Kerckhoffs's principle
expressioncryptographysecurity
A cryptosystem must remain secure even if everything about it, except the key, is public knowledge.
Auguste Kerckhoffs stated it in 1883 for military ciphers, and Claude Shannon compressed it into 'the enemy knows the system'. It is the formal argument against security through obscurity: designs leak, insiders defect, hardware gets captured, and only the key can be cheaply replaced. Every open cipher competition, from AES to the post-quantum process, is Kerckhoffs's principle institutionalized.
Kerckhoffs's principle holds that a cryptographic system should remain secure even if everything about it except the key is public knowledge. Auguste Kerckhoffs stated it in 1883, for military ciphers, and it has survived every technology change since because the reasoning is about people rather than mathematics.
The argument is that secrets leak. Algorithms are reverse-engineered from the devices that implement them, disclosed by employees, or rediscovered independently, and a design whose security depends on staying hidden fails permanently the first time any of that happens. A key can be rotated in an afternoon; an algorithm cannot. So the sensible place to concentrate secrecy is the thing you can cheaply replace.
The practical consequence is public review, and the evidence is one-sided. Every widely used modern cipher was published, attacked for years, and adopted because the attacks failed; the proprietary alternatives that avoided scrutiny have a long history of falling quickly once examined. Security through obscurity fails as a foundation, though it remains reasonable as a thin extra layer over something that would hold anyway.
Also known as: Shannon's maxim, the enemy knows the system