What is symmetric encryption, and what algorithms use it?
Quick Answer
Symmetric encryption uses the same secret key to both encrypt and decrypt data. It is fast and works well for large amounts of data. AES (Advanced Encryption Standard) is the most common symmetric algorithm today, and it is what TLS uses to encrypt your actual traffic after the handshake.
Detailed Answer
Symmetric encryption uses one shared key for both sides of the conversation.
Alice Bob
key: "K7x..." key: "K7x..." (same key)
plaintext --[encrypt]--> ciphertext --[decrypt]--> plaintext
Whoever holds the key can both lock (encrypt) and unlock (decrypt) the data. This makes symmetric encryption simple and, importantly, fast — it needs much less computing power than asymmetric encryption.
Common symmetric algorithms:
- AES (Advanced Encryption Standard) — the industry standard today. Usually seen as AES-128 or AES-256, where the number is the key size in bits. Modern TLS uses AES in GCM mode, which adds a built-in integrity check.
- ChaCha20 — a newer algorithm, often paired with Poly1305 for integrity. It is popular on mobile devices because it is fast even without special hardware support.
- 3DES / DES — older algorithms, now considered weak or outdated. Modern TLS does not use them.
The one big problem symmetric encryption does not solve: how do two strangers agree on a shared key over an open, public network without anyone else seeing it? That problem is what asymmetric encryption helps solve, which the next question covers.