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constant_time_impl.h
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1
8#ifndef MBEDTLS_CONSTANT_TIME_IMPL_H
9#define MBEDTLS_CONSTANT_TIME_IMPL_H
10
11#include <stddef.h>
12
13#include "common.h"
14
15#if defined(MBEDTLS_BIGNUM_C)
16#include "mbedtls/bignum.h"
17#endif
18
19/*
20 * To improve readability of constant_time_internal.h, the static inline
21 * definitions are here, and constant_time_internal.h has only the declarations.
22 *
23 * This results in duplicate declarations of the form:
24 * static inline void f(); // from constant_time_internal.h
25 * static inline void f() { ... } // from constant_time_impl.h
26 * when constant_time_internal.h is included.
27 *
28 * This appears to behave as if the declaration-without-definition was not present
29 * (except for warnings if gcc -Wredundant-decls or similar is used).
30 *
31 * Disable -Wredundant-decls so that gcc does not warn about this. This is re-enabled
32 * at the bottom of this file.
33 */
34#if defined(MBEDTLS_COMPILER_IS_GCC) && (__GNUC__ > 4)
35 #pragma GCC diagnostic push
36 #pragma GCC diagnostic ignored "-Wredundant-decls"
37#endif
38
39/* armcc5 --gnu defines __GNUC__ but doesn't support GNU's extended asm */
40#if defined(MBEDTLS_HAVE_ASM) && defined(__GNUC__) && (!defined(__ARMCC_VERSION) || \
41 __ARMCC_VERSION >= 6000000)
42#define MBEDTLS_CT_ASM
43#if (defined(__arm__) || defined(__thumb__) || defined(__thumb2__))
44#define MBEDTLS_CT_ARM_ASM
45#elif defined(__aarch64__)
46#define MBEDTLS_CT_AARCH64_ASM
47#elif defined(__amd64__) || defined(__x86_64__)
48#define MBEDTLS_CT_X86_64_ASM
49#elif defined(__i386__)
50#define MBEDTLS_CT_X86_ASM
51#endif
52#endif
53
54#define MBEDTLS_CT_SIZE (sizeof(mbedtls_ct_uint_t) * 8)
55
56
57/* ============================================================================
58 * Core const-time primitives
59 */
60
61/* Ensure that the compiler cannot know the value of x (i.e., cannot optimise
62 * based on its value) after this function is called.
63 *
64 * If we are not using assembly, this will be fairly inefficient, so its use
65 * should be minimised.
66 */
67
68#if !defined(MBEDTLS_CT_ASM)
69extern volatile mbedtls_ct_uint_t mbedtls_ct_zero;
70#endif
71
85static inline mbedtls_ct_uint_t mbedtls_ct_compiler_opaque(mbedtls_ct_uint_t x)
86{
87#if defined(MBEDTLS_CT_ASM)
88 asm volatile ("" : [x] "+r" (x) :);
89 return x;
90#else
91 return x ^ mbedtls_ct_zero;
92#endif
93}
94
95/*
96 * Selecting unified syntax is needed for gcc, and harmless on clang.
97 *
98 * This is needed because on Thumb 1, condition flags are always set, so
99 * e.g. "negs" is supported but "neg" is not (on Thumb 2, both exist).
100 *
101 * Under Thumb 1 unified syntax, only the "negs" form is accepted, and
102 * under divided syntax, only the "neg" form is accepted. clang only
103 * supports unified syntax.
104 *
105 * On Thumb 2 and Arm, both compilers are happy with the "s" suffix,
106 * although we don't actually care about setting the flags.
107 *
108 * For old versions of gcc (see #8516 for details), restore divided
109 * syntax afterwards - otherwise old versions of gcc seem to apply
110 * unified syntax globally, which breaks other asm code.
111 */
112#if defined(MBEDTLS_COMPILER_IS_GCC) && defined(__thumb__) && !defined(__thumb2__) && \
113 (__GNUC__ < 11) && !defined(__ARM_ARCH_2__)
114#define RESTORE_ASM_SYNTAX ".syntax divided \n\t"
115#else
116#define RESTORE_ASM_SYNTAX
117#endif
118
119/* Convert a number into a condition in constant time. */
120static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x)
121{
122 /*
123 * Define mask-generation code that, as far as possible, will not use branches or conditional instructions.
124 *
125 * For some platforms / type sizes, we define assembly to assure this.
126 *
127 * Otherwise, we define a plain C fallback which (in May 2023) does not get optimised into
128 * conditional instructions or branches by trunk clang, gcc, or MSVC v19.
