CodeBusters

North Carolina Science Olympiad

Coaches Clinic

2018

CodeBusters

• Rule summary
• Review of code types
• Additional details on new code types
• Coaching for success
• Test generation tools
• Demo
• Questions

CodeBusters - Rule Summary (but see official rules!)

Description:

• Teams will cryptanalyze (decode) encrypted messages using cryptanalysis techniques and show skill with advanced ciphers by encrypting or decrypting a message.
• Team of up to 3 students

What to bring:

• Writing utensils with erasers
• Up to three “simple” calculators: +, -, ×, ÷ (Not a scientific one!)
• Everything else will be provided, including scrap paper.

CodeBusters - Regional Code Types

• Atbash -- New this year [1]
• The alphabet is mapped to the reverse (A ↔ Z; B ↔ Y; … M ↔ N)
• Caesar cipher (shift cipher) [2]
• Mono-alphabetic substitution
• Random alphabet
• K1
• K2 -- New this year
• Like K1 -- keyword goes in the replacement
• Frequency counts encrypted letters in the K2.
• Decoding has an additional level of indirection. K2 letter is used to find the replacement letter that maps to the decoded letter (in the K2).
• (Contrast with a K1, where decoding is done directly by mapping the K1 letter to the replacement letter.)

CodeBusters - Regional Code Types

• Mono-alphabetic substitution - problem classes
• Aristocrat - contains spaces between words; may or may not have a hint. [3, 4]
• Aristocrat - contains spaces between words, but could have spelling and/or grammer[sic] errors; may or may not have a hint. [5, 6]
• Patristocrat - spaces between words are removed (historically represented in blocks of 5 letters); may or may not have a hint. [7, 8]
• Affine cipher (encryption only) [9]
• Vigenère Cipher (encryption/decryption only) [10]
• Baconian cipher [11]
• Xenocrypt - no more than one cryptogram can be in Spanish [12] (Expect one!)
• Mathematical cryptanalysis of the Hill Cipher [13]

CodeBusters - State and National Code Types

• All regional code types
• Note: Regional tests are intended to be easier than the state test.
• Running key cipher [14]
• Cryptanalysis of Viginère cipher [15]
• RSA cipher [16, 17]
• Hill cipher [18, 19]
• Xenocrypt - at least one cryptogram will be in Spanish [12]
• Mathematical cryptanalysis of the Affine cipher [20] NEW

CodeBusters - K1 vs. K2 in Mono-alphabetic Substitution

• Example: Message: Hello world Keyword/offset: keyword/0

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CodeBusters - RSA Tips

• RSA (state only) - New this year
• Math problem with logic.
• Learn the RSA “alphabet soup” https://en.wikipedia.org/wiki/RSA_(cryptosystem)
• p,q are generated and discarded
• Φ is calculated and kept private.
• Private key = (n,d)
• Public Key = (n,e)
• To encode with the recipient's public key: result = val ^ e mod n
• To decode using your own Private key: result = val ^ d mod n
• Keep track of which parts to transmit and to use
• Learn: https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm and https://en.wikipedia.org/wiki/Exponentiation_by_squaring
• The test generator has good explanations

CodeBusters - Coaching for Success

• Practice, practice, practice.
• Put three students on the team.
• Use a pencil.
• Divide and conquer ... Remove the staple, split the test up with team.
• Get really comfortable with “mod 26” and your calculator.
• Take advantage of the “up to two wrong is ok” rule.
• What works best for the team?
• spend time on one 500 point question or five 100 point questions?
• Practice, practice, practice...

CodeBusters - Test Generation Tools

• https://toebes.com/codebusters/index.html
• All Regional Tests and the State Test will be generated using this
• Great tool for coaches to generate their own practice tests
• Tool overview video: https://youtu.be/pcz_3ql8ebM
• If you think you find a bug or have suggestions for improvements, enter them at https://github.com/toebes/ciphers/issues

CodeBusters - Questions?

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CodeBusters - Additional Resources

• National event page
• https://www.soinc.org/codebusters-c
• NCSO event page
• https://www.sciencenc.com/resources/high-school/codebusters/
• Clinic binder document
• Your favorite search engine

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