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.
Also known as: Shannon's maxim, the enemy knows the system