248 lines
8.6 KiB
C
248 lines
8.6 KiB
C
/**
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* Constant-time functions
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*
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* Copyright The Mbed TLS Contributors
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* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
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*/
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/*
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* The following functions are implemented without using comparison operators, as those
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* might be translated to branches by some compilers on some platforms.
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*/
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#include <stdint.h>
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#include <limits.h>
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#include "common.h"
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#include "constant_time_internal.h"
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#include "mbedtls/constant_time.h"
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#include "mbedtls/error.h"
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#include "mbedtls/platform_util.h"
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#include <string.h>
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#if !defined(MBEDTLS_CT_ASM)
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/*
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* Define an object with the value zero, such that the compiler cannot prove that it
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* has the value zero (because it is volatile, it "may be modified in ways unknown to
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* the implementation").
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*/
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volatile mbedtls_ct_uint_t mbedtls_ct_zero = 0;
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#endif
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/*
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* Define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS where assembly is present to
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* perform fast unaligned access to volatile data.
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*
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* This is needed because mbedtls_get_unaligned_uintXX etc don't support volatile
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* memory accesses.
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*
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* Some of these definitions could be moved into alignment.h but for now they are
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* only used here.
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*/
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#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) && \
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((defined(MBEDTLS_CT_ARM_ASM) && (UINTPTR_MAX == 0xfffffffful)) || \
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defined(MBEDTLS_CT_AARCH64_ASM))
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/* We check pointer sizes to avoid issues with them not matching register size requirements */
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#define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS
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static inline uint32_t mbedtls_get_unaligned_volatile_uint32(volatile const unsigned char *p)
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{
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/* This is UB, even where it's safe:
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* return *((volatile uint32_t*)p);
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* so instead the same thing is expressed in assembly below.
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*/
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uint32_t r;
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#if defined(MBEDTLS_CT_ARM_ASM)
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asm volatile ("ldr %0, [%1]" : "=r" (r) : "r" (p) :);
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#elif defined(MBEDTLS_CT_AARCH64_ASM)
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asm volatile ("ldr %w0, [%1]" : "=r" (r) : MBEDTLS_ASM_AARCH64_PTR_CONSTRAINT(p) :);
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#else
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#error "No assembly defined for mbedtls_get_unaligned_volatile_uint32"
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#endif
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return r;
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}
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#endif /* defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) &&
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(defined(MBEDTLS_CT_ARM_ASM) || defined(MBEDTLS_CT_AARCH64_ASM)) */
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int mbedtls_ct_memcmp(const void *a,
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const void *b,
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size_t n)
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{
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size_t i = 0;
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/*
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* `A` and `B` are cast to volatile to ensure that the compiler
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* generates code that always fully reads both buffers.
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* Otherwise it could generate a test to exit early if `diff` has all
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* bits set early in the loop.
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*/
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volatile const unsigned char *A = (volatile const unsigned char *) a;
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volatile const unsigned char *B = (volatile const unsigned char *) b;
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uint32_t diff = 0;
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#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
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for (; (i + 4) <= n; i += 4) {
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uint32_t x = mbedtls_get_unaligned_volatile_uint32(A + i);
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uint32_t y = mbedtls_get_unaligned_volatile_uint32(B + i);
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diff |= x ^ y;
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}
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#endif
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for (; i < n; i++) {
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/* Read volatile data in order before computing diff.
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* This avoids IAR compiler warning:
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* 'the order of volatile accesses is undefined ..' */
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unsigned char x = A[i], y = B[i];
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diff |= x ^ y;
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}
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#if (INT_MAX < INT32_MAX)
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/* We don't support int smaller than 32-bits, but if someone tried to build
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* with this configuration, there is a risk that, for differing data, the
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* only bits set in diff are in the top 16-bits, and would be lost by a
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* simple cast from uint32 to int.
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* This would have significant security implications, so protect against it. */
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#error "mbedtls_ct_memcmp() requires minimum 32-bit ints"
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#else
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/* The bit-twiddling ensures that when we cast uint32_t to int, we are casting
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* a value that is in the range 0..INT_MAX - a value larger than this would
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* result in implementation defined behaviour.
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*
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* This ensures that the value returned by the function is non-zero iff
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* diff is non-zero.
