Symmetric Encryption & Classical Ciphers

Cryptography has a small, precise vocabulary worth getting straight before anything else. Plaintext is the readable message. Ciphertext is what it looks like once transformed. A cipher is the general procedure used for that transformation -- Caesar's shift, for instance. A key is the secret value fed into the cipher that determines exactly how a given message gets scrambled, so the same cipher run with a different key produces a different, unrelated ciphertext. Encryption turns plaintext into ciphertext; decryption reverses it. Symmetric encryption, the subject of this room, uses the same key for both directions -- whoever can encrypt with it can also decrypt with it, which is why guarding that one key is the whole game.