/**
* Constant-time functions
*
* For readability, the static inline definitions are here, and
* constant_time_internal.h has only the declarations.
*
* This results in duplicate declarations of the form:
* static inline void f() { ... }
* static inline void f();
* when constant_time_internal.h is included. This appears to behave
* exactly as if the declaration-without-definition was not present.
*
* Copyright The Mbed TLS Contributors
* SPDX-License-Identifier: Apache-2.0
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may
* not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef MBEDTLS_CONSTANT_TIME_IMPL_H
#define MBEDTLS_CONSTANT_TIME_IMPL_H
#include <stddef.h>
#include "common.h"
#if defined(MBEDTLS_BIGNUM_C)
#include "mbedtls/bignum.h"
#endif
/* constant_time_impl.h contains all the static inline implementations,
* so that constant_time_internal.h is more readable.
*
* gcc generates warnings about duplicate declarations, so disable this
* warning.
*/
#ifdef __GNUC__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wredundant-decls"
#endif
/* Disable asm under Memsan because it confuses Memsan and generates false errors */
#if defined(MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN)
#define MBEDTLS_CT_NO_ASM
#elif defined(__has_feature)
#if __has_feature(memory_sanitizer)
#define MBEDTLS_CT_NO_ASM
#endif
#endif
/* armcc5 --gnu defines __GNUC__ but doesn't support GNU's extended asm */
#if defined(MBEDTLS_HAVE_ASM) && defined(__GNUC__) && (!defined(__ARMCC_VERSION) || \
__ARMCC_VERSION >= 6000000) && !defined(MBEDTLS_CT_NO_ASM)
#define MBEDTLS_CT_ASM
#if (defined(__arm__) || defined(__thumb__) || defined(__thumb2__))
#define MBEDTLS_CT_ARM_ASM
#elif defined(__aarch64__)
#define MBEDTLS_CT_AARCH64_ASM
#endif
#endif
#define MBEDTLS_CT_SIZE (sizeof(mbedtls_ct_uint_t) * 8)
/* ============================================================================
* Core const-time primitives
*/
/* Ensure that the compiler cannot know the value of x (i.e., cannot optimise
* based on its value) after this function is called.
*
* If we are not using assembly, this will be fairly inefficient, so its use
* should be minimised.
*/
#if !defined(MBEDTLS_CT_ASM)
extern volatile mbedtls_ct_uint_t mbedtls_ct_zero;
#endif
/**
* \brief Ensure that a value cannot be known at compile time.
*
* \param x The value to hide from the compiler.
* \return The same value that was passed in, such that the compiler
* cannot prove its value (even for calls of the form
* x = mbedtls_ct_compiler_opaque(1), x will be unknown).
*
* \note This is mainly used in constructing mbedtls_ct_condition_t
* values and performing operations over them, to ensure that
* there is no way for the compiler to ever know anything about
* the value of an mbedtls_ct_condition_t.
*/
static inline mbedtls_ct_uint_t mbedtls_ct_compiler_opaque(mbedtls_ct_uint_t x)
{
#if defined(MBEDTLS_CT_ASM)
asm volatile ("" : [x] "+r" (x) :);
return x;
#else
return x ^ mbedtls_ct_zero;
#endif
}
/* Convert a number into a condition in constant time. */
static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x)
{
/*
* Define mask-generation code that, as far as possible, will not use branches or conditional instructions.
*
* For some platforms / type sizes, we define assembly to assure this.
*
* Otherwise, we define a plain C fallback which (in May 2023) does not get optimised into
* conditional instructions or branches by trunk clang, gcc, or MSVC v19.
*/
const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
#if defined(_MSC_VER)
/* MSVC has a warning about unary minus on unsigned, but this is
* well-defined and precisely what we want to do here */
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
return (mbedtls_ct_condition_t) (((mbedtls_ct_int_t) ((-xo) | -(xo >> 1))) >>
(MBEDTLS_CT_SIZE - 1));
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
static inline mbedtls_ct_uint_t mbedtls_ct_if(mbedtls_ct_condition_t condition,
mbedtls_ct_uint_t if1,
mbedtls_ct_uint_t if0)
{
mbedtls_ct_condition_t not_cond =
(mbedtls_ct_condition_t) (~mbedtls_ct_compiler_opaque(condition));
return (mbedtls_ct_uint_t) ((condition & if1) | (not_cond & if0));
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_lt(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
{
/* Ensure that the compiler cannot optimise the following operations over x and y,
* even if it knows the value of x and y.
