Information Security and Privacy

Volume 2384 of the series Lecture Notes in Computer Science pp 154-158


Cryptanalysis of Stream Cipher COS (2, 128) Mode I

  • Hongjun WuAffiliated withLaboratories for Information Technology
  • , Feng BaoAffiliated withLaboratories for Information Technology

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Filiol and Fontaine recently proposed a family of stream ciphers named COS. COS is based on nonlinear feedback shift registers and was claimed to be highly secure. Babbage showed that COS (2, 128) Mode II is extremely weak. But Babbage’s attack is very expensive to break the COS (2, 128) Mode I (the complexity is around 252). In this paper, we show that the COS (2, 128) Mode I is very weak. Secret information could be recovered easily with about 216-bit known plaintext.