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