In cryptography, a key is a secret value that controls how data is encrypted and decrypted. The same algorithm can produce very different results depending on the key, which is why the algorithm alone does not protect information. Without the correct key, ciphertext should remain unreadable.
Keys matter because they are often the real security boundary. Attackers may steal keys from endpoints, memory, backups, or poor access controls instead of breaking the cipher itself. Defenders protect keys with strong storage, access control, rotation, revocation, and hardware-backed systems such as HSMs or secure enclaves. In real-world systems, a leaked key can expose encrypted files, sessions, or messages even when the cryptography is sound.