Outline ![]()
Files ![]()
loading...
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
/**
* Constant-time functions
*
* Copyright The Mbed TLS Contributors
* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
*/
/*
* The following functions are implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
#include "common.h"
#include "constant_time_internal.h"
#include "mbedtls/constant_time.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#if defined(MBEDTLS_BIGNUM_C)
#include "mbedtls/bignum.h"
#endif
#if defined(MBEDTLS_SSL_TLS_C)
#include "mbedtls/ssl_internal.h"
#endif
#if defined(MBEDTLS_RSA_C)
#include "mbedtls/rsa.h"
#endif
#if defined(MBEDTLS_BASE64_C)
#include "constant_time_invasive.h"
#endif
#include <string.h>
int mbedtls_ct_memcmp(const void *a,
const void *b,
size_t n)
{
size_t i;
volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b;
volatile unsigned char diff = 0;
for (i = 0; i < n; i++) {
/* Read volatile data in order before computing diff.
* This avoids IAR compiler warning:
* 'the order of volatile accesses is undefined ..' */
unsigned char x = A[i], y = B[i];
diff |= x ^ y;
}
return (int) diff;
}
unsigned mbedtls_ct_uint_mask(unsigned value)
{
/* MSVC has a warning about unary minus on unsigned, but this is
* well-defined and precisely what we want to do here */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
return -((value | -value) >> (sizeof(value) * 8 - 1));
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC) || defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) || \
defined(MBEDTLS_NIST_KW_C) || defined(MBEDTLS_CIPHER_MODE_CBC)
size_t mbedtls_ct_size_mask(size_t value)
{
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
return -((value | -value) >> (sizeof(value) * 8 - 1));
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
#endif /* defined(MBEDTLS_SSL_SOME_MODES_USE_MAC) || defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) ||
defined(MBEDTLS_NIST_KW_C) || defined(MBEDTLS_CIPHER_MODE_CBC) */
#if defined(MBEDTLS_BIGNUM_C)
mbedtls_mpi_uint mbedtls_ct_mpi_uint_mask(mbedtls_mpi_uint value)
{
/* MSVC has a warning about unary minus on unsigned, but this is
* well-defined and precisely what we want to do here */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
return -((value | -value) >> (sizeof(value) * 8 - 1));
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
#endif /* MBEDTLS_BIGNUM_C */
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) || defined(MBEDTLS_NIST_KW_C) || \
defined(MBEDTLS_CIPHER_MODE_CBC)
/** Constant-flow mask generation for "less than" comparison:
* - if \p x < \p y, return all-bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return All-bits-one if \p x is less than \p y, otherwise zero.
*/
static size_t mbedtls_ct_size_mask_lt(size_t x,
size_t y)
{
/* This has the most significant bit set if and only if x < y */
const size_t sub = x - y;
/* sub1 = (x < y) ? 1 : 0 */
const size_t sub1 = sub >> (sizeof(sub) * 8 - 1);
/* mask = (x < y) ? 0xff... : 0x00... */
const size_t mask = mbedtls_ct_size_mask(sub1);
return mask;
}
size_t mbedtls_ct_size_mask_ge(size_t x,
size_t y)
{
return ~mbedtls_ct_size_mask_lt(x, y);
}
#endif /* defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) || defined(MBEDTLS_NIST_KW_C) ||
defined(MBEDTLS_CIPHER_MODE_CBC) */
#if defined(MBEDTLS_BASE64_C)
/* Return 0xff if low <= c <= high, 0 otherwise.
*
* Constant flow with respect to c.
