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
53 extern "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 */
79 static inline unsigned int constant_time_msb(unsigned int a);
80
81 /*
82 * Returns 0xff..f if a < b and 0 otherwise.
83 */
84 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
85 /* Convenience method for getting an 8-bit mask. */
86 static 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 */
91 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b);
92 /* Convenience method for getting an 8-bit mask. */
93 static 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 */
98 static inline unsigned int constant_time_is_zero(unsigned int a);
99 /* Convenience method for getting an 8-bit mask. */
100 static 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 */
106 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
107 /* Convenience method for getting an 8-bit mask. */
108 static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);
109 /* Signed integers. */
110 static inline unsigned int constant_time_eq_int(int a, int b);
111 /* Convenience method for getting an 8-bit mask. */
112 static 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 */
122 static inline unsigned int constant_time_select(unsigned int mask,
123 unsigned int a, unsigned int b);
124 /* Convenience method for unsigned chars. */
125 static inline unsigned char constant_time_select_8(unsigned char mask,
126 unsigned char a, unsigned char b);
127 /* Convenience method for signed integers. */
128 static inline int constant_time_select_int(unsigned int mask, int a, int b);
129
constant_time_msb(unsigned int a)130 static inline unsigned int constant_time_msb(unsigned int a)
131 {
132 return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
133 }
134
constant_time_lt(unsigned int a,unsigned int b)135 static 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
constant_time_lt_8(unsigned int a,unsigned int b)145 static 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
constant_time_ge(unsigned int a,unsigned int b)150 static 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
constant_time_ge_8(unsigned int a,unsigned int b)160 static 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
constant_time_is_zero(unsigned int a)165 static inline unsigned int constant_time_is_zero(unsigned int a)
166 {
167 return constant_time_msb(~a & (a - 1));
168 }
169
constant_time_is_zero_8(unsigned int a)170 static inline unsigned char constant_time_is_zero_8(unsigned int a)
171 {
172 return (unsigned char)(constant_time_is_zero(a));
173 }
174
constant_time_eq(unsigned int a,unsigned int b)175 static inline unsigned int constant_time_eq(unsigned int a, unsigned int b)
176 {
177 return constant_time_is_zero(a ^ b);
178 }
179
constant_time_eq_8(unsigned int a,unsigned int b)180 static 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
constant_time_eq_int(int a,int b)185 static inline unsigned int constant_time_eq_int(int a, int b)
186 {
187 return constant_time_eq((unsigned)(a), (unsigned)(b));
188 }
189
constant_time_eq_int_8(int a,int b)190 static 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
constant_time_select(unsigned int mask,unsigned int a,unsigned int b)195 static 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
constant_time_select_8(unsigned char mask,unsigned char a,unsigned char b)201 static 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
constant_time_select_int(unsigned int mask,int a,int b)207 inline 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