Keyword substitution

Why move every letter the same distance? A substitution cipher uses any shuffled alphabet as the key. To make it easy to remember, write a keyword first (dropping repeated letters), then the rest of the alphabet in order.

The keyword ZEBRAS gives:
ZEBRASCDFGHIJKLMNOPQTUVWXY
so A→Z, B→E, C→B, D→R …

There are 26! (about 4 × 10²⁶) ways to shuffle the alphabet. That is far too many to try one by one. Yet substitution ciphers were broken for centuries. Each plaintext letter always becomes the same cipher letter, so the letter counts survive. You will use that in World 3.