What is an indirect way to transmit information with no explicit reading of confidential information?

A.
Covert channels

B.
Backdoor

C.
Timing channels

D.
Overt channels

Explanation:
Covert channels: indirect ways for transmitting information with no explicit reading of
confidential information. This kind of difficulties induced some researchers to
re-think from scratch the whole problem of guaranteeing security in computer systems.