crypto/constant_time_locl.h

   1 /* crypto/constant_time_locl.h */
   2 /*
   3  * Utilities for constant-time cryptography.
   4  *
   5  * Author: Emilia Kasper (emilia@openssl.org)
   6  * Based on previous work by Bodo Moeller, Emilia Kasper, Adam Langley
   7  * (Google).
   8  * ====================================================================
   9  * Copyright (c) 2014 The OpenSSL Project.  All rights reserved.
  10  *
  11  * Redistribution and use in source and binary forms, with or without
  12  * modification, are permitted provided that the following conditions
  13  * are met:
  14  * 1. Redistributions of source code must retain the copyright
  15  *    notice, this list of conditions and the following disclaimer.
  16  * 2. Redistributions in binary form must reproduce the above copyright
  17  *    notice, this list of conditions and the following disclaimer in the
  18  *    documentation and/or other materials provided with the distribution.
  19  * 3. All advertising materials mentioning features or use of this software
  20  *    must display the following acknowledgement:
  21  *    "This product includes cryptographic software written by
  22  *     Eric Young (eay@cryptsoft.com)"
  23  *    The word 'cryptographic' can be left out if the rouines from the library
  24  *    being used are not cryptographic related :-).
  25  * 4. If you include any Windows specific code (or a derivative thereof) from
  26  *    the apps directory (application code) you must include an acknowledgement:
  27  *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
  28  *
  29  * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
  30  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  31  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  32  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  33  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  34  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  35  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  36  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  37  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  38  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  39  * SUCH DAMAGE.
  40  *
  41  * The licence and distribution terms for any publically available version or
  42  * derivative of this code cannot be changed.  i.e. this code cannot simply be
  43  * copied and put under another distribution licence
  44  * [including the GNU Public Licence.]
  45  */
  46
  47 #ifndef HEADER_CONSTANT_TIME_LOCL_H
  48 #define HEADER_CONSTANT_TIME_LOCL_H
  49
  50 #include "e_os.h"  /* For 'inline' */
  51
  52 #ifdef __cplusplus
  53 extern "C" {
  54 #endif
  55
  56 /*
  57  * The following methods return a bitmask of all ones (0xff...f) for true
  58  * and 0 for false. This is useful for choosing a value based on the result
  59  * of a conditional in constant time. For example,
  60  *
  61  * if (a < b) {
  62  *   c = a;
  63  * } else {
  64  *   c = b;
  65  * }
  66  *
  67  * can be written as
  68  *
  69  * unsigned int lt = constant_time_lt(a, b);
  70  * c = a & lt | b & ~lt;
  71  */
  72
  73 /*
  74  * Returns the given value with the MSB copied to all the other
  75  * bits. Uses the fact that arithmetic shift shifts-in the sign bit.
  76  * However, this is not ensured by the C standard so you may need to
  77  * replace this with something else on odd CPUs.
  78  */
  79 static inline unsigned int constant_time_msb(unsigned int a);
  80
  81 /*
  82  * Returns 0xff..f if a < b and 0 otherwise.
  83  */
  84 inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
  85 /* Convenience method for getting an 8-bit mask. */
  86 inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b);
  87
  88 /*
  89  * Returns 0xff..f if a >= b and 0 otherwise.
  90  */
  91 inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
  92 /* Convenience method for getting an 8-bit mask. */
  93 inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b);
  94
  95 /*
  96  * Returns 0xff..f if a == 0 and 0 otherwise.
  97  */
  98 inline unsigned int constant_time_is_zero(unsigned int a);
  99 /* Convenience method for getting an 8-bit mask. */
 100 inline unsigned char constant_time_is_zero_8(unsigned int a);
 101
 102
 103 /*
 104  * Returns 0xff..f if a == b and 0 otherwise.
 105  */
 106 inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
 107 /* Convenience method for getting an 8-bit mask. */
 108 inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);
 109
 110 static inline unsigned int constant_time_msb(unsigned int a)
 111         {
 112         return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
 113         }
 114
 115 inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
 116         {
 117         unsigned int lt;
 118         /* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/
 119         lt = ~(a ^ b) & (a - b);
 120         /* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */
 121         lt |= ~a & b;
 122         return constant_time_msb(lt);
 123         }
 124
 125 inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
 126         {
 127         return (unsigned char)(constant_time_lt(a, b));
 128         }
 129
 130 inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
 131         {
 132         unsigned int ge;
 133         /* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/
 134         ge = ~((a ^ b) | (a - b));
 135         /* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */
 136         ge |= a & ~b;
 137         return constant_time_msb(ge);
 138         }
 139
 140 inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
 141         {
 142         return (unsigned char)(constant_time_ge(a, b));
 143         }
 144
 145 inline unsigned int constant_time_is_zero(unsigned int a)
 146         {
 147         return constant_time_msb(~a & (a - 1));
 148         }
 149
 150 inline unsigned char constant_time_is_zero_8(unsigned int a)
 151         {
 152         return (unsigned char)(constant_time_is_zero(a));
 153         }
 154
 155 inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
 156         {
 157         return constant_time_is_zero(a ^ b);
 158         }
 159
 160 inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
 161         {
 162         return (unsigned char)(constant_time_eq(a, b));
 163         }
 164
 165 #ifdef __cplusplus
 166 }
 167 #endif
 168
 169 #endif  /* HEADER_CONSTANT_TIME_LOCL_H */