Add mpi_safe_cond_assign()
diff --git a/library/bignum.c b/library/bignum.c
index 55f7463..663d924 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -205,6 +205,31 @@
}
/*
+ * Conditionally assign X = Y, without leaking information
+ */
+int mpi_safe_cond_assign( mpi *X, 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 both MPIs have the same size */
+ if( X->n > Y->n )
+ MPI_CHK( mpi_grow( Y, X->n ) );
+ if( Y->n > X->n )
+ MPI_CHK( mpi_grow( X, Y->n ) );
+
+ /* Do the conditional assign safely */
+ for( i = 0; i < X->n; i++ )
+ X->p[i] = X->p[i] * (1 - assign) + Y->p[i] * assign;
+
+cleanup:
+ return( ret );
+}
+
+/*
* Set value from integer
*/
int mpi_lset( mpi *X, t_sint z )
