Separation of duties (SoD) is one of the key concepts of internal control and is the most difficult and sometimes the most costly control to achieve. SoD states that no single individual should have control over two or more phases of a transaction or operation, which makes deliberate fraud more difficult to perpetrate because it requires the collusion of two or more individuals or parties. 2009, Catherine Paquet, Implementing Cisco IOS Network Security (IINS), →ISBN: