Covert channels represent a security problem for information systems, since they permit illegal flows and sometimes leaks of classified data. Although numerous descriptions have been given at a concrete level, relatively few work has been carried out at a more abstract level. In this paper, we propose a definition for covert channels based on encoding and decoding binary messages with transducers, in a finite transition system, and we explain why this definition is different from a similar notion of so called iterated interference. We also show that covert channels can be detected using a restricted class of transducers.