[331] | 1 | /*
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| 2 | * Copyright 2014-2016 The OpenSSL Project Authors. All Rights Reserved.
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| 3 | *
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| 4 | * Licensed under the OpenSSL license (the "License"). You may not use
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| 5 | * this file except in compliance with the License. You can obtain a copy
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| 6 | * in the file LICENSE in the source distribution or at
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| 7 | * https://www.openssl.org/source/license.html
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| 8 | */
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| 9 |
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| 10 | #ifndef HEADER_CONSTANT_TIME_LOCL_H
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| 11 | # define HEADER_CONSTANT_TIME_LOCL_H
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| 12 |
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| 13 | # include <openssl/e_os2.h> /* For 'ossl_inline' */
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| 14 |
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| 15 | #ifdef __cplusplus
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| 16 | extern "C" {
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| 17 | #endif
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| 18 |
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| 19 | /*-
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| 20 | * The boolean methods return a bitmask of all ones (0xff...f) for true
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| 21 | * and 0 for false. This is useful for choosing a value based on the result
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| 22 | * of a conditional in constant time. For example,
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| 23 | *
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| 24 | * if (a < b) {
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| 25 | * c = a;
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| 26 | * } else {
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| 27 | * c = b;
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| 28 | * }
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| 29 | *
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| 30 | * can be written as
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| 31 | *
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| 32 | * unsigned int lt = constant_time_lt(a, b);
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| 33 | * c = constant_time_select(lt, a, b);
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| 34 | */
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| 35 |
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| 36 | /*
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| 37 | * Returns the given value with the MSB copied to all the other
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| 38 | * bits. Uses the fact that arithmetic shift shifts-in the sign bit.
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| 39 | * However, this is not ensured by the C standard so you may need to
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| 40 | * replace this with something else on odd CPUs.
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| 41 | */
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| 42 | static ossl_inline unsigned int constant_time_msb(unsigned int a);
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| 43 |
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| 44 | /*
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| 45 | * Returns 0xff..f if a < b and 0 otherwise.
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| 46 | */
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| 47 | static ossl_inline unsigned int constant_time_lt(unsigned int a,
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| 48 | unsigned int b);
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| 49 | /* Convenience method for getting an 8-bit mask. */
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| 50 | static ossl_inline unsigned char constant_time_lt_8(unsigned int a,
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| 51 | unsigned int b);
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| 52 |
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| 53 | /*
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| 54 | * Returns 0xff..f if a >= b and 0 otherwise.
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| 55 | */
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| 56 | static ossl_inline unsigned int constant_time_ge(unsigned int a,
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| 57 | unsigned int b);
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| 58 | /* Convenience method for getting an 8-bit mask. */
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| 59 | static ossl_inline unsigned char constant_time_ge_8(unsigned int a,
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| 60 | unsigned int b);
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| 61 |
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| 62 | /*
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| 63 | * Returns 0xff..f if a == 0 and 0 otherwise.
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| 64 | */
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| 65 | static ossl_inline unsigned int constant_time_is_zero(unsigned int a);
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| 66 | /* Convenience method for getting an 8-bit mask. */
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| 67 | static ossl_inline unsigned char constant_time_is_zero_8(unsigned int a);
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| 68 |
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| 69 | /*
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| 70 | * Returns 0xff..f if a == b and 0 otherwise.
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| 71 | */
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| 72 | static ossl_inline unsigned int constant_time_eq(unsigned int a,
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| 73 | unsigned int b);
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| 74 | /* Convenience method for getting an 8-bit mask. */
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| 75 | static ossl_inline unsigned char constant_time_eq_8(unsigned int a,
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| 76 | unsigned int b);
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| 77 | /* Signed integers. */
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| 78 | static ossl_inline unsigned int constant_time_eq_int(int a, int b);
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| 79 | /* Convenience method for getting an 8-bit mask. */
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| 80 | static ossl_inline unsigned char constant_time_eq_int_8(int a, int b);
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| 81 |
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| 82 | /*-
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| 83 | * Returns (mask & a) | (~mask & b).
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| 84 | *
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| 85 | * When |mask| is all 1s or all 0s (as returned by the methods above),
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| 86 | * the select methods return either |a| (if |mask| is nonzero) or |b|
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| 87 | * (if |mask| is zero).
