275 lines
9.5 KiB
C
275 lines
9.5 KiB
C
/**
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* Constant-time functions
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*
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* For readability, the static inline definitions are here, and
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* constant_time_internal.h has only the declarations.
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*
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* This results in duplicate declarations of the form:
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* static inline void f() { ... }
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* static inline void f();
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* when constant_time_internal.h is included. This appears to behave
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* exactly as if the declaration-without-definition was not present.
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*
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* Copyright The Mbed TLS Contributors
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* SPDX-License-Identifier: Apache-2.0
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*
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* Licensed under the Apache License, Version 2.0 (the "License"); you may
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* not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef MBEDTLS_CONSTANT_TIME_IMPL_H
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#define MBEDTLS_CONSTANT_TIME_IMPL_H
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#include <stddef.h>
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#include "common.h"
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#if defined(MBEDTLS_BIGNUM_C)
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#include "mbedtls/bignum.h"
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#endif
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/* Disable asm under Memsan because it confuses Memsan and generates false errors */
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#if defined(MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN)
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#define MBEDTLS_CT_NO_ASM
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#elif defined(__has_feature)
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#if __has_feature(memory_sanitizer)
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#define MBEDTLS_CT_NO_ASM
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#endif
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#endif
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/* armcc5 --gnu defines __GNUC__ but doesn't support GNU's extended asm */
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#if defined(MBEDTLS_HAVE_ASM) && defined(__GNUC__) && (!defined(__ARMCC_VERSION) || \
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__ARMCC_VERSION >= 6000000) && !defined(MBEDTLS_CT_NO_ASM)
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#define MBEDTLS_CT_ASM
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#if (defined(__arm__) || defined(__thumb__) || defined(__thumb2__))
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#define MBEDTLS_CT_ARM_ASM
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#elif defined(__aarch64__)
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#define MBEDTLS_CT_AARCH64_ASM
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#endif
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#endif
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#define MBEDTLS_CT_SIZE (sizeof(mbedtls_ct_uint_t) * 8)
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/* ============================================================================
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* Core const-time primitives
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*/
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/** Ensure that the compiler cannot know the value of x (i.e., cannot optimise
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* based on its value) after this function is called.
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*
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* If we are not using assembly, this will be fairly inefficient, so its use
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* should be minimised.
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*/
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static inline mbedtls_ct_uint_t mbedtls_ct_compiler_opaque(mbedtls_ct_uint_t x)
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{
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#if defined(MBEDTLS_CT_ASM)
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asm volatile ("" : [x] "+r" (x) :);
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return x;
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#else
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volatile mbedtls_ct_uint_t result = x;
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return result;
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#endif
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}
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/* Convert a number into a condition in constant time. */
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static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x)
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{
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/*
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* Define mask-generation code that, as far as possible, will not use branches or conditional instructions.
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*
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* For some platforms / type sizes, we define assembly to assure this.
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*
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* Otherwise, we define a plain C fallback which (in May 2023) does not get optimised into
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* conditional instructions or branches by trunk clang, gcc, or MSVC v19.
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*/
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const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
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#if defined(_MSC_VER)
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/* MSVC has a warning about unary minus on unsigned, but this is
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* well-defined and precisely what we want to do here */
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#pragma warning( push )
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#pragma warning( disable : 4146 )
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#endif
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return (mbedtls_ct_condition_t) (((mbedtls_ct_int_t) ((-xo) | -(xo >> 1))) >>
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(MBEDTLS_CT_SIZE - 1));
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#if defined(_MSC_VER)
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#pragma warning( pop )
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#endif
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}
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static inline mbedtls_ct_uint_t mbedtls_ct_if(mbedtls_ct_condition_t condition,
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mbedtls_ct_uint_t if1,
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mbedtls_ct_uint_t if0)
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{
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mbedtls_ct_condition_t not_cond =
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(mbedtls_ct_condition_t) (~mbedtls_ct_compiler_opaque(condition));
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return (mbedtls_ct_uint_t) ((condition & if1) | (not_cond & if0));
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_lt(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
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{
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/* Ensure that the compiler cannot optimise the following operations over x and y,
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* even if it knows the value of x and y.
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*/
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const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
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const mbedtls_ct_uint_t yo = mbedtls_ct_compiler_opaque(y);
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/*
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* Check if the most significant bits (MSB) of the operands are different.
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* cond is true iff the MSBs differ.
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*/
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mbedtls_ct_condition_t cond = mbedtls_ct_bool((xo ^ yo) >> (MBEDTLS_CT_SIZE - 1));
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/*
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* If the MSB are the same then the difference x-y will be negative (and
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* have its MSB set to 1 during conversion to unsigned) if and only if x<y.