*/
const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
const mbedtls_ct_uint_t yo = mbedtls_ct_compiler_opaque(y);
/*
* Check if the most significant bits (MSB) of the operands are different.
* cond is true iff the MSBs differ.
*/
mbedtls_ct_condition_t cond = mbedtls_ct_bool((xo ^ yo) >> (MBEDTLS_CT_SIZE - 1));
/*
* 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.
*
* 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.)
*/
// Select either y, or x - y
mbedtls_ct_uint_t ret = mbedtls_ct_if(cond, yo, (mbedtls_ct_uint_t) (xo - yo));
// Extract only the MSB of ret
ret = ret >> (MBEDTLS_CT_SIZE - 1);
// Convert to a condition (i.e., all bits set iff non-zero)
return mbedtls_ct_bool(ret);
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_ne(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y)
{
/* diff = 0 if x == y, non-zero otherwise */
const mbedtls_ct_uint_t diff = mbedtls_ct_compiler_opaque(x) ^ mbedtls_ct_compiler_opaque(y);
/* all ones if x != y, 0 otherwise */
return mbedtls_ct_bool(diff);
}
static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
unsigned char high,
unsigned char c,
unsigned char t)
{
const unsigned char co = (const unsigned char) mbedtls_ct_compiler_opaque(c);
const unsigned char to = (const unsigned char) mbedtls_ct_compiler_opaque(t);
/* low_mask is: 0 if low <= c, 0x...ff if low > c */
unsigned low_mask = ((unsigned) co - low) >> 8;
/* high_mask is: 0 if c <= high, 0x...ff if c > high */
unsigned high_mask = ((unsigned) high - co) >> 8;
return (unsigned char) (~(low_mask | high_mask)) & to;
}
/* ============================================================================
* Everything below here is trivial wrapper functions
*/
static inline size_t mbedtls_ct_size_if(mbedtls_ct_condition_t condition,
size_t if1,
size_t if0)
{
return (size_t) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
}
static inline unsigned mbedtls_ct_uint_if(mbedtls_ct_condition_t condition,
unsigned if1,
unsigned if0)
{
return (unsigned) mbedtls_ct_if(condition, (mbedtls_ct_uint_t) if1, (mbedtls_ct_uint_t) if0);
}
#if defined(MBEDTLS_BIGNUM_C)
static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if(mbedtls_ct_condition_t condition,
mbedtls_mpi_uint if1,
mbedtls_mpi_uint if0)
{
return (mbedtls_mpi_uint) mbedtls_ct_if(condition,
(mbedtls_ct_uint_t) if1,
(mbedtls_ct_uint_t) if0);
}
#endif
static inline size_t mbedtls_ct_size_if_else_0(mbedtls_ct_condition_t condition, size_t if1)
{
return (size_t) (condition & if1);
}
static inline unsigned mbedtls_ct_uint_if_else_0(mbedtls_ct_condition_t condition, unsigned if1)
{
return (unsigned) (condition & if1);
}
#if defined(MBEDTLS_BIGNUM_C)
static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if_else_0(mbedtls_ct_condition_t condition,
mbedtls_mpi_uint if1)
{
return (mbedtls_mpi_uint) (condition & if1);
}
#endif /* MBEDTLS_BIGNUM_C */
static inline mbedtls_ct_condition_t mbedtls_ct_uint_eq(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{
return ~mbedtls_ct_uint_ne(x, y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_gt(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{
return mbedtls_ct_uint_lt(y, x);
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_ge(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{
return ~mbedtls_ct_uint_lt(x, y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_le(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{
return ~mbedtls_ct_uint_gt(x, y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_bool_xor(mbedtls_ct_condition_t x,
mbedtls_ct_condition_t y)
{
return (mbedtls_ct_condition_t) (x ^ y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_bool_and(mbedtls_ct_condition_t x,
mbedtls_ct_condition_t y)
{
return (mbedtls_ct_condition_t) (x & y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_bool_or(mbedtls_ct_condition_t x,
mbedtls_ct_condition_t y)
{
return (mbedtls_ct_condition_t) (x | y);
}
static inline mbedtls_ct_condition_t mbedtls_ct_bool_not(mbedtls_ct_condition_t x)
{
return (mbedtls_ct_condition_t) (~x);
}
#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif
#endif /* MBEDTLS_CONSTANT_TIME_IMPL_H */