*/
MBEDTLS_STATIC_TESTABLE
unsigned char mbedtls_ct_uchar_mask_of_range(unsigned char low,
unsigned char high,
unsigned char c)
{
/* low_mask is: 0 if low <= c, 0x...ff if low > c */
unsigned low_mask = ((unsigned) c - low) >> 8;
/* high_mask is: 0 if c <= high, 0x...ff if c > high */
unsigned high_mask = ((unsigned) high - c) >> 8;
return ~(low_mask | high_mask) & 0xff;
}
#endif /* MBEDTLS_BASE64_C */
unsigned mbedtls_ct_size_bool_eq(size_t x,
size_t y)
{
/* diff = 0 if x == y, non-zero otherwise */
const size_t diff = x ^ y;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* diff_msb's most significant bit is equal to x != y */
const size_t diff_msb = (diff | (size_t) -diff);
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* diff1 = (x != y) ? 1 : 0 */
const unsigned diff1 = diff_msb >> (sizeof(diff_msb) * 8 - 1);
return 1 ^ diff1;
}
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** Constant-flow "greater than" comparison:
* return x > y
*
* This is equivalent to \p x > \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x greater than \p y, otherwise 0.
*/
static unsigned mbedtls_ct_size_gt(size_t x,
size_t y)
{
/* Return the sign bit (1 for negative) of (y - x). */
return (y - x) >> (sizeof(size_t) * 8 - 1);
}
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
#if defined(MBEDTLS_BIGNUM_C)
unsigned mbedtls_ct_mpi_uint_lt(const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y)
{
mbedtls_mpi_uint ret;
mbedtls_mpi_uint cond;
/*
* Check if the most significant bits (MSB) of the operands are different.
*/
cond = (x ^ y);
/*
* If the MSB are the same then the difference x-y will be negative (and
* have its MSB set to 1 during conversion to unsigned) if and only if x<y.
*/
ret = (x - y) & ~cond;
/*
* If the MSB are different, then the operand with the MSB of 1 is the
* bigger. (That is if y has MSB of 1, then x<y is true and it is false if
* the MSB of y is 0.)
*/
ret |= y & cond;
ret = ret >> (sizeof(mbedtls_mpi_uint) * 8 - 1);
return (unsigned) ret;
}
#endif /* MBEDTLS_BIGNUM_C */
unsigned mbedtls_ct_uint_if(unsigned condition,
unsigned if1,
unsigned if0)
{
unsigned mask = mbedtls_ct_uint_mask(condition);
return (mask & if1) | (~mask & if0);
}
#if defined(MBEDTLS_BIGNUM_C)
void mbedtls_ct_mpi_uint_cond_assign(size_t n,
mbedtls_mpi_uint *dest,
const mbedtls_mpi_uint *src,
unsigned char condition)
{
size_t i;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* all-bits 1 if condition is 1, all-bits 0 if condition is 0 */
const mbedtls_mpi_uint mask = -condition;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
for (i = 0; i < n; i++) {
dest[i] = (src[i] & mask) | (dest[i] & ~mask);
}
}
#endif /* MBEDTLS_BIGNUM_C */
#if defined(MBEDTLS_BASE64_C)
unsigned char mbedtls_ct_base64_enc_char(unsigned char value)
{
unsigned char digit = 0;
/* For each range of values, if value is in that range, mask digit with
* the corresponding value. Since value can only be in a single range,
* only at most one masking will change digit. */
digit |= mbedtls_ct_uchar_mask_of_range(0, 25, value) & ('A' + value);
digit |= mbedtls_ct_uchar_mask_of_range(26, 51, value) & ('a' + value - 26);
digit |= mbedtls_ct_uchar_mask_of_range(52, 61, value) & ('0' + value - 52);
digit |= mbedtls_ct_uchar_mask_of_range(62, 62, value) & '+';
digit |= mbedtls_ct_uchar_mask_of_range(63, 63, value) & '/';
return digit;
}
signed char mbedtls_ct_base64_dec_value(unsigned char c)
{
unsigned char val = 0;
/* For each range of digits, if c is in that range, mask val with
* the corresponding value. Since c can only be in a single range,
* only at most one masking will change val. Set val to one plus
* the desired value so that it stays 0 if c is in none of the ranges. */
val |= mbedtls_ct_uchar_mask_of_range('A', 'Z', c) & (c - 'A' + 0 + 1);
val |= mbedtls_ct_uchar_mask_of_range('a', 'z', c) & (c - 'a' + 26 + 1);
val |= mbedtls_ct_uchar_mask_of_range('0', '9', c) & (c - '0' + 52 + 1);
val |= mbedtls_ct_uchar_mask_of_range('+', '+', c) & (c - '+' + 62 + 1);
val |= mbedtls_ct_uchar_mask_of_range('/', '/', c) & (c - '/' + 63 + 1);
/* At this point, val is 0 if c is an invalid digit and v+1 if c is
* a digit with the value v. */
return val - 1;
}
#endif /* MBEDTLS_BASE64_C */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** Shift some data towards the left inside a buffer.
