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Security Fundamentals & Crypto · Intermediate

Kerckhoffs's Principle

A cryptosystem should remain secure even if everything about it except the key is public knowledge.

Definition

Formulated by Auguste Kerckhoffs in 1883, this principle underlies modern cryptographic practice of using open, peer-reviewed algorithms. Security must rest on secret keys, not secret algorithms. Claude Shannon restated it as 'the enemy knows the system.'

Example

AES's design is fully published, yet data encrypted with it remains secure as long as the key is secret.

Exam tip

Proprietary 'secret' algorithms violate Kerckhoffs's principle and are a red flag on exams.

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