Definition
Open Design
Open design is the security principle that the security of a system should not depend on the secrecy of its protection mechanisms.
Rationale
No security through obscurity
Security should depend only on possession of secrets (e.g. passwords, keys), not on the attacker being ignorant of the system’s design. Securing a door does not rely on an attacker’s ignorance of how to operate a door, but on possession of the key and the security of the lock mechanism.
This principle is also known as Kerckhoffs’ principle in cryptography. Modern cryptographic mechanisms are all developed with open design.