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mbedtls_mpi_cmp_mpi_ct: remove multiplications

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Multiplication is known to have measurable timing variations based on
the operands. For example it typically is much faster if one of the
operands is zero. Remove them from constant time code.
This commit is contained in:
Janos Follath 2019-10-11 10:43:40 +01:00
parent 3d826456f5
commit 4f6cf38016
1 changed files with 10 additions and 6 deletions
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16
library/bignum.c
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@ -1098,6 +1098,11 @@ static int ct_lt_mpi_uint( const mbedtls_mpi_uint x, const mbedtls_mpi_uint y )
return ret;
}
static int ct_bool_get_mask( unsigned int b )
{
return ~( b - 1 );
}
/*
* Compare signed values in constant time
*/
@ -1129,7 +1134,7 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
sign_X = X->s;
sign_Y = Y->s;
cond = ( ( sign_X ^ sign_Y ) >> ( sizeof( unsigned int ) * 8 - 1 ) );
*ret = cond * X->s;
*ret = ct_bool_get_mask( cond ) & X->s;
done = cond;
for( i = X->n; i > 0; i-- )
@ -1142,8 +1147,8 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
* }
*/
cond = ct_lt_mpi_uint( Y->p[i - 1], X->p[i - 1] );
*ret |= ( cond * ( 1 - done ) ) * X->s;
done |= cond * ( 1 - done );
*ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & X->s;
done |= cond & ( 1 - done );
/*
* if( ( X->p[i - 1] < Y->p[i - 1] ) && !done )
@ -1153,9 +1158,8 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
* }
*/
cond = ct_lt_mpi_uint( X->p[i - 1], Y->p[i - 1] );
*ret |= ( cond * ( 1 - done ) ) * -X->s;
done |= cond * ( 1 - done );
*ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & -X->s;
done |= cond & ( 1 - done );
}
return( 0 );