[Resource Topic] 2002/005: Cryptanalysis of Stream Cipher COS (2,128) Mode I

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

Authors: Hongjun Wu, Feng Bao


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).

ePrint: https://eprint.iacr.org/2002/005

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