constant_time_locl.h revision 296465 
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
53extern "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 */
79static inline unsigned int constant_time_msb(unsigned int a);
80
81/*
82 * Returns 0xff..f if a < b and 0 otherwise.
83 */
84static inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
85/* Convenience method for getting an 8-bit mask. */
86static 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 */
92static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
93/* Convenience method for getting an 8-bit mask. */
94static 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 */
100static inline unsigned int constant_time_is_zero(unsigned int a);
101/* Convenience method for getting an 8-bit mask. */
102static inline unsigned char constant_time_is_zero_8(unsigned int a);
103
104/*
105 * Returns 0xff..f if a == b and 0 otherwise.
106 */
107static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
108/* Convenience method for getting an 8-bit mask. */
109static inline unsigned char constant_time_eq_8(unsigned int a,
110                                               unsigned int b);
111/* Signed integers. */
112static inline unsigned int constant_time_eq_int(int a, int b);
113/* Convenience method for getting an 8-bit mask. */
114static 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 */
123static inline unsigned int constant_time_select(unsigned int mask,
124                                                unsigned int a,
125                                                unsigned int b);
126/* Convenience method for unsigned chars. */
127static inline unsigned char constant_time_select_8(unsigned char mask,
128                                                   unsigned char a,
129                                                   unsigned char b);
130/* Convenience method for signed integers. */
131static inline int constant_time_select_int(unsigned int mask, int a, int b);
132
133static inline unsigned int constant_time_msb(unsigned int a)
134{
135    return 0 - (a >> (sizeof(a) * 8 - 1));
136}
137
138static 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
143static 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
148static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
149{
150    return ~constant_time_lt(a, b);
151}
152
153static 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
158static inline unsigned int constant_time_is_zero(unsigned int a)
159{
160    return constant_time_msb(~a & (a - 1));
161}
162
163static inline unsigned char constant_time_is_zero_8(unsigned int a)
164{
165    return (unsigned char)(constant_time_is_zero(a));
166}
167
168static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
169{
170    return constant_time_is_zero(a ^ b);
171}
172
173static 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
178static inline unsigned int constant_time_eq_int(int a, int b)
179{
180    return constant_time_eq((unsigned)(a), (unsigned)(b));
181}
182
183static 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
188static 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
195static 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
202static 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 */
212