129 */
130#if defined(MBEDTLS_CT_AARCH64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
131 mbedtls_ct_uint_t s;
132 asm volatile ("neg %x[s], %x[x] \n\t"
133 "orr %x[x], %x[s], %x[x] \n\t"
134 "asr %x[x], %x[x], 63 \n\t"
135 :
136 [s] "=&r" (s),
137 [x] "+&r" (x)
138 :
139 :
140 );
141 return (mbedtls_ct_condition_t) x;
142#elif defined(MBEDTLS_CT_ARM_ASM) && defined(MBEDTLS_CT_SIZE_32)
143 uint32_t s;
144 asm volatile (".syntax unified \n\t"
145 "negs %[s], %[x] \n\t"
146 "orrs %[x], %[x], %[s] \n\t"
147 "asrs %[x], %[x], #31 \n\t"
148 RESTORE_ASM_SYNTAX
149 :
150 [s] "=&l" (s),
151 [x] "+&l" (x)
152 :
153 :
154 "cc" /* clobbers flag bits */
155 );
156 return (mbedtls_ct_condition_t) x;
157#elif defined(MBEDTLS_CT_X86_64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
158 uint64_t s;
159 asm volatile ("mov %[x], %[s] \n\t"
160 "neg %[s] \n\t"
161 "or %[x], %[s] \n\t"
162 "sar $63, %[s] \n\t"
163 :
164 [s] "=&a" (s)
165 :
166 [x] "D" (x)
167 :
168 );
169 return (mbedtls_ct_condition_t) s;
170#elif defined(MBEDTLS_CT_X86_ASM) && defined(MBEDTLS_CT_SIZE_32)
171 uint32_t s;
172 asm volatile ("mov %[x], %[s] \n\t"
173 "neg %[s] \n\t"
174 "or %[s], %[x] \n\t"
175 "sar $31, %[x] \n\t"
176 :
177 [s] "=&c" (s),
178 [x] "+&a" (x)
179 :
180 :
181 );
182 return (mbedtls_ct_condition_t) x;
183#else
184 const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
185#if defined(_MSC_VER)
186 /* MSVC has a warning about unary minus on unsigned, but this is
187 * well-defined and precisely what we want to do here */
188#pragma warning( push )
189#pragma warning( disable : 4146 )
190#endif
191 // y is negative (i.e., top bit set) iff x is non-zero
192 mbedtls_ct_int_t y = (-xo) | -(xo >> 1);
193
194 // extract only the sign bit of y so that y == 1 (if x is non-zero) or 0 (if x is zero)
195 y = (((mbedtls_ct_uint_t) y) >> (MBEDTLS_CT_SIZE - 1));
196
197 // -y has all bits set (if x is non-zero), or all bits clear (if x is zero)
198 return (mbedtls_ct_condition_t) (-y);
199#if defined(_MSC_VER)
200#pragma warning( pop )
201#endif
202#endif
203}
204
205static inline mbedtls_ct_uint_t mbedtls_ct_if(mbedtls_ct_condition_t condition,
206 mbedtls_ct_uint_t if1,
207 mbedtls_ct_uint_t if0)
208{
209#if defined(MBEDTLS_CT_AARCH64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
210 asm volatile ("and %x[if1], %x[if1], %x[condition] \n\t"
211 "mvn %x[condition], %x[condition] \n\t"
212 "and %x[condition], %x[condition], %x[if0] \n\t"
213 "orr %x[condition], %x[if1], %x[condition]"
214 :
215 [condition] "+&r" (condition),
216 [if1] "+&r" (if1)
217 :
218 [if0] "r" (if0)
219 :
220 );
221 return (mbedtls_ct_uint_t) condition;
222#elif defined(MBEDTLS_CT_ARM_ASM) && defined(MBEDTLS_CT_SIZE_32)
223 asm volatile (".syntax unified \n\t"
224 "ands %[if1], %[if1], %[condition] \n\t"
225 "mvns %[condition], %[condition] \n\t"
226 "ands %[condition], %[condition], %[if0] \n\t"
227 "orrs %[condition], %[if1], %[condition] \n\t"
228 RESTORE_ASM_SYNTAX
229 :
230 [condition] "+&l" (condition),
231 [if1] "+&l" (if1)
232 :
233 [if0] "l" (if0)
234 :
235 "cc"
236 );
