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Add mpi_safe_cond_swap()

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This commit is contained in:
Manuel Pégourié-Gonnard
2013-12-04 21:41:50 +01:00
parent 97871ef236
commit a60fe8943d
4 changed files with 110 additions and 7 deletions
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47
library/bignum.c
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@@ -214,16 +214,16 @@ int mpi_safe_cond_assign( mpi *X, const mpi *Y, unsigned char assign )
int ret = 0;
size_t i;
if( assign * ( 1 - assign ) != 0 )
return( POLARSSL_ERR_MPI_BAD_INPUT_DATA );
/* make sure assign is 0 or 1 */
assign = ( assign != 0 );
if( Y->n > X->n )
MPI_CHK( mpi_grow( X, Y->n ) );
MPI_CHK( mpi_grow( X, Y->n ) );
/* Do the conditional assign safely */
X->s = X->s * (1 - assign) + Y->s * assign;
for( i = 0; i < Y->n; i++ )
X->p[i] = X->p[i] * (1 - assign) + Y->p[i] * assign;
for( ; i < X->n; i++ )
X->p[i] *= (1 - assign);
@@ -231,6 +231,43 @@ cleanup:
return( ret );
}
/*
* Conditionally swap X and Y, without leaking information
* about whether the swap was made or not.
* Here it is not ok to simply swap the pointers, which whould lead to
* different memory access patterns when X and Y are used afterwards.
*/
int mpi_safe_cond_swap( mpi *X, mpi *Y, unsigned char swap )
{
int ret, s;
size_t i;
t_uint tmp;
if( X == Y )
return( 0 );
/* make sure swap is 0 or 1 */
swap = ( swap != 0 );
MPI_CHK( mpi_grow( X, Y->n ) );
MPI_CHK( mpi_grow( Y, X->n ) );
s = X->s;
X->s = X->s * (1 - swap) + Y->s * swap;
Y->s = Y->s * (1 - swap) + s * swap;
for( i = 0; i < X->n; i++ )
{
tmp = X->p[i];
X->p[i] = X->p[i] * (1 - swap) + Y->p[i] * swap;
Y->p[i] = Y->p[i] * (1 - swap) + tmp * swap;
}
cleanup:
return( ret );
}
/*
* Set value from integer
*/