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| 88 | */
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| 89 | static ossl_inline unsigned int constant_time_select(unsigned int mask,
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| 90 | unsigned int a,
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| 91 | unsigned int b);
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| 92 | /* Convenience method for unsigned chars. */
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| 93 | static ossl_inline unsigned char constant_time_select_8(unsigned char mask,
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| 94 | unsigned char a,
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| 95 | unsigned char b);
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| 96 | /* Convenience method for signed integers. */
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| 97 | static ossl_inline int constant_time_select_int(unsigned int mask, int a,
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| 98 | int b);
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| 99 |
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| 100 | static ossl_inline unsigned int constant_time_msb(unsigned int a)
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| 101 | {
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| 102 | return 0 - (a >> (sizeof(a) * 8 - 1));
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| 103 | }
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| 104 |
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| 105 | static ossl_inline unsigned int constant_time_lt(unsigned int a,
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| 106 | unsigned int b)
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| 107 | {
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| 108 | return constant_time_msb(a ^ ((a ^ b) | ((a - b) ^ b)));
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| 109 | }
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| 110 |
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| 111 | static ossl_inline unsigned char constant_time_lt_8(unsigned int a,
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| 112 | unsigned int b)
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| 113 | {
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| 114 | return (unsigned char)(constant_time_lt(a, b));
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| 115 | }
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| 116 |
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| 117 | static ossl_inline unsigned int constant_time_ge(unsigned int a,
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| 118 | unsigned int b)
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| 119 | {
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| 120 | return ~constant_time_lt(a, b);
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| 121 | }
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| 122 |
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| 123 | static ossl_inline unsigned char constant_time_ge_8(unsigned int a,
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| 124 | unsigned int b)
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| 125 | {
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| 126 | return (unsigned char)(constant_time_ge(a, b));
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| 127 | }
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| 128 |
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| 129 | static ossl_inline unsigned int constant_time_is_zero(unsigned int a)
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| 130 | {
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| 131 | return constant_time_msb(~a & (a - 1));
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| 132 | }
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| 133 |
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| 134 | static ossl_inline unsigned char constant_time_is_zero_8(unsigned int a)
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| 135 | {
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| 136 | return (unsigned char)(constant_time_is_zero(a));
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| 137 | }
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| 138 |
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| 139 | static ossl_inline unsigned int constant_time_eq(unsigned int a,
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| 140 | unsigned int b)
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| 141 | {
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| 142 | return constant_time_is_zero(a ^ b);
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| 143 | }
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| 144 |
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| 145 | static ossl_inline unsigned char constant_time_eq_8(unsigned int a,
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| 146 | unsigned int b)
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| 147 | {
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| 148 | return (unsigned char)(constant_time_eq(a, b));
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| 149 | }
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| 150 |
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| 151 | static ossl_inline unsigned int constant_time_eq_int(int a, int b)
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| 152 | {
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| 153 | return constant_time_eq((unsigned)(a), (unsigned)(b));
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| 154 | }
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| 155 |
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| 156 | static ossl_inline unsigned char constant_time_eq_int_8(int a, int b)
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| 157 | {
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| 158 | return constant_time_eq_8((unsigned)(a), (unsigned)(b));
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| 159 | }
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| 160 |
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| 161 | static ossl_inline unsigned int constant_time_select(unsigned int mask,
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| 162 | unsigned int a,
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| 163 | unsigned int b)
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| 164 | {
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| 165 | return (mask & a) | (~mask & b);
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| 166 | }
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| 167 |
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| 168 | static ossl_inline unsigned char constant_time_select_8(unsigned char mask,
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| 169 | unsigned char a,
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| 170 | unsigned char b)
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| 171 | {
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| 172 | return (unsigned char)(constant_time_select(mask, a, b));
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| 173 | }
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| 174 |
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| 175 | static ossl_inline int constant_time_select_int(unsigned int mask, int a,
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| 176 | int b)
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| 177 | {
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| 178 | return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
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| 179 | }
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| 180 |
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| 181 | #ifdef __cplusplus
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| 182 | }
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| 183 | #endif
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| 184 |
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| 185 | #endif /* HEADER_CONSTANT_TIME_LOCL_H */
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