CS 251 Ciphers - Breaking Classical Codes with C++
A cryptography tool for encryption, decryption, and heuristic key recovery.
CS Project • C++ • Solo Developer • 3 weeks • File I/O • Algorithms
Overview
For my CS 251 course at UIC, I built a C++ command-line tool that encrypts, decrypts, and breaks classical ciphers using heuristic search. The program supports Caesar and substitution ciphers, English-ness scoring, hill-climbing, dictionary-based scoring, and quadgram evaluation.
Context Problem
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Built for CS 251: Data Structures Algorithms.
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Goal: Encrypt, decrypt, and crack ciphers.
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Solve unknown substitution keys using heuristics.
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Work with real text files and datasets.
Process
Menu System
Designed a simple text-based interface allowing users to choose cipher type, encrypt/decrypt, load files, and run the substitution attack.
Caesar Cipher
Implemented clean helper functions for character shifting, alphabet wraparound, and brute forcing all keys when the shift is unknown.
Substitution Cipher + Hill Climbing
Built a heuristic solver using scoring functions, dictionary matching, quadgrams, random key generation, letter swapping, and hill-climbing optimization.
File I/O + Testing
Used file reading, writing, and unit testing to validate encryption, decryption, and hill climbing using large real-text datasets.
Challenges: What I Learned
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Balancing readability vs correctness.
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Debugging subtle scoring + indexing bugs.
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Managing large file I/O and long strings efficiently.
My Role
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Solo developer.
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Implemented Caesar + Substitution logic.
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Built hill-climbing key recovery.
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Added dictionary + quadgram scoring.
Outcome
Built a full cryptography tool capable of encrypting, decrypting, and cracking classical ciphers. Strengthened my understanding of heuristic search, C++ architecture, and multi-file projects.