*
* `mbedtls_ct_mem_move_to_left(start, total, offset)` is functionally
* equivalent to
* ```
* memmove(start, start + offset, total - offset);
* memset(start + offset, 0, total - offset);
* ```
* but it strives to use a memory access pattern (and thus total timing)
* that does not depend on \p offset. This timing independence comes at
* the expense of performance.
*
* \param start Pointer to the start of the buffer.
* \param total Total size of the buffer.
* \param offset Offset from which to copy \p total - \p offset bytes.
*/
static void mbedtls_ct_mem_move_to_left(void *start,
size_t total,
size_t offset)
{
volatile unsigned char *buf = start;
size_t i, n;
if (total == 0) {
return;
}
for (i = 0; i < total; i++) {
unsigned no_op = mbedtls_ct_size_gt(total - offset, i);
/* The first `total - offset` passes are a no-op. The last
* `offset` passes shift the data one byte to the left and
* zero out the last byte. */
for (n = 0; n < total - 1; n++) {
unsigned char current = buf[n];
unsigned char next = buf[n+1];
buf[n] = mbedtls_ct_uint_if(no_op, current, next);
}
buf[total-1] = mbedtls_ct_uint_if(no_op, buf[total-1], 0);
}
}
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
#if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
void mbedtls_ct_memcpy_if_eq(unsigned char *dest,
const unsigned char *src,
size_t len,
size_t c1,
size_t c2)
{
/* mask = c1 == c2 ? 0xff : 0x00 */
const size_t equal = mbedtls_ct_size_bool_eq(c1, c2);
const unsigned char mask = (unsigned char) mbedtls_ct_size_mask(equal);
/* dest[i] = c1 == c2 ? src[i] : dest[i] */
for (size_t i = 0; i < len; i++) {
dest[i] = (src[i] & mask) | (dest[i] & ~mask);
}
}
void mbedtls_ct_memcpy_offset(unsigned char *dest,
const unsigned char *src,
size_t offset,
size_t offset_min,
size_t offset_max,
size_t len)
{
size_t offsetval;
for (offsetval = offset_min; offsetval <= offset_max; offsetval++) {
mbedtls_ct_memcpy_if_eq(dest, src + offsetval, len,
offsetval, offset);
}
}
int mbedtls_ct_hmac(mbedtls_md_context_t *ctx,
const unsigned char *add_data,
size_t add_data_len,
const unsigned char *data,
size_t data_len_secret,
size_t min_data_len,
size_t max_data_len,
unsigned char *output)
{
/*
* This function breaks the HMAC abstraction and uses the md_clone()
* extension to the MD API in order to get constant-flow behaviour.
*
* HMAC(msg) is defined as HASH(okey + HASH(ikey + msg)) where + means
* concatenation, and okey/ikey are the XOR of the key with some fixed bit
* patterns (see RFC 2104, sec. 2), which are stored in ctx->hmac_ctx.
*
* We'll first compute inner_hash = HASH(ikey + msg) by hashing up to
* minlen, then cloning the context, and for each byte up to maxlen
* finishing up the hash computation, keeping only the correct result.
*
* Then we only need to compute HASH(okey + inner_hash) and we're done.
