Red/black concept
The red/black concept, named in comparison to a typewriter ribbon and sometimes called the red–black architecture or red/black engineering, refers to the careful segregation in cryptographic systems of signals that contain sensitive or classified plaintext information (red signals) from those that carry encrypted information, or ciphertext (black signals). Therefore, the red side is usually considered the internal side, and the black side the more public side, with often some sort of guard, firewall or data-diode between the two.