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http://en.wikipedia.org/wiki/ISAAC_(cipher)

This question was asked before but the answers seem vague, and I want to know about ISAAC specifically, not ISAAC+.

It seems some cryptanalysis was performed on it and the only seemingly major drawback to security is that some initial rounds may be biased (although on the official website, looking at the implementations the author of ISAAC purposefully skips the first round).

Also, is there anything unique to seeding this cipher? Can I not just pass it any securely generate random seed?

forest
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user3100783
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1 Answers1

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It somewhat depends how you define “secure“, but as far as I know, it has not been broken yet.

Potential attacks and/or weaknesses:

  • Brute force to find 256-bit key.
    ~ Obvious.
  • Paul and Preneel (2006) published an alleged distinguishing attack using $2^{17}$ bytes of output
    ~ Turned out to be an attack on a cipher different from ISAAC.
  • Aumasson (2006) labelled various states of ISAAC as "weak."
    ~ Pointing to weaknesses, but no statements of any attack.

Therefore, I would categorize it to be cryptographically secure indeed. Yet, we have to remember that ISAAC didn’t get as much attention as (for example) AES did. So there may well be weaknesses that have not yet been found and/or published. But looking at what is known, it should be safe. (ISAAC+ would be more recommendable though, as it fixes some weaknesses pointed out by Aumasson).

As to seeding: a securely generated random seed should indeed suffice, as long as it is generated cryptographically secure.

Mike Edward Moras
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