Commit 5e4b9c6b authored by Nick Mathewson's avatar Nick Mathewson 🦀
Browse files

Remove minor biasing problem from crypto_pseudo_rand_int 


svn:r799
parent 7e4cb9a7

src/common/crypto.c

+15 −7
Original line number Diff line number Diff line
@@ -16,6 +16,7 @@ 
#include <stdlib.h>

#include <assert.h>

#include <stdio.h>

#include <limits.h>



#include "crypto.h"

#include "../or/or.h"
@@ -1008,15 +1009,22 @@ void crypto_pseudo_rand(unsigned int n, unsigned char *to) 
  }

}



int crypto_pseudo_rand_int(int max) {

int crypto_pseudo_rand_int(unsigned int max) {

  unsigned int val;

  unsigned int cutoff;

  assert(max < UINT_MAX);



  /* We ignore any values that are >= 'cutoff,' to avoid biasing the

   * distribution with clipping at the upper end of unsigned int's

   * range.

   */

  cutoff = UINT_MAX - (UINT_MAX%max);

  while(1) {

    crypto_pseudo_rand(sizeof(val), (unsigned char*) &val);

  /* Bug: Low values are _slightly_ favored over high values because

   * ((unsigned)-1)%max != max-1 .  This shouldn't matter if max is

   * significantly smaller than ((unsigned)-1).

   **/

    if (val < cutoff)

      return val % max;

  }

}



/* errors */

char *crypto_perror()

src/common/crypto.h

+1 −1
Original line number Diff line number Diff line
@@ -101,7 +101,7 @@ int crypto_SHA_digest(unsigned char *m, int len, unsigned char *digest); 
int crypto_seed_rng();

int crypto_rand(unsigned int n, unsigned char *to);

void crypto_pseudo_rand(unsigned int n, unsigned char *to);

int crypto_pseudo_rand_int(int max);

int crypto_pseudo_rand_int(unsigned int max);



/* errors */

char *crypto_perror();