Codebusters Practice Tests
Free, AI-generated Codebusters practice tests for Science Olympiad. Choose Invitational, Regional, State, or National difficulty, import past exam PDFs, and track your scores over time.
Codebusters runs in both Division B and Division C for the 2027 season.
Try some questions · No sign-in required
Codebusters Sample Practice Questions
1. Decode "FRPSXWHU" (Caesar shift of 3).
- COMPUTER
- CALCULATOR
- CONSUMER
- COMPILER
Show answer and explanation
Answer: COMPUTER
Shift each letter back by 3: F→C, R→O, P→M, S→P, X→U, W→T, H→E, U→R. Caesar shifts have only 25 keys, so trying shifts until words appear works quickly.
2. Atbash decodes "GSV" as:
- THE
- AND
- NOT
- FOR
Show answer and explanation
Answer: THE
Atbash mirrors the alphabet (A↔Z, B↔Y, ...): G→T, S→H, V→E. Applying Atbash twice returns the original, so it is its own inverse.
3. Using the Vigenère keyword KEY (A=0 convention), the plaintext "AT" encrypts to:
- KX
- OD
- KY
- LB
Show answer and explanation
Answer: KX
A(0)+K(10)=K and T(19)+E(4)=X mod 26. Under Science Olympiad rules the key advances only on letters, not spaces.
4. Which value of "a" is INVALID in an affine cipher x → (ax + b) mod 26?
- a = 1
- a = 3
- a = 2
- a = 5
Show answer and explanation
Answer: a = 2
"a" must be coprime with 26 so every letter maps to a unique letter and decryption exists. Even values share the factor 2 with 26 and fail; 1, 3, and 5 all work.
5. In a Spanish xenocrypt, the most frequent letters are typically:
- E, then A and O
- T, then A and O
- E, then T and A
- A, then E and I
Show answer and explanation
Answer: E, then A and O
Spanish letter frequency runs roughly E, A, O - different from English ETAOIN. Common Spanish words to look for: DE, LA, EL, QUE, EN.
Want the full timed version with scoring? Generate a free Codebusters practice test at Invitational, Regional, State, or National difficulty.
What Codebusters Tests Cover
cryptanalysis for the 2027 season: decoding messages encrypted with historical and modern ciphers, from Caesar shifts to RSA.
Event format 2027
- Both Division B and Division C
- Question types: timed question (bonus for first correct decode), questions worth varying points, and unlimited-time sections
- Decoders allowed: one binder or sheet per rules; calculators per rules
Monoalphabetic substitution ciphers
- Caesar (shift): each letter shifted by fixed amount; 25 possible keys, brute-forceable
- Atbash: A↔Z, B↔Y reversal (its own inverse)
- Affine: x → (ax + b) mod 26; 'a' must be coprime with 26; encrypt letter numbers A=0...Z=25
- Aristocrat: substitution with word spacing kept; Patristocrat: no spacing (harder)
- Keyword/mixed alphabet: keyword letters first, unused letters follow
- Decode with frequency analysis: English letter order approx ETAOIN SHRDLU; most common digraph TH; common short words A, I, THE, AND
Polyalphabetic and key ciphers
- Vigenère: keyword picks shifted alphabets via tabula recta; key advances only on plaintext LETTERS (spaces are skipped under SO rules); decryption = shift backward by key letter
- Letter 1 of plaintext uses letter 1 of keyword; keyword repeats when exhausted
- Vigenère defeats naive frequency analysis until key length is found (Kasiski examination)
Encoding systems
- Morse code: dots/dashes; E = single dot, T = single dash, SOS = ··· ··· ··· ; short words have short codes
- Binary: base 2; ASCII/letters mapped to bit strings; converting binary ↔ decimal
- Baconian: 5-bit groups (A = aaaaa, B = aaaba, ...); any two symbols work (font faces, positions)
- Pigpen/Masonic: symbols from grid shapes
- Xenocrypt: cipher in SPANISH - know Spanish letter frequencies (E, A, O most common) and common words (DE, LA, EL, QUE, EN)
Math and modern ciphers (state/national)
- Modular arithmetic: clock math; 27 mod 26 = 1
- Division algorithm and inverses mod 26 (e.g., 3×9 = 27 ≡ 1, so 9 is the inverse of 3)
- Hill cipher: matrix multiplication mod 26
- RSA: public key (n, e); encryption c = m^e mod n; security rests on difficulty of factoring n = p×q; d is the private exponent with e·d ≡ 1 mod φ(n)
- One-time pad: key as long as message, used once = unbreakable in principle
Common mistakes
- Letting the Vigenère key advance on spaces (SO convention: skip spaces)
- Using A=1 instead of A=0 in affine and Hill ciphers (SO uses A=0 convention)
- Forgetting the affine multiplier must be coprime to 26 (a = 2, 4, 6... are invalid)
- Transcribing Morse with wrong spacing or forgetting 5-bit grouping in Baconian
- Giving up on Aristocrats without writing out letter counts first