*/
const mbedtls_md_type_t md_alg = mbedtls_md_get_type(ctx->md_info);
/* TLS 1.0-1.2 only support SHA-384, SHA-256, SHA-1, MD-5,
* all of which have the same block size except SHA-384. */
const size_t block_size = md_alg == MBEDTLS_MD_SHA384 ? 128 : 64;
const unsigned char * const ikey = ctx->hmac_ctx;
const unsigned char * const okey = ikey + block_size;
const size_t hash_size = mbedtls_md_get_size(ctx->md_info);
unsigned char aux_out[MBEDTLS_MD_MAX_SIZE];
mbedtls_md_context_t aux;
size_t offset;
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
mbedtls_md_init(&aux);
#define MD_CHK(func_call) \
do { \
ret = (func_call); \
if (ret != 0) \
goto cleanup; \
} while (0)
MD_CHK(mbedtls_md_setup(&aux, ctx->md_info, 0));
/* After hmac_start() of hmac_reset(), ikey has already been hashed,
* so we can start directly with the message */
MD_CHK(mbedtls_md_update(ctx, add_data, add_data_len));
MD_CHK(mbedtls_md_update(ctx, data, min_data_len));
/* Fill the hash buffer in advance with something that is
* not a valid hash (barring an attack on the hash and
* deliberately-crafted input), in case the caller doesn't
* check the return status properly. */
memset(output, '!', hash_size);
/* For each possible length, compute the hash up to that point */
for (offset = min_data_len; offset <= max_data_len; offset++) {
MD_CHK(mbedtls_md_clone(&aux, ctx));
MD_CHK(mbedtls_md_finish(&aux, aux_out));
/* Keep only the correct inner_hash in the output buffer */
mbedtls_ct_memcpy_if_eq(output, aux_out, hash_size,
offset, data_len_secret);
if (offset < max_data_len) {
MD_CHK(mbedtls_md_update(ctx, data + offset, 1));
}
}
/* The context needs to finish() before it starts() again */
MD_CHK(mbedtls_md_finish(ctx, aux_out));
/* Now compute HASH(okey + inner_hash) */
MD_CHK(mbedtls_md_starts(ctx));
MD_CHK(mbedtls_md_update(ctx, okey, block_size));
MD_CHK(mbedtls_md_update(ctx, output, hash_size));
MD_CHK(mbedtls_md_finish(ctx, output));
/* Done, get ready for next time */
MD_CHK(mbedtls_md_hmac_reset(ctx));
#undef MD_CHK
cleanup:
mbedtls_md_free(&aux);
return ret;
}
#endif /* MBEDTLS_SSL_SOME_MODES_USE_MAC */
#if defined(MBEDTLS_BIGNUM_C)
#define MPI_VALIDATE_RET(cond) \
MBEDTLS_INTERNAL_VALIDATE_RET(cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA)
/*
* Conditionally assign X = Y, without leaking information
* about whether the assignment was made or not.
* (Leaking information about the respective sizes of X and Y is ok however.)
*/
#if defined(_MSC_VER) && defined(_M_ARM64) && (_MSC_FULL_VER < 193131103)
/*
* MSVC miscompiles this function if it's inlined prior to Visual Studio 2022 version 17.1. See:
* https://developercommunity.visualstudio.com/t/c-compiler-miscompiles-part-of-mbedtls-library-on/1646989
*/
__declspec(noinline)
#endif
int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
const mbedtls_mpi *Y,
unsigned char assign)
{
int ret = 0;
size_t i;
mbedtls_mpi_uint limb_mask;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
limb_mask = mbedtls_ct_mpi_uint_mask(assign);;
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
X->s = (int) mbedtls_ct_uint_if(assign, Y->s, X->s);
mbedtls_ct_mpi_uint_cond_assign(Y->n, X->p, Y->p, assign);
for (i = Y->n; i < X->n; i++) {
X->p[i] &= ~limb_mask;
}
cleanup:
return ret;
}
/*
* Conditionally swap X and Y, without leaking information
* about whether the swap was made or not.
* Here it is not ok to simply swap the pointers, which would lead to
* different memory access patterns when X and Y are used afterwards.