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*
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* If the MSB are different, then the operand with the MSB of 1 is the
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* bigger. (That is if y has MSB of 1, then x<y is true and it is false if
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* the MSB of y is 0.)
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*/
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// Select either y, or x - y
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mbedtls_ct_uint_t ret = mbedtls_ct_if(cond, yo, (mbedtls_ct_uint_t) (xo - yo));
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// Extract only the MSB of ret
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ret = ret >> (MBEDTLS_CT_SIZE - 1);
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// Convert to a condition (i.e., all bits set iff non-zero)
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return mbedtls_ct_bool(ret);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_ne(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
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{
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/* diff = 0 if x == y, non-zero otherwise */
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const mbedtls_ct_uint_t diff = mbedtls_ct_compiler_opaque(x) ^ mbedtls_ct_compiler_opaque(y);
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/* all ones if x != y, 0 otherwise */
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return mbedtls_ct_bool(diff);
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}
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static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
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unsigned char high,
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unsigned char c,
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unsigned char t)
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{
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const unsigned char co = (const unsigned char) mbedtls_ct_compiler_opaque(c);
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const unsigned char to = (const unsigned char) mbedtls_ct_compiler_opaque(t);
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/* low_mask is: 0 if low <= c, 0x...ff if low > c */
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unsigned low_mask = ((unsigned) co - low) >> 8;
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/* high_mask is: 0 if c <= high, 0x...ff if c > high */
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unsigned high_mask = ((unsigned) high - co) >> 8;
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return (unsigned char) (~(low_mask | high_mask)) & to;
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}
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/* ============================================================================
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* Everything below here is trivial wrapper functions
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*/
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_eq(mbedtls_ct_uint_t x,
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mbedtls_ct_uint_t y)
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{
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return ~mbedtls_ct_bool_ne(x, y);
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}
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static inline size_t mbedtls_ct_size_if(mbedtls_ct_condition_t condition,
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size_t if1,
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size_t if0)
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{
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return (size_t) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
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}
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static inline unsigned mbedtls_ct_uint_if(mbedtls_ct_condition_t condition,
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unsigned if1,
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unsigned if0)
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{
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return (unsigned) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
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}
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#if defined(MBEDTLS_BIGNUM_C)
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static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if(mbedtls_ct_condition_t condition, \
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mbedtls_mpi_uint if1, \
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mbedtls_mpi_uint if0)
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{
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return (mbedtls_mpi_uint) mbedtls_ct_if(condition,
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(mbedtls_ct_uint_t) if1,
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(mbedtls_ct_uint_t) if0);
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}
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#endif
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static inline size_t mbedtls_ct_size_if0(mbedtls_ct_condition_t condition, size_t if1)
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{
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return (size_t) (condition & if1);
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}
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static inline unsigned mbedtls_ct_uint_if0(mbedtls_ct_condition_t condition, unsigned if1)
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{
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return (unsigned) (condition & if1);
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}
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#if defined(MBEDTLS_BIGNUM_C)
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static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if0(mbedtls_ct_condition_t condition,
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mbedtls_mpi_uint if1)
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{
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return (mbedtls_mpi_uint) (condition & if1);
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}
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#endif /* MBEDTLS_BIGNUM_C */
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_gt(mbedtls_ct_uint_t x,
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mbedtls_ct_uint_t y)
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{
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return mbedtls_ct_bool_lt(y, x);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_ge(mbedtls_ct_uint_t x,
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mbedtls_ct_uint_t y)
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{
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return ~mbedtls_ct_bool_lt(x, y);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_le(mbedtls_ct_uint_t x,
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mbedtls_ct_uint_t y)
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{
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return ~mbedtls_ct_bool_gt(x, y);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_xor(mbedtls_ct_condition_t x,
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mbedtls_ct_condition_t y)
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{
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return (mbedtls_ct_condition_t) (x ^ y);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_and(mbedtls_ct_condition_t x,
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mbedtls_ct_condition_t y)
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{
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return (mbedtls_ct_condition_t) (x & y);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_or(mbedtls_ct_condition_t x,
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mbedtls_ct_condition_t y)
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{
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return (mbedtls_ct_condition_t) (x | y);
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}
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static inline mbedtls_ct_condition_t mbedtls_ct_bool_not(mbedtls_ct_condition_t x)
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{
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return (mbedtls_ct_condition_t) (~x);
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}
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#endif /* MBEDTLS_CONSTANT_TIME_IMPL_H */
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