What is Codebusters?

• A Science Olympiad event about encoding and decoding secret messages
• Three-person teams compete against the clock — 30 minutes per test
• Questions range from simple decryption (key is given) to full cryptanalysis (no key!)
• Math-based ciphers (Cryptarithm) also appear
• We compete at Invitationals, Regionals, and hopefully State

Event Format

• Written test — paper and pencil, no computers
• Up to 3 teammates in the room, unlimited collaboration
• Tiebreaker: one team member solves a cipher solo, on the spot
• Scoring: points per correct plaintext letter plus bonus for full solves
• Strategy matters — divide and conquer between teammates

What We'll Learn This Year

• Monoalphabetic — Caesar, Atbash, Aristocrat, Patristocrat
• Polyalphabetic / Keyed — Porta, Nihilist, Homophonic
• Transposition — Complete Columnar
• Encoding / Steganographic — Baconian, Fractionated Morse, Xenocrypt, Checkerboard
• Math — Cryptarithm

How the Caesar Cipher Works

• Replace each plaintext letter with the letter N positions later in the alphabet
• Wrap around: Z + 1 = A
• The key is the shift amount (0–25)
• Classic Caesar used shift = 3

Encrypting with Caesar (shift 3)

Decrypting Caesar

• Reverse the shift — subtract N instead of adding
• In a Codebusters D-type problem the key (shift) is given to you

The Alphabet Strip Trick

• Write the alphabet twice in a row
• Slide the bottom row to align A with the shift letter
• Read off substitutions directly — no arithmetic needed

• Top row = plaintext  ·  Bottom row = ciphertext (shift 3)

Practice — Decrypt This

• Work through each letter: subtract 7 (wrap around at A)

Atbash Background

• One of the oldest ciphers — used in the Hebrew Bible (~600 BCE)
• Name from Hebrew: Aleph-Tav-Bet-Shin (first + last two letters)
• Simple rule: A↔Z, B↔Y, C↔X, … — reverse the alphabet
• Self-inverse: apply it twice and you're back to the original
• Always appears as Decrypt (D) — no key needed, just the mapping

The Atbash Mapping

• Top row = plaintext  ·  Bottom row = ciphertext
• Quick check: every plain + cipher letter pair sums to 27 (A=1 … Z=26)

Atbash Example

• To decrypt, apply the same mapping in reverse — it's identical.

Practice — Decrypt This Atbash Message

Key Takeaways

• Caesar: shift every letter by a fixed amount; key = shift number (D-type)
• Atbash: reverse the alphabet (A↔Z); no key — just apply the mirror (D-type)
• Both are monoalphabetic and both are always Decrypt — key or mapping is given
• Caesar has 25 possible keys; Atbash has exactly one — memorize it cold
• Next class: Aristocrat — full scrambled alphabet + cryptanalysis