*/
int mbedtls_mpi_safe_cond_swap(mbedtls_mpi *X,
mbedtls_mpi *Y,
unsigned char swap)
{
int ret, s;
size_t i;
mbedtls_mpi_uint limb_mask;
mbedtls_mpi_uint tmp;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
if (X == Y) {
return 0;
}
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
limb_mask = mbedtls_ct_mpi_uint_mask(swap);
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Y, X->n));
s = X->s;
X->s = (int) mbedtls_ct_uint_if(swap, Y->s, X->s);
Y->s = (int) mbedtls_ct_uint_if(swap, s, Y->s);
for (i = 0; i < X->n; i++) {
tmp = X->p[i];
X->p[i] = (X->p[i] & ~limb_mask) | (Y->p[i] & limb_mask);
Y->p[i] = (Y->p[i] & ~limb_mask) | (tmp & limb_mask);
}
cleanup:
return ret;
}
/*
* Compare signed values in constant time
*/
int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
const mbedtls_mpi *Y,
unsigned *ret)
{
size_t i;
/* The value of any of these variables is either 0 or 1 at all times. */
unsigned cond, done, X_is_negative, Y_is_negative;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
MPI_VALIDATE_RET(ret != NULL);
if (X->n != Y->n) {
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
}
/*
* Set sign_N to 1 if N >= 0, 0 if N < 0.
* We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
*/
X_is_negative = (X->s & 2) >> 1;
Y_is_negative = (Y->s & 2) >> 1;
/*
* If the signs are different, then the positive operand is the bigger.
* That is if X is negative (X_is_negative == 1), then X < Y is true and it
* is false if X is positive (X_is_negative == 0).
*/
cond = (X_is_negative ^ Y_is_negative);
*ret = cond & X_is_negative;
/*
* This is a constant-time function. We might have the result, but we still
* need to go through the loop. Record if we have the result already.
*/
done = cond;
for (i = X->n; i > 0; i--) {
/*
* If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both
* X and Y are negative.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = mbedtls_ct_mpi_uint_lt(Y->p[i - 1], X->p[i - 1]);
*ret |= cond & (1 - done) & X_is_negative;
done |= cond;
/*
* If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both
* X and Y are positive.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = mbedtls_ct_mpi_uint_lt(X->p[i - 1], Y->p[i - 1]);
*ret |= cond & (1 - done) & (1 - X_is_negative);
done |= cond;
}
return 0;
}
#endif /* MBEDTLS_BIGNUM_C */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
int mbedtls_ct_rsaes_pkcs1_v15_unpadding(int mode,
unsigned char *input,
size_t ilen,
unsigned char *output,
size_t output_max_len,
size_t *olen)
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i, plaintext_max_size;
/* The following variables take sensitive values: their value must
* not leak into the observable behavior of the function other than
* the designated outputs (output, olen, return value). Otherwise
* this would open the execution of the function to
* side-channel-based variants of the Bleichenbacher padding oracle
* attack. Potential side channels include overall timing, memory
* access patterns (especially visible to an adversary who has access
* to a shared memory cache), and branches (especially visible to
* an adversary who has access to a shared code cache or to a shared
* branch predictor). */
size_t pad_count = 0;
unsigned bad = 0;
unsigned char pad_done = 0;
size_t plaintext_size = 0;
unsigned output_too_large;
plaintext_max_size = (output_max_len > ilen - 11) ? ilen - 11
: output_max_len;
/* Check and get padding length in constant time and constant
* memory trace. The first byte must be 0. */
bad |= input[0];
if (mode == MBEDTLS_RSA_PRIVATE) {
/* Decode EME-PKCS1-v1_5 padding: 0x00 || 0x02 || PS || 0x00
* where PS must be at least 8 nonzero bytes. */
bad |= input[1] ^ MBEDTLS_RSA_CRYPT;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count. */
for (i = 2; i < ilen; i++) {
pad_done |= ((input[i] | (unsigned char) -input[i]) >> 7) ^ 1;
pad_count += ((pad_done | (unsigned char) -pad_done) >> 7) ^ 1;
}
} else {
/* Decode EMSA-PKCS1-v1_5 padding: 0x00 || 0x01 || PS || 0x00
* where PS must be at least 8 bytes with the value 0xFF. */
bad |= input[1] ^ MBEDTLS_RSA_SIGN;
/* Read the whole buffer. Set pad_done to nonzero if we find
* the 0x00 byte and remember the padding length in pad_count.
