Security evaluation of Rakaposhi Stream Cipher

Orumiehchiha, Mohammad Ali, Pieprzyk, Josef, Shakour, Elham, & Steinfeld, Ron (2013) Security evaluation of Rakaposhi Stream Cipher. Lecture Notes in Computer Science : Information Security Practice and Experience, 7863, pp. 361-371.

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Rakaposhi is a synchronous stream cipher, which uses three main components: a non-linear feedback shift register (NLFSR), a dynamic linear feedback shift register (DLFSR) and a non-linear filtering function (NLF). NLFSR consists of 128 bits and is initialised by the secret key K. DLFSR holds 192 bits and is initialised by an initial vector (IV). NLF takes 8-bit inputs and returns a single output bit. The work identifies weaknesses and properties of the cipher. The main observation is that the initialisation procedure has the so-called sliding property. The property can be used to launch distinguishing and key recovery attacks. The distinguisher needs four observations of the related (K,IV) pairs. The key recovery algorithm allows to discover the secret key K after observing 29 pairs of (K,IV). Based on the proposed related-key attack, the number of related (K,IV) pairs is 2(128 + 192)/4 pairs.

Further the cipher is studied when the registers enter short cycles. When NLFSR is set to all ones, then the cipher degenerates to a linear feedback shift register with a non-linear filter. Consequently, the initial state (and Secret Key and IV) can be recovered with complexity 263.87.

If DLFSR is set to all zeros, then NLF reduces to a low non-linearity filter function. As the result, the cipher is insecure allowing the adversary to distinguish it from a random cipher after 217 observations of keystream bits. There is also the key recovery algorithm that allows to find the secret key with complexity 2 54.

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ID Code: 69686
Item Type: Journal Article
Refereed: Yes
Additional Information: 9th International Conference, ISPEC 2013, Lanzhou, China, May 12-14, 2013. Proceedings
Keywords: Rakaposhi Stream Cipher, Related Key Attack, Weak State, Cryptanalysis, Distinguishing Attack, Key Recovery Attack
DOI: 10.1007/978-3-642-38033-4_26
ISSN: 0302-9743
Divisions: Current > Schools > School of Electrical Engineering & Computer Science
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Copyright Owner: Copyright 2013 Springer-Verlag Berlin Heidelberg
Deposited On: 03 Apr 2014 02:00
Last Modified: 18 Jul 2014 01:07

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