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Add conditional swap and assign function for MPI core

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Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Gabor Mezei 2022-09-12 16:25:24 +02:00
parent 845de0898e
commit e1d31c4aad
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GPG Key ID: F072ACA227ACD71D
2 changed files with 120 additions and 0 deletions
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56
library/bignum_core.c
View File

@ -25,6 +25,7 @@
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include "constant_time_internal.h"
#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
@ -161,6 +162,61 @@ void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *A,
}
}
int mbedtls_mpi_core_cond_assign( mbedtls_mpi_uint *X,
size_t X_limbs,
const mbedtls_mpi_uint *Y,
size_t Y_limbs,
unsigned char assign )
{
if( X_limbs < Y_limbs )
return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
if( X != NULL && Y != NULL )
{
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
mbedtls_mpi_uint limb_mask = mbedtls_ct_mpi_uint_mask( assign );
mbedtls_ct_mpi_uint_cond_assign( X_limbs, X, Y, assign );
for( size_t i = Y_limbs; i < X_limbs; i++ )
X[i] &= ~limb_mask;
return( 0 );
}
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
}
int mbedtls_mpi_core_cond_swap( mbedtls_mpi_uint *X,
size_t X_limbs,
mbedtls_mpi_uint *Y,
size_t Y_limbs,
unsigned char swap )
{
if( X == Y )
return( 0 );
if( X_limbs != Y_limbs )
return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
if( X != NULL && Y != NULL )
{
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
mbedtls_mpi_uint limb_mask = mbedtls_ct_mpi_uint_mask( swap );
for( size_t i = 0; i < X_limbs; i++ )
{
mbedtls_mpi_uint tmp = X[i];
X[i] = ( X[i] & ~limb_mask ) | ( Y[i] & limb_mask );
Y[i] = ( Y[i] & ~limb_mask ) | ( tmp & limb_mask );
}
return( 0 );
}
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
}
int mbedtls_mpi_core_read_le( mbedtls_mpi_uint *X,
size_t X_limbs,
const unsigned char *input,

64
library/bignum_core.h
View File

@ -74,6 +74,70 @@ size_t mbedtls_mpi_core_bitlen( const mbedtls_mpi_uint *A, size_t A_limbs );
void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *A,
size_t A_limbs );
/**
* \brief Perform a safe conditional copy of MPI which doesn't reveal whether
* the condition was true or not.
*
* \param[OUT] X The address of the first MPI. This must be initialized.
* \param X_limbs The number of limbs of \p X.
* \param[IN] Y The address of the second MPI. This must be initialized.
* \param Y_limbs The number of limbs of \p Y.
* \param assign The condition deciding whether to perform the
* assignment or not. Must be either 0 or 1:
* * \c 1: Perform the assignment `X = Y`.
* * \c 0: Keep the original value of \p X.
*
* \note This function avoids leaking any information about whether
* the assignment was done or not.
*
* \warning If \p assign is neither 0 nor 1, the result of this function
* is indeterminate, and the resulting value in \p X might be
* neither its original value nor the value in \p Y.
*
* \return \c 0 if successful.
* \return #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if \p X isn't
* large enough to hold the value in \p Y.
* \return #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if \p X or \p Y is invalid.
*/
int mbedtls_mpi_core_cond_assign( mbedtls_mpi_uint *X,
size_t X_limbs,
const mbedtls_mpi_uint *Y,
size_t Y_limbs,
unsigned char assign );
/**
* \brief Perform a safe conditional copy of MPI which doesn't reveal whether
* the condition was true or not.
*
* \param[IN,OUT] X The address of the first MPI.
* This must be initialized.
* \param X_limbs The number of limbs of \p X.
* \param[IN,OUT] Y The address of the second MPI.
* This must be initialized.
* \param Y_limbs The number of limbs of \p Y.
* \param swap The condition deciding whether to perform
* the swap or not. Must be either 0 or 1:
* * \c 1: Swap the values of \p X and \p Y.
* * \c 0: Keep the original values of \p X and \p Y.
*
* \note This function avoids leaking any information about whether
* the swap was done or not.
*
* \warning If \p swap is neither 0 nor 1, the result of this function
* is indeterminate, and both \p X and \p Y might end up with
* values different to either of the original ones.
*
* \return \c 0 if successful.
* \return #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if the size of
* \p X and \p Y is differ.
* \return #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if \p X or \p Y is invalid.
*/
int mbedtls_mpi_core_cond_swap( mbedtls_mpi_uint *X,
size_t X_limbs,
mbedtls_mpi_uint *Y,
size_t Y_limbs,
unsigned char swap );
/** Import X from unsigned binary data, little-endian.
*
* The MPI needs to have enough limbs to store the full value (including any