Cryptology ePrint Archive: Report 2019/289

Cryptanalysis of ForkAES

Subhadeep Banik and Jannis Bossert and Amit Jana and Eik List and Stefan Lucks and Willi Meier and Mostafizar Rahman and Dhiman Saha and Yu Sasaki

Abstract: Forkciphers are a new kind of primitive proposed recently by Andreeva et al. for efficient encryption and authentication of small messages. They fork the middle state of a cipher and encrypt it twice under two smaller independent permutations. Thus, forkciphers produce two output blocks in one primitive call.

Andreeva et al. proposed ForkAES, a tweakable AES-based forkcipher that splits the state after five out of ten rounds. While their authenticated encrypted schemes were accompanied by proofs, the security discussion for ForkAES was not provided, and founded on existing results on the AES and KIASU-BC. Forkciphers provide a unique interface called reconstruction queries that use one ciphertext block as input and compute the respective other ciphertext block. Thus, they deserve a careful security analysis.

This work fosters the understanding of the security of ForkAES with three contributions: (1) We observe that security in reconstruction queries differs strongly from the existing results on the AES. This allows to attack nine out of ten rounds with differential, impossible-differential and yoyo attacks. (2) We observe that some forkcipher modes may lack the interface of reconstruction queries, so that attackers must use encryption queries. We show that nine rounds can still be attacked with rectangle and impossible-differential attacks. (3) We present forgery attacks on the AE modes proposed by Andreeva et al. with nine-round ForkAES.

Category / Keywords: secret-key cryptography / Symmetric-key cryptography, cryptanalysis, tweakable block cipher, impossible differential, boomerang, yoyo and AE

Original Publication (with major differences): ACNS 2019

Date: received 13 Mar 2019, last revised 18 Apr 2019

Contact author: sasaki yu at lab ntt co jp

Available format(s): PDF | BibTeX Citation

Version: 20190418:184354 (All versions of this report)

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