what is a covert channel?

In Trojan terminology, what is a covert channel?

A.
A channel that transfers information within a computer system or network in a way that violates the security policy

B.
A legitimate communication path within a computer system or network for transfer of data

C.
It is a kernel operation that hides boot processes and services to mask detection

D.
It is Reverse tunneling technique that uses HTTPS protocol instead of HTTP protocol to establish connections