Transmission alone is often not enough; encryption accomplishes privacy, identification, and integrity. But remember: encryption cannot compensate for human weakness (e.g. sharing passwords).

• A cipher / cryptosystem encrypts the plaintext; the result is ciphertext, which we decrypt to recover the plaintext.
• A key configures the cryptosystem. A symmetric-key cryptosystem uses the same key to encrypt and decrypt; a public-key cryptosystem uses a public key to encrypt and a private key to decrypt.