On Sep 4, 2013, at 10:45 AM, Faré <fah...@gmail.com> wrote:
>>> Can't you trivially transform a hash into a PRNG, a PRNG into a
>>> cypher, and vice versa?
>> No.
>>
>
>> Let H(X) = SHA-512(X) || SHA-512(X)
>> where '||' is concatenation. Assuming SHA-512 is a cryptographically secure
>> hash H trivially is as well. (Nothing in the definition of a cryptographic
>> hash function says anything about minimality.) But H(X) is clearly not
>> useful for producing a PRNG.
>>
> Just because it's trivial to produce bogus crypto doesn't mean it's
> non-trivial to produce good crypto, given a few universal recipes.
Look, if you want to play around a produce things that look secure to you and a
few of your buddies - feel free to go ahead. If your system is only used by
you and a few friends, it's unlikely anyone with the appropriate skills will
ever care enough to attack your system, and you'll be "secure". As always,
"security" is mainly an *economic* question, not a purely technical one.
But if you want to play in the crypto game as it's actually played today - if
you want something that will survive even if you use it to protect information
that has significant value to someone willing to make the investment to get it
from you - well, you're going to have to up your game. You're playing at
1980's levels. The world has moved on - your opponents won't feel constrained
to do the same.
-- Jerry
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