What is the difference between a cryptographic algorithm and a cryptographic key?

The algorithm is the fixed procedure (recipe) that's publicly known; the key is the secret parameter that makes each instance unique.

Algorithm (Verfahren):

• The mathematical procedure or recipe
• Should be publicly known (Kerckhoffs' principle)
• Examples: AES, RSA, SHA-256

Key (Schlüssel):

• A specific parameter that customizes the algorithm
• Must be kept secret (in symmetric crypto) or private (in asymmetric crypto)
• The security depends entirely on the key, not on algorithm secrecy

Analogy: Think of a combination lock — the algorithm is the lock mechanism (publicly known how it works), the key is the specific combination (secret).

Kerckhoffs' principle (1883): "A cryptosystem should be secure even if everything about the system, except the key, is public knowledge." This is the opposite of "security through obscurity."

Go deeper:

• Kerckhoffs's principle — why the algorithm is public and only the key is secret.