International Association for Cryptologic Research

International Association
for Cryptologic Research


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

Hongjun Wu
Feng Bao
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Abstract: 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 with high cryptographic strength. Babbage showed that COS $(2,128)$ Mode II is extremely weak. But Babbage's attack is too expensive to break the COS $(2,128)$ Mode I (the complexity is around $2^{52}$). In this paper, we show that the COS $(2,128)$ Mode I is too weak. With about $2^{16}$-bit known plaintext, the secret information could be recovered with small amount of memory and computation time (less than one second on a Pentium IV Processor).
  title={Cryptanalysis of Stream Cipher COS (2,128) Mode I},
  booktitle={IACR Eprint archive},
  keywords={secret-key cryptography / stream ciphers, cryptanalysis, COS},
  note={ 11691 received 3 Jan 2002},
  author={Hongjun Wu and Feng Bao},