Codebusters 🔐

Overview

Codebusters is a competitive event focusing on cryptography and code-breaking. Participants demonstrate their skills in encoding and decoding information using various cipher techniques, from historical classical ciphers to more modern cryptographic methods. The event tests speed, accuracy, and knowledge of cryptographic systems.

Rules & Format

Event Format

Codebusters typically consists of several cryptographic challenges:

1. Encoding Tasks: Convert plaintext messages into ciphertext using specified techniques
2. Decoding Tasks: Convert ciphertext messages into plaintext using specified techniques
3. Cryptanalysis: Decode messages without being told what cipher was used
4. Aristocrat and Patristocrat: Special types of substitution ciphers with specific solving techniques
5. Speed Round: Timed decryption of a message for bonus points

Materials Allowed

According to the Science Olympiad rules, participants may bring:

• One 8.5" × 11" sheet of paper with information on both sides per team member
• Non-graphing calculators (one per team member)
• Writing utensils

Content Guide

Codebusters covers a wide range of cryptographic techniques and analysis methods. Here's a breakdown of the major content areas:

Classical Ciphers

Caesar Cipher

The Caesar cipher is a substitution cipher where each letter in the plaintext is shifted a certain number of places down the alphabet:

Where:

• C = Ciphertext letter (0-25)
• P = Plaintext letter (0-25)
• K = Key (shift value)

Atbash Cipher

The Atbash cipher is a substitution cipher that maps each letter to its reverse in the alphabet:

Where:

• A becomes Z, B becomes Y, etc.
• No key is needed

Affine Cipher

The Affine cipher uses mathematical functions for encryption:

Where:

• a and b are the keys
• a must be coprime with 26
• Decryption: P = a^-1(C - b) mod 26

Vigenère Cipher

The Vigenère cipher uses a keyword to determine multiple shift values:

Where:

• K is the keyword converted to numbers (A=0, B=1, etc.)
• m is the length of the keyword
• i is the position in the message

Hill Cipher

The Hill cipher uses matrix multiplication for encryption:

Where:

• K is the key matrix
• P is the plaintext vector
• C is the ciphertext vector

Modern Techniques

Frequency Analysis

Letter Frequency in English

Understanding the frequency distribution of letters in English text is crucial for breaking substitution ciphers:

• Most Common Letters: E (12.7%), T (9.1%), A (8.2%), O (7.5%), I (7.0%)
• Least Common Letters: Z (0.1%), Q (0.1%), X (0.2%), J (0.2%), K (0.8%)

Common Digraphs and Trigraphs

Frequent letter pairs and triplets can provide valuable clues:

• Common Digraphs: TH, HE, IN, ER, AN, RE, ED, ON, ES, ST, EN, AT, TO
• Common Trigraphs: THE, AND, THA, ENT, ION, TIO, FOR, NDE, HAS, NCE, EDT, TIS

Solving Aristocrats

Aristocrats are monoalphabetic substitution ciphers with word divisions preserved:

• Identify Short Words: Focus on one, two, and three-letter words (A, I, AN, THE, AND)
• Pattern Recognition: Look for repeated letter patterns
• Double Letters: Common doubles include SS, EE, TT, FF, LL, MM, OO
• Word Endings: Common endings include -ING, -ED, -LY, -TION

Solving Patristocrats

Patristocrats are monoalphabetic substitution ciphers without word divisions:

• Pure Frequency Analysis: Compare letter frequencies with English language norms
• Digraph/Trigraph Analysis: Identify common pairs and triplets
• Word Division Reconstruction: After some substitutions, try to identify word boundaries

Practice Tips

Preparation Strategies

Practice Resources