mbedtls/library/constant_time.c
gabor-mezei-arm b2dbf2c113
Move mbedtls_cf_uint_if function to the constant-time module
Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
2021-09-28 16:21:10 +02:00

291 lines
8.7 KiB
C

/**
* Constant-time functions
*
* 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.
*/
#include "common.h"
#include "constant_time.h"
#if defined(MBEDTLS_BIGNUM_C)
#include "mbedtls/bignum.h"
#endif
/* constant-time buffer comparison */
int mbedtls_ssl_safer_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( diff );
}
/* Compare the contents of two buffers in constant time.
* Returns 0 if the contents are bitwise identical, otherwise returns
* a non-zero value.
* This is currently only used by GCM and ChaCha20+Poly1305.
*/
int mbedtls_constant_time_memcmp( const void *v1, const void *v2,
size_t len )
{
const unsigned char *p1 = (const unsigned char*) v1;
const unsigned char *p2 = (const unsigned char*) v2;
size_t i;
unsigned char diff;
for( diff = 0, i = 0; i < len; i++ )
diff |= p1[i] ^ p2[i];
return( (int)diff );
}
/* constant-time buffer comparison */
unsigned char mbedtls_nist_kw_safer_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( diff );
}
/* constant-time buffer comparison */
int mbedtls_safer_memcmp( const void *a, const void *b, size_t n )
{
size_t i;
const unsigned char *A = (const unsigned char *) a;
const unsigned char *B = (const unsigned char *) b;
unsigned char diff = 0;
for( i = 0; i < n; i++ )
diff |= A[i] ^ B[i];
return( diff );
}
/** Turn zero-or-nonzero into zero-or-all-bits-one, without branches.
*
* \param value The value to analyze.
* \return Zero if \p value is zero, otherwise all-bits-one.
*/
unsigned mbedtls_cf_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
}
/*
* Turn a bit into a mask:
* - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
* - if bit == 0, return the all-bits 0 mask, aka 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_size_mask( size_t bit )
{
/* 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 -bit;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
/*
* Constant-flow mask generation for "less than" comparison:
* - if x < 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.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_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_cf_size_mask( sub1 );
return( mask );
}
/*
* Constant-flow mask generation for "greater or equal" comparison:
* - if x >= 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.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_size_mask_ge( size_t x, size_t y )
{
return( ~mbedtls_cf_size_mask_lt( x, y ) );
}
/*
* Constant-flow boolean "equal" comparison:
* return x == y
*
* This function can be used to write constant-time code by replacing branches
* with bit operations - it can be used in conjunction with
* mbedtls_ssl_cf_mask_from_bit().
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
size_t mbedtls_cf_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 size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
return( 1 ^ diff1 );
}
/** Check whether a size is out of bounds, without branches.
*
* This is equivalent to `size > max`, but is likely to be compiled to
* to code using bitwise operation rather than a branch.
*
* \param size Size to check.
* \param max Maximum desired value for \p size.
* \return \c 0 if `size <= max`.
* \return \c 1 if `size > max`.
*/
unsigned mbedtls_cf_size_gt( size_t size, size_t max )
{
/* Return the sign bit (1 for negative) of (max - size). */
return( ( max - size ) >> ( sizeof( size_t ) * 8 - 1 ) );
}
#if defined(MBEDTLS_BIGNUM_C)
/** Decide if an integer is less than the other, without branches.
*
* \param x First integer.
* \param y Second integer.
*
* \return 1 if \p x is less than \p y, 0 otherwise
*/
unsigned mbedtls_cf_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 */
/** Choose between two integer values, without branches.
*
* This is equivalent to `cond ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param cond Condition to test.
* \param if1 Value to use if \p cond is nonzero.
* \param if0 Value to use if \p cond is zero.
* \return \c if1 if \p cond is nonzero, otherwise \c if0.
*/
unsigned mbedtls_cf_uint_if( unsigned cond, unsigned if1, unsigned if0 )
{
unsigned mask = mbedtls_cf_uint_mask( cond );
return( ( mask & if1 ) | (~mask & if0 ) );
}