🔐 Cryptography Decoder (MATLAB)

This repository contains a cryptographic decoding project written in MATLAB. It was developed as part of the CMOR220 course at Rice University during Spring 2024.

📜 Project Overview

The goal of this project is to decrypt an encoded text using letter frequency analysis and probabilistic modeling. The decoder uses a transition matrix of letter probabilities and iteratively searches for the most likely decoding key by maximizing a log-likelihood function.

📂 Files Included

• Project3.m — Main script with all decoding functions.
• encodedtext.txt — File containing the encrypted message.
• letterprob.mat — Matrix of letter pair probabilities used for scoring.

⚙️ Features

• Converts text to numerical ASCII values for processing.
• Implements a stochastic optimization technique (random swaps) to improve the decryption key.
• Uses a transition matrix to evaluate the likelihood of letter pairings.
• Decodes and cleans output by mapping back to readable text.

💡 Key Concepts

• ASCII encoding and decoding
• Markov transition matrices
• Log-likelihood scoring
• Monte Carlo-style optimization
• Basic cryptanalysis techniques

🧪 How to Run

Make sure all files (Project3.m, encodedtext.txt, and letterprob.mat) are in the same directory. Then run in MATLAB:

Project3