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 boolean 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 = constant_time_select(lt, a, b);
  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 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
  85 /* Convenience method for getting an 8-bit mask. */
  86 static inline unsigned char constant_time_lt_8(unsigned int a,
  87                                                unsigned int b);
  88
  89 /*
  90  * Returns 0xff..f if a >= b and 0 otherwise.
  91  */
  92 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
  93 /* Convenience method for getting an 8-bit mask. */
  94 static inline unsigned char constant_time_ge_8(unsigned int a,
  95                                                unsigned int b);
  96
  97 /*
  98  * Returns 0xff..f if a == 0 and 0 otherwise.
  99  */
 100 static inline unsigned int constant_time_is_zero(unsigned int a);
 101 /* Convenience method for getting an 8-bit mask. */
 102 static inline unsigned char constant_time_is_zero_8(unsigned int a);
 103
 104 /*
 105  * Returns 0xff..f if a == b and 0 otherwise.
 106  */
 107 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
 108 /* Convenience method for getting an 8-bit mask. */
 109 static inline unsigned char constant_time_eq_8(unsigned int a,
 110                                                unsigned int b);
 111 /* Signed integers. */
 112 static inline unsigned int constant_time_eq_int(int a, int b);
 113 /* Convenience method for getting an 8-bit mask. */
 114 static inline unsigned char constant_time_eq_int_8(int a, int b);
 115
 116 /*-
 117  * Returns (mask & a) | (~mask & b).
 118  *
 119  * When |mask| is all 1s or all 0s (as returned by the methods above),
 120  * the select methods return either |a| (if |mask| is nonzero) or |b|
 121  * (if |mask| is zero).
 122  */
 123 static inline unsigned int constant_time_select(unsigned int mask,
 124                                                 unsigned int a,
 125                                                 unsigned int b);
 126 /* Convenience method for unsigned chars. */
 127 static inline unsigned char constant_time_select_8(unsigned char mask,
 128                                                    unsigned char a,
 129                                                    unsigned char b);
 130 /* Convenience method for signed integers. */
 131 static inline int constant_time_select_int(unsigned int mask, int a, int b);
 132
 133 static inline unsigned int constant_time_msb(unsigned int a)
 134 {
 135     return 0 - (a >> (sizeof(a) * 8 - 1));
 136 }
 137
 138 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
 139 {
 140     return constant_time_msb(a ^ ((a ^ b) | ((a - b) ^ b)));
 141 }
 142
 143 static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
 144 {
 145     return (unsigned char)(constant_time_lt(a, b));
 146 }
 147
 148 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
 149 {
 150     return ~constant_time_lt(a, b);
 151 }
 152
 153 static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
 154 {
 155     return (unsigned char)(constant_time_ge(a, b));
 156 }
 157
 158 static inline unsigned int constant_time_is_zero(unsigned int a)
 159 {
 160     return constant_time_msb(~a & (a - 1));
 161 }
 162
 163 static inline unsigned char constant_time_is_zero_8(unsigned int a)
 164 {
 165     return (unsigned char)(constant_time_is_zero(a));
 166 }
 167
 168 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
 169 {
 170     return constant_time_is_zero(a ^ b);
 171 }
 172
 173 static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
 174 {
 175     return (unsigned char)(constant_time_eq(a, b));
 176 }
 177
 178 static inline unsigned int constant_time_eq_int(int a, int b)
 179 {
 180     return constant_time_eq((unsigned)(a), (unsigned)(b));
 181 }
 182
 183 static inline unsigned char constant_time_eq_int_8(int a, int b)
 184 {
 185     return constant_time_eq_8((unsigned)(a), (unsigned)(b));
 186 }
 187
 188 static inline unsigned int constant_time_select(unsigned int mask,
 189                                                 unsigned int a,
 190                                                 unsigned int b)
 191 {
 192     return (mask & a) | (~mask & b);
 193 }
 194
 195 static inline unsigned char constant_time_select_8(unsigned char mask,
 196                                                    unsigned char a,
 197                                                    unsigned char b)
 198 {
 199     return (unsigned char)(constant_time_select(mask, a, b));
 200 }
 201
 202 static inline int constant_time_select_int(unsigned int mask, int a, int b)
 203 {
 204     return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
 205 }
 206
 207 #ifdef __cplusplus
 208 }
 209 #endif
 210
 211 #endif                          /* HEADER_CONSTANT_TIME_LOCL_H */