* If there's a non-0xff byte in the padding, the padding is bad. */
for (i = 2; i < ilen; i++) {
pad_done |= mbedtls_ct_uint_if(input[i], 0, 1);
pad_count += mbedtls_ct_uint_if(pad_done, 0, 1);
bad |= mbedtls_ct_uint_if(pad_done, 0, input[i] ^ 0xFF);
}
}
/* If pad_done is still zero, there's no data, only unfinished padding. */
bad |= mbedtls_ct_uint_if(pad_done, 0, 1);
/* There must be at least 8 bytes of padding. */
bad |= mbedtls_ct_size_gt(8, pad_count);
/* If the padding is valid, set plaintext_size to the number of
* remaining bytes after stripping the padding. If the padding
* is invalid, avoid leaking this fact through the size of the
* output: use the maximum message size that fits in the output
* buffer. Do it without branches to avoid leaking the padding
* validity through timing. RSA keys are small enough that all the
* size_t values involved fit in unsigned int. */
plaintext_size = mbedtls_ct_uint_if(
bad, (unsigned) plaintext_max_size,
(unsigned) (ilen - pad_count - 3));
/* Set output_too_large to 0 if the plaintext fits in the output
* buffer and to 1 otherwise. */
output_too_large = mbedtls_ct_size_gt(plaintext_size,
plaintext_max_size);
/* Set ret without branches to avoid timing attacks. Return:
* - INVALID_PADDING if the padding is bad (bad != 0).
* - OUTPUT_TOO_LARGE if the padding is good but the decrypted
* plaintext does not fit in the output buffer.
* - 0 if the padding is correct. */
ret = -(int) mbedtls_ct_uint_if(
bad, -MBEDTLS_ERR_RSA_INVALID_PADDING,
mbedtls_ct_uint_if(output_too_large,
-MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE,
0));
/* If the padding is bad or the plaintext is too large, zero the
* data that we're about to copy to the output buffer.
* We need to copy the same amount of data
* from the same buffer whether the padding is good or not to
* avoid leaking the padding validity through overall timing or
* through memory or cache access patterns. */
bad = mbedtls_ct_uint_mask(bad | output_too_large);
for (i = 11; i < ilen; i++) {
input[i] &= ~bad;
}
/* If the plaintext is too large, truncate it to the buffer size.
* Copy anyway to avoid revealing the length through timing, because
* revealing the length is as bad as revealing the padding validity
* for a Bleichenbacher attack. */
plaintext_size = mbedtls_ct_uint_if(output_too_large,
(unsigned) plaintext_max_size,
(unsigned) plaintext_size);
/* Move the plaintext to the leftmost position where it can start in
* the working buffer, i.e. make it start plaintext_max_size from
* the end of the buffer. Do this with a memory access trace that
* does not depend on the plaintext size. After this move, the
* starting location of the plaintext is no longer sensitive
* information. */
mbedtls_ct_mem_move_to_left(input + ilen - plaintext_max_size,
plaintext_max_size,
plaintext_max_size - plaintext_size);
/* Finally copy the decrypted plaintext plus trailing zeros into the output
* buffer. If output_max_len is 0, then output may be an invalid pointer
* and the result of memcpy() would be undefined; prevent undefined
* behavior making sure to depend only on output_max_len (the size of the
* user-provided output buffer), which is independent from plaintext
* length, validity of padding, success of the decryption, and other
* secrets. */
if (output_max_len != 0) {
memcpy(output, input + ilen - plaintext_max_size, plaintext_max_size);
}
/* Report the amount of data we copied to the output buffer. In case
* of errors (bad padding or output too large), the value of *olen
* when this function returns is not specified. Making it equivalent
* to the good case limits the risks of leaking the padding validity. */
*olen = plaintext_size;
return ret;
}
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
Details ![]()
Show: from Types: Columns: ![]( "Show Project Names")
![]( "Show Locations")
![]( "Show Referring Symbols")
![]( "Show Scope")
This file uses the notable symbols shown below. Click anywhere in the file to view more details.