Equivalent Keys in RC6-32/20/176

Hiroshi MIZUNO
Hatsukazu TANAKA

IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E84-A    No.10    pp.2474-2481
Publication Date: 2001/10/01
Online ISSN: 
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Information Theory and Its Applications)
Category: Information Security
cryptanalysis,  RC6,  key schedule,  equivalent key,  Davies-Meyer hash function,  

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RC6 is a common-key block cipher that was proposed as one of the AES candidates. Although any weakness of RC6 in the use of the confidentiality is not known, Saarinen pointed out the existence of almost equivalent keys in RC6 with 176-byte keys. This means that the Davies-Meyer hash function based on RC6 with 176-byte keys is not a good collision-resistance function. However, Saarinen could not find a precise collision of it. In this paper, we propose a practical method for obtaining a collision of the Davies-Meyer hash function based on RC6-32/20/176. In other words, there exist equivalent user supplied keys in RC6-32/20/176, and it is possible to obtain them practically. This means that the essential key space of RC6-32/20/176 is smaller than the space provided by 176-byte keys. Our computer simulation shows that a collision can be found in about 100 minutes. We should notice that the result of this paper does not affect the security of the AES version of RC6 because RC6-32/20/176 discussed in this paper is different from the parameter of the AES version.