/s22218263","type":"journal-article","created":{"date-parts":[[2022,10,30]],"date-time":"2022-10-30T10:47:57Z","timestamp":1667126877000},"page":"8263","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["DAG-Based Blockchain Sharding for Secure Federated Learning with Non-IID Data"],"prefix":"10.3390","volume":"22","author":[{"given":"Jungjae","family":"Lee","sequence":"first","affiliation":[{"name":"Computer Engineering Department, Gachon University, Seongnam 1342, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0955-3421","authenticated-orcid":false,"given":"Wooseong","family":"Kim","sequence":"additional","affiliation":[{"name":"Computer Engineering Department, Gachon University, Seongnam 1342, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2022,10,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"11","DOI":"10.1016\/j.neucom.2016.12.038","article-title":"A survey of deep neural network architectures and their applications","volume":"234","author":"Liu","year":"2017","journal-title":"Neurocomputing"},{"key":"ref_2","first-page":"1","article-title":"A survey on deep learning: Algorithms, techniques, and applications","volume":"51","author":"Pouyanfar","year":"2018","journal-title":"ACM Comput. 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