Mir-1 is a stream cipher proposed for Profile 1 at the ECRYPT Stream Cipher Project (eSTREAM). The Mir-1 designer claims a security level of at least 2{sup}128, meaning that the secret key cannot be recovered or that the Mir-1 output sequence cannot be distinguished from a truly random number sequence more efficiently than an exhaustive search. At SASC 2006, however, a distinguishing attack on Mir-1 was proposed making use of vulnerabilities in Mir-1 initialization. This paper shows that unknown entries in the key-dependent S-box used by Mir-1 can be classified into partially equivalent pairs by extending the SASC 2006 technique. It also demonstrates an attack that applies that information to recovering the Mir-1 secret key more efficiently than an exhaustive search. To the best of the authors' knowledge, the results described in this paper represent the first successful key recovery attack on Mir-1.
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