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*/
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return (int) ((diff & 0xffff) | (diff >> 16));
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#endif
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}
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#if defined(MBEDTLS_NIST_KW_C)
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int mbedtls_ct_memcmp_partial(const void *a,
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const void *b,
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size_t n,
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size_t skip_head,
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size_t skip_tail)
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{
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unsigned int diff = 0;
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volatile const unsigned char *A = (volatile const unsigned char *) a;
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volatile const unsigned char *B = (volatile const unsigned char *) b;
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size_t valid_end = n - skip_tail;
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for (size_t i = 0; i < n; i++) {
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unsigned char x = A[i], y = B[i];
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unsigned int d = x ^ y;
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mbedtls_ct_condition_t valid = mbedtls_ct_bool_and(mbedtls_ct_uint_ge(i, skip_head),
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mbedtls_ct_uint_lt(i, valid_end));
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diff |= mbedtls_ct_uint_if_else_0(valid, d);
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}
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/* Since we go byte-by-byte, the only bits set will be in the bottom 8 bits, so the
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* cast from uint to int is safe. */
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return (int) diff;
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}
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#endif
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#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
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void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset)
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{
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volatile unsigned char *buf = start;
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for (size_t i = 0; i < total; i++) {
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mbedtls_ct_condition_t no_op = mbedtls_ct_uint_gt(total - offset, i);
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/* The first `total - offset` passes are a no-op. The last
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* `offset` passes shift the data one byte to the left and
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* zero out the last byte. */
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for (size_t n = 0; n < total - 1; n++) {
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unsigned char current = buf[n];
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unsigned char next = buf[n+1];
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buf[n] = mbedtls_ct_uint_if(no_op, current, next);
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}
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buf[total-1] = mbedtls_ct_uint_if_else_0(no_op, buf[total-1]);
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}
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}
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#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
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void mbedtls_ct_memcpy_if(mbedtls_ct_condition_t condition,
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unsigned char *dest,
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const unsigned char *src1,
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const unsigned char *src2,
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size_t len)
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{
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#if defined(MBEDTLS_CT_SIZE_64)
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const uint64_t mask = (uint64_t) condition;
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const uint64_t not_mask = (uint64_t) ~mbedtls_ct_compiler_opaque(condition);
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#else
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const uint32_t mask = (uint32_t) condition;
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const uint32_t not_mask = (uint32_t) ~mbedtls_ct_compiler_opaque(condition);
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#endif
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/* If src2 is NULL, setup src2 so that we read from the destination address.
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*
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* This means that if src2 == NULL && condition is false, the result will be a
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* no-op because we read from dest and write the same data back into dest.
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*/
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if (src2 == NULL) {
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src2 = dest;
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}
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/* dest[i] = c1 == c2 ? src[i] : dest[i] */
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size_t i = 0;
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#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
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#if defined(MBEDTLS_CT_SIZE_64)
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for (; (i + 8) <= len; i += 8) {
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uint64_t a = mbedtls_get_unaligned_uint64(src1 + i) & mask;
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uint64_t b = mbedtls_get_unaligned_uint64(src2 + i) & not_mask;
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mbedtls_put_unaligned_uint64(dest + i, a | b);
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}
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#else
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for (; (i + 4) <= len; i += 4) {
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uint32_t a = mbedtls_get_unaligned_uint32(src1 + i) & mask;
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uint32_t b = mbedtls_get_unaligned_uint32(src2 + i) & not_mask;
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mbedtls_put_unaligned_uint32(dest + i, a | b);
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}
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#endif /* defined(MBEDTLS_CT_SIZE_64) */
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#endif /* MBEDTLS_EFFICIENT_UNALIGNED_ACCESS */
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for (; i < len; i++) {
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dest[i] = (src1[i] & mask) | (src2[i] & not_mask);
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}
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}
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void mbedtls_ct_memcpy_offset(unsigned char *dest,
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const unsigned char *src,
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size_t offset,
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size_t offset_min,
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size_t offset_max,
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size_t len)
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{
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size_t offsetval;
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for (offsetval = offset_min; offsetval <= offset_max; offsetval++) {
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mbedtls_ct_memcpy_if(mbedtls_ct_uint_eq(offsetval, offset), dest, src + offsetval, NULL,
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len);
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}
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}
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#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
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void mbedtls_ct_zeroize_if(mbedtls_ct_condition_t condition, void *buf, size_t len)
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{
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uint32_t mask = (uint32_t) ~condition;
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uint8_t *p = (uint8_t *) buf;
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size_t i = 0;
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#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
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for (; (i + 4) <= len; i += 4) {
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mbedtls_put_unaligned_uint32((void *) (p + i),
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mbedtls_get_unaligned_uint32((void *) (p + i)) & mask);
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}
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#endif
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for (; i < len; i++) {
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p[i] = p[i] & mask;
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}
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}
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#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
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