How does encryption keep data secure?

Encryption scrambles data into an unreadable form (ciphertext) using mathematical algorithms and a secret key, so only someone with the correct key can unscramble it. In practice, a cryptographic key (a string of bits) is used in an algorithm to transform plaintext into ciphertext. Without the key, the ciphertext appears random and conveys no information. For example, with symmetric encryption both parties share a secret key, while with public-key (asymmetric) crypto a public key encrypts and a private key decrypts. Intercepted encrypted data is useless without the key. Thus encryption ensures confidentiality: even if hackers capture the data, they cannot read it without the key.