Secure updating of integrated circuits

A method of attempting a write to an entity to cause performance of an action is provided in which a first message is sent to the entity which causes performance of the action and adjustment of initial values in respective security fields of the entity to respective first adjusted values, and a second message is sent to the entity which causes adjustment of the initial values to respective second adjusted values. The security fields have write restrictions which prevent values in the security fields being adjusted, in accordance with the first message, if the initial values have been adjusted in accordance with the second message, and vice versa. The action is only performed when the initial values have been adjusted in accordance with the first message. The respective first adjusted values are different than the respective second adjusted values.