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In the Random Oracle Model (ROM) all parties have oracle access to a common random function, and the parties are limited in the number of queries they can make to the oracle. The Merkle’s Puzzles protocol, introduced by Merkle [CACM '78], is a key-agreement protocol in the ROM with a quadratic gap between the query complexity of the honest parties and the eavesdropper. This quadratic gap is known to be optimal, by the works of Impagliazzo and Rudich [STOC ’89] and Barak and Mahmoody [Crypto ’09]. When the oracle function is injective or a permutation, Merkle’s Puzzles has perfect completeness. That is, it is certain that the protocol results in agreement between the parties. However, without such an assumption on the random function, there is a small error probability, and the parties may end up holding different keys. This fact raises the question: Is there a key-agreement protocol with perfect completeness and super-linear security in the ROM? In this paper we give a positive answer to the above question, showing that changes to the query distribution of the parties in Merkle’s Puzzles, yield a protocol with perfect completeness and roughly the same security.
@InProceedings{mazor:LIPIcs.ITC.2025.12,
author = {Mazor, Noam},
title = {{Key-Agreement with Perfect Completeness from Random Oracles}},
booktitle = {6th Conference on Information-Theoretic Cryptography (ITC 2025)},
pages = {12:1--12:11},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-385-0},
ISSN = {1868-8969},
year = {2025},
volume = {343},
editor = {Gilboa, Niv},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2025.12},
URN = {urn:nbn:de:0030-drops-243628},
doi = {10.4230/LIPIcs.ITC.2025.12},
annote = {Keywords: Key-Agreement, Random Oracle, Merkle’s Puzzles, Perfect Completeness}
}