Is this secure?

Lie Ryan lie.1296 at gmail.com
Wed Feb 24 04:41:26 CET 2010


On 02/24/10 14:09, Robert Kern wrote:
> On 2010-02-23 20:43 , Steven D'Aprano wrote:
>> On Wed, 24 Feb 2010 02:40:13 +0000, Steven D'Aprano wrote:
>>
>>> On Tue, 23 Feb 2010 15:36:02 +0100, mk wrote:
>>>
>>>> The question is: is this secure? That is, can the string generated this
>>>> way be considered truly random?
>>>
>>> Putting aside the philosophical question of what "truly random" means, I
>>> presume you mean that the letters are uniformly distributed. The answer
>>> to that is, they don't like uniformly distributed.
>>
>> Er, they don't *look* uniformly distributed.
>>
>> (Of course, being random, perhaps they are and I just got unlucky.)
> 
> You'd have to be very, *very* unlucky to get a sample of that size so
> far from uniformly distributed if the generating process actually were
> uniform.
> 
> Of course, uniformity isn't really necessary. You just need enough
> entropy in the distribution (amongst other things like protection of the
> seed from being known or guessed). A skewed distribution of characters
> is perfectly fine provided that you had enough characters in the
> password to meet the desired entropy requirement. A skewed distribution
> does require more characters to meet a specified entropy requirement
> than a uniform distribution, of course.
> 
> That said, for a naive strategy like "pick an independent random
> character, repeat", you should just use a uniform distribution. It makes
> the analysis easier. Worthwhile generators that give skewed
> distributions usually do so for a good reason, like generating
> pronounceable passwords.

If an attacker knows the that the random number generator have an
extreme skew and he knows the distribution of the letters, how much
advantage would it give the attacker? My initial guess is that the more
skewed the letters are, the better the advantage, since an attacker
using brute-force can write his program to prefer the most likely letters?



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