1 /* $OpenBSD: constant_time.h,v 1.3 2023/04/06 18:43:47 tb Exp $ */ 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_H 48 #define HEADER_CONSTANT_TIME_H 49 50 __BEGIN_HIDDEN_DECLS 51 52 /*- 53 * The boolean methods return a bitmask of all ones (0xff...f) for true 54 * and 0 for false. This is useful for choosing a value based on the result 55 * of a conditional in constant time. For example, 56 * 57 * if (a < b) { 58 * c = a; 59 * } else { 60 * c = b; 61 * } 62 * 63 * can be written as 64 * 65 * unsigned int lt = constant_time_lt(a, b); 66 * c = constant_time_select(lt, a, b); 67 */ 68 69 /* 70 * Returns the given value with the MSB copied to all the other 71 * bits. Uses the fact that arithmetic shift shifts-in the sign bit. 72 * However, this is not ensured by the C standard so you may need to 73 * replace this with something else on odd CPUs. 74 */ 75 static inline unsigned int constant_time_msb(unsigned int a); 76 77 /* 78 * Returns 0xff..f if a < b and 0 otherwise. 79 */ 80 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b); 81 /* Convenience method for getting an 8-bit mask. */ 82 static inline unsigned char constant_time_lt_8(unsigned int a, 83 unsigned int b); 84 85 /* 86 * Returns 0xff..f if a >= b and 0 otherwise. 87 */ 88 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b); 89 /* Convenience method for getting an 8-bit mask. */ 90 static inline unsigned char constant_time_ge_8(unsigned int a, 91 unsigned int b); 92 93 /* 94 * Returns 0xff..f if a == 0 and 0 otherwise. 95 */ 96 static inline unsigned int constant_time_is_zero(unsigned int a); 97 /* Convenience method for getting an 8-bit mask. */ 98 static inline unsigned char constant_time_is_zero_8(unsigned int a); 99 100 /* 101 * Returns 0xff..f if a == b and 0 otherwise. 102 */ 103 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b); 104 /* Convenience method for getting an 8-bit mask. */ 105 static inline unsigned char constant_time_eq_8(unsigned int a, 106 unsigned int b); 107 /* Signed integers. */ 108 static inline unsigned int constant_time_eq_int(int a, int b); 109 /* Convenience method for getting an 8-bit mask. */ 110 static inline unsigned char constant_time_eq_int_8(int a, int b); 111 112 /*- 113 * Returns (mask & a) | (~mask & b). 114 * 115 * When |mask| is all 1s or all 0s (as returned by the methods above), 116 * the select methods return either |a| (if |mask| is nonzero) or |b| 117 * (if |mask| is zero). 118 */ 119 static inline unsigned int constant_time_select(unsigned int mask, 120 unsigned int a, 121 unsigned int b); 122 /* Convenience method for unsigned chars. */ 123 static inline unsigned char constant_time_select_8(unsigned char mask, 124 unsigned char a, 125 unsigned char b); 126 /* Convenience method for signed integers. */ 127 static inline int constant_time_select_int(unsigned int mask, int a, int b); 128 129 static inline unsigned int constant_time_msb(unsigned int a) 130 { 131 return 0 - (a >> (sizeof(a) * 8 - 1)); 132 } 133 134 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b) 135 { 136 return constant_time_msb(a ^ ((a ^ b) | ((a - b) ^ b))); 137 } 138 139 static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b) 140 { 141 return (unsigned char)(constant_time_lt(a, b)); 142 } 143 144 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b) 145 { 146 return ~constant_time_lt(a, b); 147 } 148 149 static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b) 150 { 151 return (unsigned char)(constant_time_ge(a, b)); 152 } 153 154 static inline unsigned int constant_time_is_zero(unsigned int a) 155 { 156 return constant_time_msb(~a & (a - 1)); 157 } 158 159 static inline unsigned char constant_time_is_zero_8(unsigned int a) 160 { 161 return (unsigned char)(constant_time_is_zero(a)); 162 } 163 164 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b) 165 { 166 return constant_time_is_zero(a ^ b); 167 } 168 169 static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b) 170 { 171 return (unsigned char)(constant_time_eq(a, b)); 172 } 173 174 static inline unsigned int constant_time_eq_int(int a, int b) 175 { 176 return constant_time_eq((unsigned)(a), (unsigned)(b)); 177 } 178 179 static inline unsigned char constant_time_eq_int_8(int a, int b) 180 { 181 return constant_time_eq_8((unsigned)(a), (unsigned)(b)); 182 } 183 184 static inline unsigned int constant_time_select(unsigned int mask, 185 unsigned int a, 186 unsigned int b) 187 { 188 return (mask & a) | (~mask & b); 189 } 190 191 static inline unsigned char constant_time_select_8(unsigned char mask, 192 unsigned char a, 193 unsigned char b) 194 { 195 return (unsigned char)(constant_time_select(mask, a, b)); 196 } 197 198 static inline int constant_time_select_int(unsigned int mask, int a, int b) 199 { 200 return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b))); 201 } 202 203 void err_clear_last_constant_time(int clear); 204 205 __END_HIDDEN_DECLS 206 207 #endif /* !HEADER_CONSTANT_TIME_H */ 208