git.distorted.org.uk Git - u/mdw/catacomb/blob - base/ct.c

   1 /* -*-c-*-
   2  *
   3  * Constant-time operations
   4  *
   5  * (c) 2013 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include <mLib/bits.h>
  31
  32 #include "ct.h"
  33
  34 /*----- Main code ---------------------------------------------------------*/
  35
  36 #define MASK(a) (U32(~((a) - 1)))
  37
  38 /* --- @ct_inteq@ --- *
  39  *
  40  * Arguments:   @uint32 x, y@ = two 32-bit unsigned integers
  41  *
  42  * Returns:     One if @x@ and @y@ are equal, zero if they differ.
  43  *
  44  * Use:         Answers whether two integers are equal, in constant time.
  45  */
  46
  47 int ct_inteq(uint32 x, uint32 y)
  48 {
  49   uint32 a = U32(~(x ^ y));
  50
  51   a &= a >> 16;
  52   a &= a >>  8;
  53   a &= a >>  4;
  54   a &= a >>  2;
  55   a &= a >>  1;
  56   return (a & 1);
  57 }
  58
  59 /* --- @ct_intle@ --- *
  60  *
  61  * Arguments:   @uint32 x, y@ = two 32-bit unsigned integers
  62  *
  63  * Returns:     One if %$x \le y$% are equal, zero if @x@ is greater.
  64  *
  65  * Use:         Answers whether two integers are ordered, in constant time.
  66  */
  67
  68 int ct_intle(uint32 x, uint32 y)
  69 {
  70   /* --- This bit's a little fiddly --- *
  71    *
  72    * If the top bits of @x@ and @y@ are the same then %$x \le y$% if and only
  73    * if %$y - x$% has its top bit clear; otherwise, %$x \le y$% if and only
  74    * if the top bit of @x@ is clear and the top bit of @y@ is set.
  75    *
  76    * This assumes we can do subtraction in constant time, which seems like a
  77    * safe enough bet.
  78    */
  79
  80   uint32 xx = x >> 31, yy = y >> 31, zz = (y - x) >> 31;
  81   return ((~xx&yy) | (~(xx^yy)&~zz)) & 1;
  82 }
  83
  84 /* --- @ct_pick@ --- *
  85  *
  86  * Arguments:   @uint32 a@ = a switch, either zero or one
  87  *              @uint32 x0, x1@ = two 32-bit unsigned integers
  88  *
  89  * Returns:     @x0@ if @a@ is zero; @x1@ if @a@ is one.  Other values of @a@
  90  *              will give you unhelpful results.
  91  *
  92  * Use:         Picks one of two results according to a switch variable, in
  93  *              constant time.
  94  */
  95
  96 int ct_pick(uint32 a, uint32 x0, uint32 x1)
  97   { uint32 m = MASK(a); return (x0&~m) | (x1&m); }
  98
  99 /* --- @ct_condcopy@ --- *
 100  *
 101  * Arguments:   @uint32 a@ = a switch, either zero or one
 102  *              @void *d@ = destination pointer
 103  *              @const void *s@ = source pointer
 104  *              @size_t n@ amount to copy
 105  *
 106  * Returns:     ---
 107  *
 108  * Use:         If @a@ is one then copy the @n@ bytes starting at @s@ to
 109  *              @d@; if @a@ is zero then leave @d@ unchanged (but it will
 110  *              still be written).  All of this is done in constant time.
 111  */
 112
 113 void ct_condcopy(uint32 a, void *d, const void *s, size_t n)
 114 {
 115   octet *dd = d;
 116   const octet *ss = s;
 117   uint32 m = MASK(a);
 118
 119   while (n--) {
 120     *dd = (*ss++&m) | (*dd&~m);
 121     dd++;
 122   }
 123 }
 124
 125 /* --- @ct_memeq@ ---
 126  *
 127  * Arguments:   @const void *p, *q@ = two pointers to buffers
 128  *              @size_t n@ = the (common) size of the buffers
 129  *
 130  * Returns:     One if the two buffers are equal, zero if they aren't.
 131  *
 132  * Use:         Compares two chunks of memory, in constant time.
 133  */
 134
 135 int ct_memeq(const void *p, const void *q, size_t n)
 136 {
 137   const octet *pp = p, *qq = q;
 138   octet a = 0;
 139
 140   while (n--) a |= *pp++ ^ *qq++;
 141   return (ct_inteq(a, 0));
 142 }
 143
 144 /*----- That's all, folks -------------------------------------------------*/