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Paper Abstract and Keywords
Presentation 2026-03-17 10:50
Revisitied Distinguishers for Stream Cipher ChaCha
Yurie Okada, Atsuko Miyaji (UOsaka) IT2025-124 ISEC2025-132 WBS2025-106 RCC2025-105
Abstract (in Japanese) (See Japanese page) 
(in English) ChaCha is a stream cipher standardized by the IETF and is the only stream cipher adopted in TLS 1.3.
At ASIACRYPT 2022, Coutinho et al. derived a 7-round distinguisher with complexity $2^{214}$, and at FSE 2024, Bellini et al. derived a 7-round distinguisher with complexity $2^{189.9}$. However, since these complexities exceed the adversary’s practical data limit of $2^{128}$, the best practical distinguisher to date remains the 6-round distinguisher with complexity $2^{51}$ proposed by Coutinho et al. at EUROCRYPT 2021.
In this work, by applying the two-addition carry formula proposed by Okada–Nagai–Miyaji at Indocrypt 2025, we introduce new linear approximations covering 3–6.5 rounds and 3.5–6.5 rounds. As a result, we construct 6.5-round distinguishers with complexities $2^{109.75}$ and $2^{105.82}$, respectively. Furthermore, for the latter case, by applying the Differential Linear Hull technique, which considers multiple differential paths, we further reduce the complexity and obtain a 6.5-round distinguisher with complexity $2^{105.64}$.
To the best of the authors’ knowledge, this $2^{105.64}$ complexity attack on 6.5 rounds is currently the most efficient distinguisher against ChaCha.
Keyword (in Japanese) (See Japanese page) 
(in English) Differential Linear Cryptanalysis / Distinguisher / ChaCha / / / / /  
Reference Info. IEICE Tech. Rep., vol. 125, no. 405, ISEC2025-132, pp. 290-297, March 2026.
Paper # ISEC2025-132 
Date of Issue 2026-03-09 (IT, ISEC, WBS, RCC) 
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 IT2025-124 ISEC2025-132 WBS2025-106 RCC2025-105

Conference Information
Committee IT WBS ISEC RCC  
Conference Date 2026-03-16 - 2026-03-17 
Place (in Japanese) (See Japanese page) 
Place (in English) Nagano Campus, Shinshu University 
Topics (in Japanese) (See Japanese page) 
Topics (in English) Joint Workshop of ISEC, IT, RCC, and WBS 
Paper Information
Registration To ISEC 
Conference Code 2026-03-IT-WBS-ISEC-RCC 
Language Japanese 
Title (in Japanese) (See Japanese page) 
Sub Title (in Japanese) (See Japanese page) 
Title (in English) Revisitied Distinguishers for Stream Cipher ChaCha 
Sub Title (in English)  
Keyword(1) Differential Linear Cryptanalysis  
Keyword(2) Distinguisher  
Keyword(3) ChaCha  
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1st Author's Name Yurie Okada  
1st Author's Affiliation University of Osaka (UOsaka)
2nd Author's Name Atsuko Miyaji  
2nd Author's Affiliation University of Osaka (UOsaka)
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Speaker Author-1 
Date Time 2026-03-17 10:50:00 
Presentation Time 25 minutes 
Registration for ISEC 
Paper # IT2025-124, ISEC2025-132, WBS2025-106, RCC2025-105 
Volume (vol) vol.125 
Number (no) no.404(IT), no.405(ISEC), no.406(WBS), no.407(RCC) 
Page pp.290-297 
#Pages 8 
Date of Issue 2026-03-09 (IT, ISEC, WBS, RCC) 

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