Joe is an analyst on a high profile client case that deals with sensitive information. Joe has colluded with another individual named Sam in the organization to exfiltrate information in exchange for money. However, due to the high security surrounding the client environment, Joe has been tapping his desk in morse code so that Sam can have access to the information and assist in exfiltration. This type of technique is most commonly referred to as a:

Obfuscation

Side Channel Attack

Key Exchange

Covert Channel Attack