Add mpi_safe_cond_assign()

include/polarssl/bignum.h

@@ -231,6 +231,25 @@ int mpi_copy( mpi *X, const mpi *Y );
  */
 void mpi_swap( mpi *X, mpi *Y );
 
+/**
+ * \brief          Safe conditional assignement X = Y if assign is 1
+ *
+ * \param X        MPI to conditionally assign to
+ * \param Y        Value to be assigned
+ * \param assign   1: perform the assignment, 0: leave X untouched
+ *
+ * \return         0 if successful,
+ *                 POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed,
+ *                 POLARSSL_ERR_MPI_BAD_INPUT_DATA if assing is not 0 or 1
+ *
+ * \note           This function is equivalent to
+ *                      if( assign ) mpi_copy( X, Y );
+ *                 except that it avoids leaking any information about whether
+ *                 the assignment was done or not (the above code may leak
+ *                 information through branch prediction analysis).
+ */
+int mpi_safe_cond_assign( mpi *X, mpi *Y, unsigned char assign );
+
 /**
  * \brief          Set value from integer
  *