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Cryptography

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  • $\begingroup$ Thanks. This explains why the given stream cipher is not perfectly secure. My question was: why it is not computationally secure against brute force. $\endgroup$ Commented Dec 23, 2014 at 23:43
  • $\begingroup$ Roughly speaking, if a cipher is computationally secure, it cannot be broken much more efficiently than via bruteforce. Computational security however allows being vulnerable to brute-force, but it should take very very much time. Note that correct stream ciphers (i.e. you have to use CSPRNG, not just any PRNG) are believed to be computationally secure if used properly. The only problem for computational security seems to be not mentioning some conditions (e.g. needing CSPRNG and not reusing the $k$.). $\endgroup$ Commented Dec 24, 2014 at 9:05