Codebusters

Type of Event: Study Event

Competitors: 3 participants

Materials Required: No study guide allowed, scratch paper will be provided

Competition Checklist: 3 four-function calculators (scientific calculators are NOT allowed), 3 pencils + erasers

Exam Format: The exam consists of 15-20 questions stapled together; the pages CAN be separated and the questions answered in any order. There is a coversheet provided with tables (which you can see on practice tests). The timed question is an aristocrat, and a significant bonus is given depending on the time taken to solve it, up to ten minutes (raise your hand to call the proctor to check it). If it is solved after 10 minutes, points are given, but usually not very many. The timed question is relatively important, given the high number of points that can be achieved. Aim for a solve below 5 minutes.

Below are some resources to get started but once you learn the codes, improving comes only with practice.

The wiki gives a good overview of everything except the Morse ciphers (and I've never seen Dancing Men so ignore that):

• https://scioly.org/wiki/index.php/Codebusters

Morbit and Pollux

• https://www.dcode.fr/morbit-cipher

• https://toebes.com/codebusters/Samples/Solving_Pollux.htm

Toebes is useful; you can make your own ciphers to have additional practice.

• https://toebes.com/codebusters/

More resources/explanations:

• https://www.soinc.org/codebusters-c

• https://www.sciencenc.com/resources/high-school/codebusters/

• https://docs.google.com/presentation/d/1BiVZW4nVo4qN22MMutppn-FzFuZ_dNmQSwLnIL74L9g/edit#slide=id.g4272e164cf_0_46

• https://www.sciencenc.com/wp-content/uploads/2018-Codebusters-Coaches-Institute-Notes.pdf

Additional Tests:

North Carolina Regional

• Test 1: https://www.sciencenc.com/wp-content/uploads/NC-Regional-Test-1.pdf

• Answer Key 1: https://www.sciencenc.com/wp-content/uploads/NC-Regional-Test-1-Answer-Key.pdf

• Test 2: https://www.sciencenc.com/wp-content/uploads/NC-Regional-Test-2.pdf

• Answer Key 2: https://www.sciencenc.com/wp-content/uploads/NC-Regional-Test-2-Answer-Key.pdf

I usually use capital letters for ciphertext and lowercase for plaintext, just as a safeguard against confusion.

Atbash: Honestly, it's easier just to solve it as an aristocrat (after you fill in e = V or a = Z).

Caesar: Usually, I take a three-letter word and use the Vigenere table to go through the possibilities until it actually makes a real word. Like Atbash, it's usually faster to just solve it with aristocrat strategies, with the added benefit of being able to look up unknown letters if necessary (find whatever maps to "the," fill in letters, guess words).

Aristocrat: Another word I see a lot is "every" (the first and third letters are the same). Only way to improve is practice.

Patristocrat: Practice, practice, practice.

Affine: Tedious, but can be solved with aristocrat strategies as well (although they're usually short so sometimes it's harder to do that). To take 67 mod 26 on a 4-function calculator, do "67/26 = 2.577," subtract the integer portion and multiply by 26 to get ".577 * 26 = 15." Also, make sure you understand negatives (-3 mod 26 = -3+26 = 23, NOT 3 mod 26.)

Vigenere: Write the key a bunch of times above the ciphertext. Don't do it in order. If the key is SCIENCE, do all the S's first, then the C's, then the I's -- it's a LOT faster. Also, make sure you're doing the correct operation (encoding vs decoding).

Baconian: Often there will be words instead of a's and b's (look at the practice tests from 2020). Sometimes it will be alternating (A = a, B = b, C = a, D = b) or consonant/vowel. Words never start with two b's in a row. Also, scioly always uses the 24 letter alphabet (it's on the Baconian table).

Xenocrypt: Just have to practice. Two-letter words are helpful: "a la" is common, and words ending in "ión," like "educación" seem to be common.

Hill: It just takes a long time. I would advise multiplying everything first, then adding them and taking them mod 26. Remember that "Z" is added at the end to make the plaintext length a multiple of 2 or 3, and also two mistakes are allowed without any penalty. so if you're in a time crunch, just leave the last two blank.

RSA: Kind of complicated. Use the extended Euclidean algorithm to find the inverse of d mod (p-1)(q-1). I would look up a different source (the wiki isn't great).

Since being able to solve the ciphers is mostly dependent on how much you've practiced, the strategy largely depends on how experienced the team is.

Strategy: At the beginning, 2 people work on the timed question while the last person works on a different question.

• Experienced teams: Divide up different ciphers, with at least one person responsible for each. Patristocrats and xenocrypts can be very point-heavy; aim to complete these first. If stuck on a word, ask someone else: they often see words you don't see and you don't want to waste time. Also, mono-alphabetic substitution ciphers with lower points such as Caesar, Atbash, and even Affine can often be solved like aristocrats (it's faster than consulting the tables for each letter if you are experienced.) Don't waste time filling out the letter charts but make sure if you have T = a, fill in every T in the ciphertext, so you don't miss words.

  • Aim to solve the timed question in under 5 minutes since a bunch of bonus points are added

• Newer competitors: It might be difficult to solve questions with higher points (patristocrats especially) without a decent amount of experience. There's no guarantee that you can solve aristocrats, so unless you're reasonably sure you can get it, don't waste your time. Aim to get surefire points from questions such as Hill, Vigenere, Caesar, Atbash, Affine, and so on (and make sure you know how to solve these with practice questions, since it can be a little confusing if you just read the descriptions). Filling out the letter charts can help.