การเข้ารหัส (encryption) คืออะไร และใช้งานอย่างไร?

SYMMETRIC → ONE shared key encrypts AND decrypts (e.g. AES):
  ✓ fast, efficient → for bulk data (encrypting files, databases)
  ✗ both parties need the same key (key distribution is the challenge)
ASYMMETRIC → a KEY PAIR: PUBLIC key (encrypt / verify) + PRIVATE key (decrypt / sign):
  → encrypt with the public key, only the private key decrypts (e.g. RSA, ECC)
  ✓ solves key distribution (share the public key freely); enables signatures
  ✗ slower → used for key exchange, signatures, not bulk data
→ Often COMBINED: asymmetric to exchange a symmetric key, then symmetric for the data
  (e.g. how TLS works).