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Paper Abstract and Keywords
Presentation 2025-03-06 16:00
Bridging Theory and Experiment in ChaCha Linear Approximation Analysis: A More Accurate Bias Estimation
Atsuki Nagai, Atsuko Miyaji (Osaka Univ.) ICSS2024-90
Abstract (in Japanese) (See Japanese page) 
(in English) ChaCha is a symmetric-key cipher and an encryption communication protocol standardized by the IETF. It is also the only stream cipher adopted in TLS 1.3. In the differential-linear cryptanalysis of ChaCha, Coutinho et al. reported that a distinguisher attack based on 7 rounds is possible with a computational complexity of 2^(214). The theoretical bias of the linear approximation used to derive the aforementioned 7-round result is 2^(−53), which is derived from four partial linear approximations I–IV. It has been reported that the theoretical values of the two partial linear approximations, II and III, are significantly lower than the experimental values. In this study, we rigorously reexamined these partial linear approximations, II and III, and constructed more accurate theoretical linear approximations. With the improvement of the theoretical values of the partial linear approximations, the theoretical bias of the linear approximation used in the above 7-round distinguisher attack
was refined to 5^2 · 3^(−5) · 2^(−46).
Keyword (in Japanese) (See Japanese page) 
(in English) stream cipher / ChaCha / linear approximations / distinguisher attack / / / /  
Reference Info. IEICE Tech. Rep., vol. 124, no. 422, ICSS2024-90, pp. 162-169, March 2025.
Paper # ICSS2024-90 
Date of Issue 2025-02-27 (ICSS) 
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 ICSS2024-90

Conference Information
Committee ICSS IPSJ-SPT  
Conference Date 2025-03-06 - 2025-03-07 
Place (in Japanese) (See Japanese page) 
Place (in English) Okinawa Prefectural Museum & Art Museum 
Topics (in Japanese) (See Japanese page) 
Topics (in English) Security, Trust, etc. 
Paper Information
Registration To ICSS 
Conference Code 2025-03-ICSS-SPT 
Language Japanese 
Title (in Japanese) (See Japanese page) 
Sub Title (in Japanese) (See Japanese page) 
Title (in English) Bridging Theory and Experiment in ChaCha Linear Approximation Analysis: A More Accurate Bias Estimation 
Sub Title (in English)  
Keyword(1) stream cipher  
Keyword(2) ChaCha  
Keyword(3) linear approximations  
Keyword(4) distinguisher attack  
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1st Author's Name Atsuki Nagai  
1st Author's Affiliation Osaka University (Osaka Univ.)
2nd Author's Name Atsuko Miyaji  
2nd Author's Affiliation Osaka University (Osaka Univ.)
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Speaker Author-1 
Date Time 2025-03-06 16:00:00 
Presentation Time 20 minutes 
Registration for ICSS 
Paper # ICSS2024-90 
Volume (vol) vol.124 
Number (no) no.422 
Page pp.162-169 
#Pages 8 
Date of Issue 2025-02-27 (ICSS) 

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