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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 */
75static inline unsigned int constant_time_msb(unsigned int a);
76
77/*
78 * Returns 0xff..f if a < b and 0 otherwise.
79 */
80static inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
81/* Convenience method for getting an 8-bit mask. */
82static 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 */
88static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
89/* Convenience method for getting an 8-bit mask. */
90static 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 */
96static inline unsigned int constant_time_is_zero(unsigned int a);
97/* Convenience method for getting an 8-bit mask. */
98static inline unsigned char constant_time_is_zero_8(unsigned int a);
99
100/*
101 * Returns 0xff..f if a == b and 0 otherwise.
102 */
103static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
104/* Convenience method for getting an 8-bit mask. */
105static inline unsigned char constant_time_eq_8(unsigned int a,
106 unsigned int b);
107/* Signed integers. */
108static inline unsigned int constant_time_eq_int(int a, int b);
109/* Convenience method for getting an 8-bit mask. */
110static 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 */
119static inline unsigned int constant_time_select(unsigned int mask,
120 unsigned int a,
121 unsigned int b);
122/* Convenience method for unsigned chars. */
123static inline unsigned char constant_time_select_8(unsigned char mask,
124 unsigned char a,
125 unsigned char b);
126/* Convenience method for signed integers. */
127static inline int constant_time_select_int(unsigned int mask, int a, int b);
128
129static inline unsigned int constant_time_msb(unsigned int a)
130{
131 return 0 - (a >> (sizeof(a) * 8 - 1));
132}
133
134static 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
139static 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
144static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
145{
146 return ~constant_time_lt(a, b);
147}
148
149static 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
154static inline unsigned int constant_time_is_zero(unsigned int a)
155{
156 return constant_time_msb(~a & (a - 1));
157}
158
159static inline unsigned char constant_time_is_zero_8(unsigned int a)
160{
161 return (unsigned char)(constant_time_is_zero(a));
162}
163
164static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
165{
166 return constant_time_is_zero(a ^ b);
167}
168
169static 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
174static inline unsigned int constant_time_eq_int(int a, int b)
175{
176 return constant_time_eq((unsigned)(a), (unsigned)(b));
177}
178
179static 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
184static 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
191static 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
198static 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
203void err_clear_last_constant_time(int clear);
204
205__END_HIDDEN_DECLS
206
207#endif /* !HEADER_CONSTANT_TIME_H */
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