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Date: Thu Jun 30 16:05:22 2005
From: anthrax101 at gmail.com (Aaron Horst)
Subject: Random number prediction
This is an interesting method of reducing the keyspace of attack, but
rand() is still a linear congruent PRNG. It should never be used where
cryptographically secure pseudo-random numbers are needed. I would
suggest using Blum Blum Shub or some method based on an existing
cipher in counter mode.
AnthraX101
On 6/30/05, Gabriele Avosani <avosani.gabriele@...ero.it> wrote:
>
> Hello there folks,
> take a look at this source, its called sidis.c
> /*****************************************************************************************************/
> /* Sidis
>
> */
> /* CRT rand() function random number predictor
> */
> /*
>
> */
> /* works with every windows and unix random generator
> */
> /*
>
> */
> /* this is a full service random number predictor
> */
> /* the function of CRT rand is the following
> */
> /* int rand (void)
> */
> /* {
>
> */
> /* _ptiddata ptd = _getptd();
> */
> /* return( ((ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L) >>
> 16) & 0x7fff ); */
> /* }
>
> */
> /* As you can see the seed starts to set the variable that is returned from
> the pointer */
> /* then we have a little mathematic and we get 15 bits
> */
> /* ......
>
> */
> /* but, from what i discovered, you can read the explaination in sci.math, i
> have made a post there, */
> /* we can do this operation
> */
> /* x1 = (seed * y + z)
> */
> /* x2 = (x1 * y + z)
> */
> /* x3 = (x2 * y + z)
> */
> /* the random numbers are: x1 >> 15, x2 >> 15, x3 >> 15
> */
> /* now .... if we apply a little phormula, i can assure you that:
> */
> /* (x2 >> 15 - z >> 15) * y >> 15 gives us x1 >> 15
> */
> /* now we can easily brute force the remaining 17 bits
> */
> /*
>
> */
> /* Zuc
>
> */
> /*
>
> */
> /*****************************************************************************************************/
>
> #include <stdlib.h>
> #include <stdio.h>
>
> #define rand1 666
> #define rand2 32767
>
> unsigned long next=-1,w;
>
> void main(void)
> {
> unsigned long x = (((rand2 >> 15) - (2531011 >> 15) >> 15) * 214013 >> 15)
> >> 15;
> for(long y=0;y<2^17;y++)
> {
> w = y * 2^17 + x;
> next = (w * 214013 + 2531011) >> 32;
> if( (( next * 214013 + 2531011)>> 15 ) == rand1 )break;
> }
> if(next==-1)
> {
> printf("Sorry, number not found.\n");
> return;
> }
> for(unsigned short i=0;i<10;i++)
> {
> printf("Next number n.%i ---
> %i\n",i,(next*214013+2531011)>>15);
> next=next*214013+2531011;
> }
> printf("Sic transit gloria mundi.\n");
> }
>
> Zuc
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--
AnthraX101 -- PGP Key ID# 0x4CD6D0BD
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