Symmetric encryption is a method where the same secret key is used to encrypt and decrypt data. Common algorithms include AES and ChaCha20, and both sides must hold the key before they can communicate.
Why it matters
Symmetric ciphers are much faster than public-key methods, so they do the bulk work of protecting files, disks, databases and HTTPS traffic. The hard part is key distribution: both parties need the same key without an attacker learning it. Real systems solve this by using asymmetric cryptography to agree on a short-lived symmetric key, then encrypting the actual data with it.
Example
During a TLS handshake the browser and server negotiate a session key. From then on, every page is encrypted with a symmetric cipher such as AES-256-GCM using that key, which is discarded when the session ends.
Common mistakes
- Reusing the same nonce or initialization vector with the same key, which can leak plaintext in modes like GCM.
- Sharing the key over email or chat, defeating its secrecy.
- Using an outdated cipher such as DES or RC4 because it still works.
Related
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