237 return (mbedtls_ct_uint_t) condition;
238#elif defined(MBEDTLS_CT_X86_64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
239 asm volatile ("and %[condition], %[if1] \n\t"
240 "not %[condition] \n\t"
241 "and %[condition], %[if0] \n\t"
242 "or %[if1], %[if0] \n\t"
243 :
244 [condition] "+&D" (condition),
245 [if1] "+&S" (if1),
246 [if0] "+&a" (if0)
247 :
248 :
249 );
250 return if0;
251#elif defined(MBEDTLS_CT_X86_ASM) && defined(MBEDTLS_CT_SIZE_32)
252 asm volatile ("and %[condition], %[if1] \n\t"
253 "not %[condition] \n\t"
254 "and %[if0], %[condition] \n\t"
255 "or %[condition], %[if1] \n\t"
256 :
257 [condition] "+&c" (condition),
258 [if1] "+&a" (if1)
259 :
260 [if0] "b" (if0)
261 :
262 );
263 return if1;
264#else
265 mbedtls_ct_condition_t not_cond =
266 (mbedtls_ct_condition_t) (~mbedtls_ct_compiler_opaque(condition));
267 return (mbedtls_ct_uint_t) ((condition & if1) | (not_cond & if0));
268#endif
269}
270
271static inline mbedtls_ct_condition_t mbedtls_ct_uint_lt(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
272{
273#if defined(MBEDTLS_CT_AARCH64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
274 uint64_t s1;
275 asm volatile ("eor %x[s1], %x[y], %x[x] \n\t"
276 "sub %x[x], %x[x], %x[y] \n\t"
277 "bic %x[x], %x[x], %x[s1] \n\t"
278 "and %x[s1], %x[s1], %x[y] \n\t"
279 "orr %x[s1], %x[x], %x[s1] \n\t"
280 "asr %x[x], %x[s1], 63"
281 :
282 [s1] "=&r" (s1),
283 [x] "+&r" (x)
284 :
285 [y] "r" (y)
286 :
287 );
288 return (mbedtls_ct_condition_t) x;
289#elif defined(MBEDTLS_CT_ARM_ASM) && defined(MBEDTLS_CT_SIZE_32)
290 uint32_t s1;
291 asm volatile (
292 ".syntax unified \n\t"
293#if defined(__thumb__) && !defined(__thumb2__)
294 "movs %[s1], %[x] \n\t"
295 "eors %[s1], %[s1], %[y] \n\t"
296#else
297 "eors %[s1], %[x], %[y] \n\t"
298#endif
299 "subs %[x], %[x], %[y] \n\t"
300 "bics %[x], %[x], %[s1] \n\t"
301 "ands %[y], %[s1], %[y] \n\t"
302 "orrs %[x], %[x], %[y] \n\t"
303 "asrs %[x], %[x], #31 \n\t"
304 RESTORE_ASM_SYNTAX
305 :
306 [s1] "=&l" (s1),
307 [x] "+&l" (x),
308 [y] "+&l" (y)
309 :
310 :
311 "cc"
312 );
313 return (mbedtls_ct_condition_t) x;
314#elif defined(MBEDTLS_CT_X86_64_ASM) && (defined(MBEDTLS_CT_SIZE_32) || defined(MBEDTLS_CT_SIZE_64))
315 uint64_t s;
316 asm volatile ("mov %[x], %[s] \n\t"
317 "xor %[y], %[s] \n\t"
318 "sub %[y], %[x] \n\t"
319 "and %[s], %[y] \n\t"
320 "not %[s] \n\t"
321 "and %[s], %[x] \n\t"
322 "or %[y], %[x] \n\t"
323 "sar $63, %[x] \n\t"
324 :
325 [s] "=&a" (s),
326 [x] "+&D" (x),
327 [y] "+&S" (y)
328 :
329 :
330 );
331 return (mbedtls_ct_condition_t) x;
332#elif defined(MBEDTLS_CT_X86_ASM) && defined(MBEDTLS_CT_SIZE_32)
333 uint32_t s;
334 asm volatile ("mov %[x], %[s] \n\t"
335 "xor %[y], %[s] \n\t"
336 "sub %[y], %[x] \n\t"
337 "and %[s], %[y] \n\t"
338 "not %[s] \n\t"
339 "and %[s], %[x] \n\t"
340 "or %[y], %[x] \n\t"
341 "sar $31, %[x] \n\t"
342 :
343 [s] "=&b" (s),
344 [x] "+&a" (x),
345 [y] "+&c" (y)
346 :
347 :
348 );
349 return (mbedtls_ct_condition_t) x;
350#else
351 /* Ensure that the compiler cannot optimise the following operations over x and y,
352 * even if it knows the value of x and y.
