Symmetric-key encryption hides a message using one secret key. The sender transforms plaintext into ciphertext that looks like random noise; the recipient uses the same key to recover the plaintext. The shared secret must remain secret. section 1.2 section 4.1
Alice, Bob, and the nosy messenger
Alice wants to send gossip to Bob through dangerous territory without the messenger learning its contents. She encrypts the letter with their shared key, sends the ciphertext, and Bob decrypts it with that same key. The messenger can carry the strange-looking data but cannot read the original message. section 1.2
Keep these distinctions and cautions in view:
- Treat symmetric encryption as a cryptographic primitive: AES is a specific symmetric encryption algorithm, while symmetric encryption names the broader technique. section 1.9
- Distinguish it from asymmetric cryptography, which gives participants different views of the keys rather than one shared secret. section 1.5
- Use authenticated encryption rather than encryption alone for secure applications, because unauthenticated ciphertexts can be tampered with. section 4.7
- Plan how participants obtain the same unpredictable key; establishing shared secrets is often a major practical challenge. section 4.1
