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Paper Abstract and Keywords
Presentation 2026-05-26 10:50
[Invited Talk] Key Recovery Attacks on Unpatched MEGA from Four Queries: Solving Approximate Divisor Problem with Help of Approximation of Squared Divisor (from ACNS 2025)
Naoki Shimoe, Noboru Kunihiro (Tsukuba Univ.) ISEC2026-3
Abstract (in Japanese) (See Japanese page) 
(in English) MEGA is a cloud storage platform that supports end-to-end encryption and aims to be designed to guarantee data confidentiality and integrity even against hostile servers. However, Backendal et al. (IEEE S&P 2023) and Heninger and Ryan (PKC2023) showed that a server can exploit the session ID exchange to recover the user's RSA private key with 512 and 6 login attempts, respectively. Furthermore, Albrecht et al. (Eurocrypt2023) constructed an ECB encryption oracle from MEGAdrop and reduced the number of required login attempts to two. To evaluate the security of the session ID exchange protocol alone in more detail, we evaluate the minimum number of login attempts required to recover the user's RSA private key without an ECB encryption oracle. Based on Heninger and Ryan's attack, we propose an attack that can recover the user's RSA private key with four login attempts without the ECB encryption oracle. To reduce the number of required login attempts, we introduce a problem of recovering an unknown divisor of a composite number given its approximation and an approximate of its square and propose an algorithm that solves this problem by refining its approximation.
Keyword (in Japanese) (See Japanese page) 
(in English) Cryptanalysis / E2EE / MEGA / Approximate divisor problem / RSA / / /  
Reference Info. IEICE Tech. Rep., vol. 126, no. 40, ISEC2026-3, pp. 3-3, May 2026.
Paper # ISEC2026-3 
Date of Issue 2026-05-19 (ISEC) 
ISSN Online edition: ISSN 2432-6380
Copyright
and
reproduction		 All rights are reserved and no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Notwithstanding, instructors are permitted to photocopy isolated articles for noncommercial classroom use without fee. (License No.: 10GA0019/12GB0052/13GB0056/17GB0034/18GB0034)
Download PDF ISEC2026-3

Conference Information
Committee ISEC  
Conference Date 2026-05-26 - 2026-05-26 
Place (in Japanese) (See Japanese page) 
Place (in English) Kikai-Shinko-Kaikan Bldg. 
Topics (in Japanese) (See Japanese page) 
Topics (in English)  
Paper Information
Registration To ISEC 
Conference Code 2026-05-ISEC 
Language Japanese 
Title (in Japanese) (See Japanese page) 
Sub Title (in Japanese) (See Japanese page) 
Title (in English) Key Recovery Attacks on Unpatched MEGA from Four Queries: Solving Approximate Divisor Problem with Help of Approximation of Squared Divisor (from ACNS 2025) 
Sub Title (in English)  
Keyword(1) Cryptanalysis  
Keyword(2) E2EE  
Keyword(3) MEGA  
Keyword(4) Approximate divisor problem  
Keyword(5) RSA  
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Keyword(8)  
1st Author's Name Naoki Shimoe  
1st Author's Affiliation University of Tsukuba (Tsukuba Univ.)
2nd Author's Name Noboru Kunihiro  
2nd Author's Affiliation University of Tsukuba (Tsukuba Univ.)
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Speaker Author-1 
Date Time 2026-05-26 10:50:00 
Presentation Time 25 minutes 
Registration for ISEC 
Paper # ISEC2026-3 
Volume (vol) vol.126 
Number (no) no.40 
Page p.3 
#Pages 1 
Date of Issue 2026-05-19 (ISEC) 

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