353 */
354 const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
355 const mbedtls_ct_uint_t yo = mbedtls_ct_compiler_opaque(y);
356 /*
357 * Check if the most significant bits (MSB) of the operands are different.
358 * cond is true iff the MSBs differ.
359 */
360 mbedtls_ct_condition_t cond = mbedtls_ct_bool((xo ^ yo) >> (MBEDTLS_CT_SIZE - 1));
361
362 /*
363 * If the MSB are the same then the difference x-y will be negative (and
364 * have its MSB set to 1 during conversion to unsigned) if and only if x<y.
365 *
366 * If the MSB are different, then the operand with the MSB of 1 is the
367 * bigger. (That is if y has MSB of 1, then x<y is true and it is false if
368 * the MSB of y is 0.)
369 */
370
371 // Select either y, or x - y
372 mbedtls_ct_uint_t ret = mbedtls_ct_if(cond, yo, (mbedtls_ct_uint_t) (xo - yo));
373
374 // Extract only the MSB of ret
375 ret = ret >> (MBEDTLS_CT_SIZE - 1);
376
377 // Convert to a condition (i.e., all bits set iff non-zero)
378 return mbedtls_ct_bool(ret);
379#endif
380}
381
382static inline mbedtls_ct_condition_t mbedtls_ct_uint_ne(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
383{
384 /* diff = 0 if x == y, non-zero otherwise */
385 const mbedtls_ct_uint_t diff = mbedtls_ct_compiler_opaque(x) ^ mbedtls_ct_compiler_opaque(y);
386
387 /* all ones if x != y, 0 otherwise */
388 return mbedtls_ct_bool(diff);
389}
390
391static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
392 unsigned char high,
393 unsigned char c,
394 unsigned char t)
395{
396 const unsigned char co = (unsigned char) mbedtls_ct_compiler_opaque(c);
397 const unsigned char to = (unsigned char) mbedtls_ct_compiler_opaque(t);
398
399 /* low_mask is: 0 if low <= c, 0x...ff if low > c */
400 unsigned low_mask = ((unsigned) co - low) >> 8;
401 /* high_mask is: 0 if c <= high, 0x...ff if c > high */
402 unsigned high_mask = ((unsigned) high - co) >> 8;
403
404 return (unsigned char) (~(low_mask | high_mask)) & to;
405}
406
407/* ============================================================================
408 * Everything below here is trivial wrapper functions
409 */
410
411static inline size_t mbedtls_ct_size_if(mbedtls_ct_condition_t condition,
412 size_t if1,
413 size_t if0)
414{
415 return (size_t) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
416}
417
418static inline unsigned mbedtls_ct_uint_if(mbedtls_ct_condition_t condition,
419 unsigned if1,
420 unsigned if0)
421{
422 return (unsigned) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
423}
424
425static inline mbedtls_ct_condition_t mbedtls_ct_bool_if(mbedtls_ct_condition_t condition,
426 mbedtls_ct_condition_t if1,
427 mbedtls_ct_condition_t if0)
428{
429 return (mbedtls_ct_condition_t) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1,
430 (mbedtls_ct_uint_t) if0);
431}
432
433#if defined(MBEDTLS_BIGNUM_C)
434
435static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if(mbedtls_ct_condition_t condition,
436 mbedtls_mpi_uint if1,
437 mbedtls_mpi_uint if0)
438{
439 return (mbedtls_mpi_uint) mbedtls_ct_if(condition,
440 (mbedtls_ct_uint_t) if1,
441 (mbedtls_ct_uint_t) if0);
442}
443
444#endif
445
446static inline size_t mbedtls_ct_size_if_else_0(mbedtls_ct_condition_t condition, size_t if1)
447{
448 return (size_t) (condition & if1);
449}
450
451static inline unsigned mbedtls_ct_uint_if_else_0(mbedtls_ct_condition_t condition, unsigned if1)
452{
453 return (unsigned) (condition & if1);
454}
455
456static inline mbedtls_ct_condition_t mbedtls_ct_bool_if_else_0(mbedtls_ct_condition_t condition,
457 mbedtls_ct_condition_t if1)
458{
459 return (mbedtls_ct_condition_t) (condition & if1);
460}
461
462#if defined(MBEDTLS_BIGNUM_C)
463
464static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if_else_0(mbedtls_ct_condition_t condition,
465 mbedtls_mpi_uint if1)
466{
467 return (mbedtls_mpi_uint) (condition & if1);
468}
469
470#endif /* MBEDTLS_BIGNUM_C */
471
472static inline int mbedtls_ct_error_if(mbedtls_ct_condition_t condition, int if1, int if0)
473{
474 /* Coverting int -> uint -> int here is safe, because we require if1 and if0 to be
475 * in the range -32767..0, and we require 32-bit int and uint types.
