Definition
Encryption is the process of transforming data into a form that is unreadable without a secret key. A cipher is the algorithm; the key is the secret parameter that selects one specific transformation out of an astronomical family of possibilities. Anyone with the matching key can decrypt the data back to its original form; anyone without it sees only noise, even if they know exactly which cipher was used. That distinction — public algorithm, secret key — is Kerckhoffs's principle, and modern cryptography is built on it.
Encryption is the technical mechanism behind most everyday privacy and security: HTTPS, encrypted disks, end-to-end messaging, signed software updates, password storage. It also has a social dimension explored in surveillance studies and criminology: who can read whose data, and who decides, is a question of power and law as well as mathematics.
Why it matters
How it works
A symmetric cipher uses the same key to encrypt and decrypt. Modern symmetric ciphers like AES operate on fixed-size blocks of bytes, mixing the key into the data through many rounds of substitution and permutation until the output is statistically indistinguishable from random noise. They are extremely fast — gigabytes per second on commodity hardware — but the parties involved must somehow share the key first, which is the original problem in miniature.
Asymmetric ciphers solve that bootstrap problem. Each party generates a key pair: a public key that anyone may know, and a private key kept secret. Anything encrypted with the public key can only be decrypted with the matching private key. Strangers can exchange ciphertext on a fully public channel without ever sharing a secret. The price is speed — asymmetric algorithms like RSA and elliptic-curve schemes are orders of magnitude slower than AES — so real protocols use them only briefly: to authenticate identity, negotiate a fresh symmetric session key, then hand off bulk encryption to AES for the rest of the conversation. The wider social question — who holds the keys, who can compel disclosure, who watches the channel — sits on top of the math and shapes the politics of digital privacy.