A bank B allows its customers to withdraw cash from its network of automated teller machines $($ATMs$).$ The ATMs operate in the following way. The bank asks each customer C to select a secret number called a pin and denoted by Pin$($C$).$ Then the bank issues each customer C a special magnetized card that contains the following two pieces of information $($on separate portions of the magnetized strip on the card$)$:

$$ The customer$$s account number at the bank $($call it Account$($C$)).$

$$ EB$($Pin$($C$))$ where EB refers to encryption with the bank$$s secret key in a symmetric

cryptosystem.