476 *
477 * This means that (0 <= -if0 < INT_MAX), so negating if0 is safe, and similarly for
478 * converting back to int.
479 */
480 return -((int) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) (-if1),
481 (mbedtls_ct_uint_t) (-if0)));
482}
483
484static inline int mbedtls_ct_error_if_else_0(mbedtls_ct_condition_t condition, int if1)
485{
486 return -((int) (condition & (-if1)));
487}
488
489static inline mbedtls_ct_condition_t mbedtls_ct_uint_eq(mbedtls_ct_uint_t x,
490 mbedtls_ct_uint_t y)
491{
492 return ~mbedtls_ct_uint_ne(x, y);
493}
494
495static inline mbedtls_ct_condition_t mbedtls_ct_uint_gt(mbedtls_ct_uint_t x,
496 mbedtls_ct_uint_t y)
497{
498 return mbedtls_ct_uint_lt(y, x);
499}
500
501static inline mbedtls_ct_condition_t mbedtls_ct_uint_ge(mbedtls_ct_uint_t x,
502 mbedtls_ct_uint_t y)
503{
504 return ~mbedtls_ct_uint_lt(x, y);
505}
506
507static inline mbedtls_ct_condition_t mbedtls_ct_uint_le(mbedtls_ct_uint_t x,
508 mbedtls_ct_uint_t y)
509{
510 return ~mbedtls_ct_uint_gt(x, y);
511}
512
513static inline mbedtls_ct_condition_t mbedtls_ct_bool_ne(mbedtls_ct_condition_t x,
514 mbedtls_ct_condition_t y)
515{
516 return (mbedtls_ct_condition_t) (x ^ y);
517}
518
519static inline mbedtls_ct_condition_t mbedtls_ct_bool_and(mbedtls_ct_condition_t x,
520 mbedtls_ct_condition_t y)
521{
522 return (mbedtls_ct_condition_t) (x & y);
523}
524
525static inline mbedtls_ct_condition_t mbedtls_ct_bool_or(mbedtls_ct_condition_t x,
526 mbedtls_ct_condition_t y)
527{
528 return (mbedtls_ct_condition_t) (x | y);
529}
530
531static inline mbedtls_ct_condition_t mbedtls_ct_bool_not(mbedtls_ct_condition_t x)
532{
533 return (mbedtls_ct_condition_t) (~x);
534}
535
536#if defined(MBEDTLS_COMPILER_IS_GCC) && (__GNUC__ > 4)
537/* Restore warnings for -Wredundant-decls on gcc */
538 #pragma GCC diagnostic pop
539#endif
540
541#endif /* MBEDTLS_CONSTANT_TIME_IMPL_H */
t
XETime t
Definition main.cpp:53
uint32_t
unsigned int uint32_t
Definition acefiex.h:163
uint64_t
COMPILER_DEPENDENT_UINT64 uint64_t
Definition acefiex.h:165
bignum.h
Multi-precision integer library.
mbedtls_mpi_uint
uint32_t mbedtls_mpi_uint
The type of machine digits in a bignum, called limbs.
Definition bignum.h:170
common.h
Utility macros for internal use in the library.
RESTORE_ASM_SYNTAX
#define RESTORE_ASM_SYNTAX
Definition constant_time_impl.h:116
MBEDTLS_CT_SIZE
#define MBEDTLS_CT_SIZE
Definition constant_time_impl.h:54
mbedtls_ct_zero
volatile mbedtls_ct_uint_t mbedtls_ct_zero
Definition constant_time.c:30
mbedtls_ct_condition_t
size_t mbedtls_ct_condition_t
Definition constant_time_internal.h:67
mbedtls_ct_uint_t
size_t mbedtls_ct_uint_t
Definition constant_time_internal.h:68
mbedtls_ct_int_t
ptrdiff_t mbedtls_ct_int_t
Definition constant_time_internal.h:69
y
int y
Definition layout.c:118
x
int x
Definition layout.c:118
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