constant_time_locl.h revision c64f6fe2be99cb3fa8e491b5bede9a217de87a4c
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, unsigned int b);
87
88/*
89 * Returns 0xff..f if a >= b and 0 otherwise.
90 */
91static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
92/* Convenience method for getting an 8-bit mask. */
93static inline unsigned char constant_time_ge_8(unsigned int a, 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/*
104 * Returns 0xff..f if a == b and 0 otherwise.
105 */
106static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
107/* Convenience method for getting an 8-bit mask. */
108static inline unsigned char constant_time_eq_8(unsigned int a, 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/*
116 * Returns (mask & a) | (~mask & b).
117 *
118 * When |mask| is all 1s or all 0s (as returned by the methods above),
119 * the select methods return either |a| (if |mask| is nonzero) or |b|
120 * (if |mask| is zero).
121 */
122static inline unsigned int constant_time_select(unsigned int mask,
123	unsigned int a, unsigned int b);
124/* Convenience method for unsigned chars. */
125static inline unsigned char constant_time_select_8(unsigned char mask,
126	unsigned char a, unsigned char b);
127/* Convenience method for signed integers. */
128static inline int constant_time_select_int(unsigned int mask, int a, int b);
129
130static inline unsigned int constant_time_msb(unsigned int a)
131	{
132	return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
133	}
134
135static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
136	{
137	unsigned int lt;
138	/* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/
139	lt = ~(a ^ b) & (a - b);
140	/* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */
141	lt |= ~a & b;
142	return constant_time_msb(lt);
143	}
144
145static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
146	{
147	return (unsigned char)(constant_time_lt(a, b));
148	}
149
150static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
151	{
152	unsigned int ge;
153	/* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/
154	ge = ~((a ^ b) | (a - b));
155	/* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */
156	ge |= a & ~b;
157	return constant_time_msb(ge);
158	}
159
160static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)
161	{
162	return (unsigned char)(constant_time_ge(a, b));
163	}
164
165static inline unsigned int constant_time_is_zero(unsigned int a)
166	{
167	return constant_time_msb(~a & (a - 1));
168	}
169
170static inline unsigned char constant_time_is_zero_8(unsigned int a)
171	{
172	return (unsigned char)(constant_time_is_zero(a));
173	}
174
175static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
176	{
177	return constant_time_is_zero(a ^ b);
178	}
179
180static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
181	{
182	return (unsigned char)(constant_time_eq(a, b));
183	}
184
185static inline unsigned int constant_time_eq_int(int a, int b)
186	{
187	return constant_time_eq((unsigned)(a), (unsigned)(b));
188	}
189
190static inline unsigned char constant_time_eq_int_8(int a, int b)
191	{
192	return constant_time_eq_8((unsigned)(a), (unsigned)(b));
193	}
194
195static inline unsigned int constant_time_select(unsigned int mask,
196	unsigned int a, unsigned int b)
197	{
198	return (mask & a) | (~mask & b);
199	}
200
201static inline unsigned char constant_time_select_8(unsigned char mask,
202	unsigned char a, unsigned char b)
203	{
204	return (unsigned char)(constant_time_select(mask, a, b));
205	}
206
207inline int constant_time_select_int(unsigned int mask, int a, int b)
208	{
209	return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
210	}
211
212#ifdef __cplusplus
213}
214#endif
215
216#endif  /* HEADER_CONSTANT_TIME_LOCL_H */
217