Re: streql - Constant-time string comparison

[Joel Rees <joel.rees@gmail.com>]

Here's the result of my work to this point:

---------------------------
/* Near-constant run time string/memory compare, with test frame.
** by Joel Rees,
** derived from work by Peter Scott, Riley Baird, et. al., see
** https://lists.debian.org/debian-security/2014/10/msg00060.html
** https://github.com/PeterScott/streql
**
** Use allowed under GPL v. 3, see
** http://www.gnu.org/copyleft/gpl.html
*/

#include <string.h>
#include <stdio.h>
#include <stdlib.h>


/* dbg */ static int gcount = 0;

// The core function: test two regions of memory for bytewise equality
with constant time.
// If cmplength is less than min( xlen, ylen ), comparison is incomplete.
// Lengths should be signed to make conditionals more balanced.
static int equals_internal_constime(
const char *x, int xlen,
const char *y, int ylen,
int cmplength) {

  int result = 0;

  while ( --cmplength >= 0 ) {
    char xtemp;
    char ytemp;

    /* Using unsigned lengths here would induce unbalanced conditionals:
    ** unsigned int xlen;
    ** if ( xlen > 0 ) {
    **   xtemp = *x++;
    **   --xlen;
    ** }
    ** else {
    **   xtemp = 0;
    ** }
    ** While this might be a problem with 16 bit ints,
    ** you really aren't going to be comparing strings > 2^31 bytes
with this function.
    */
    xtemp = ( --xlen >= 0 ) ? *x++ : 0;
    ytemp = ( --ylen >= 0 ) ? *y++ : 0;

/* dbg */    ++gcount;
    result |= xtemp ^ ytemp;
/* dbg  printf( "%c(%d):%c(%d) =>%d@%d\n", xtemp, xlen, ytemp, ylen,
result, gcount ); */
  }

  return (xlen == ylen) && (result == 0);
}


int main( int argc, char * argv[] ) {

    char * left, * right;
    int max = 32;
    int result = 0;

    if ( argc > 2 ) {
      left = argv[ 1 ];
      right = argv[ 2 ];
      if ( argc > 3 ) max = strtol( argv[ 3 ], NULL, 0 );
      gcount = 0;
      result = equals_internal_constime( left, strlen( left ), right,
strlen( right ), max );
      printf( "result: %d, strcmp: %d, count: %d\n", result, strcmp(
left, right ), gcount );
      if ( result != ( strcmp( left, right ) == 0 ) ) {
        fputs( "*** failed ***\n", stdout );
      }
    }


    return EXIT_SUCCESS;
}
---------------------------

Note the change to signed lengths and the reasoning.

--
Joel Rees