Which of the following statements about cryptographic controls in the database subsystem is false?

a. If little or no sharing of data among users occurs, each user can protect their own data using a personal cryptographic key

b. Block encryption is more likely to be used to protect stored data than stream encryption

c. One user's data can be protected from another user by implementing a secure encryption device in the controller for the storage medium

d. If a hierarchy of keys is used to protect stored data, file keys are encrypted under secondary keys, which in turn are encrypted under a master key