🔐 Caesar Cipher — Custom Alphabet (a-z / 0-9)

Text encryption, decryption, frequency analysis, and shift detection in Python.
The project reads text from .txt files, writes results to .txt files, uses a custom alphabet consisting of letters + digits, and leaves Polish diacritics / special characters unchanged.

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🎯 Goal

This project focuses on:

• encrypting text with a Caesar-style shift cipher,
• decrypting text using a selected shift,
• analyzing character frequencies for the custom alphabet,
• finding the most likely shift for an encrypted text using a reference text.

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🧠 Cipher rules

🔹 Custom alphabet

The cipher uses the following alphabet:

abcdefghijklmnopqrstuvwxyz0123456789

🔹 Supported characters

The program transforms:

• lowercase English letters a-z
• digits 0-9

🔹 Unchanged characters

The following characters remain unchanged:

• Polish letters: ą ć ę ł ń ó ś ź ż
• spaces
• punctuation
• line breaks
• all other characters outside the custom alphabet

🔹 Letter handling

Uppercase and lowercase letters are treated as the same during processing.
The output is saved in lowercase.

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⚙️ Project features

• file-based encryption and decryption,
• frequency analysis of letters and digits,
• shift detection based on frequency comparison,
• support for both:
  • English version of the script,
  • Polish version of the script.

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📂 Repository structure

.
├─ maineng.py
├─ mainpl.py
├─ README.md
├─ LICENSE
├─ .gitignore
├─ input.txt
├─ output.txt
├─ reference.txt
├─ encrypted.txt
├─ decrypted.txt
├─ analysis.txt
└─ ranking.txt

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▶️ Running the project

Run the English version:

python maineng.py

Run the Polish version:

python mainpl.py

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🧪 Program options

The program provides three main functions:

1. Encrypt / decrypt a file

• reads text from a .txt file,
• encrypts or decrypts it using a given shift,
• saves the result to another .txt file.

2. Character frequency analysis

• counts how many times each character from the custom alphabet appears,
• calculates the percentage share of each character,
• ignores spaces, punctuation, and other unsupported symbols.

3. Find shift and decrypt

• compares an encrypted text with a reference text,
• checks all possible shifts,
• selects the shift with the smallest frequency difference,
• saves the decrypted result and ranking to files.

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📄 Example files

Suggested example files for testing:

• input.txt — plain text to encrypt
• reference.txt — reference text for frequency comparison
• encrypted.txt — encrypted text
• decrypted.txt — recovered plaintext
• analysis.txt — frequency report
• ranking.txt — list of shifts sorted by similarity

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✨ Notes

• This project implements a Caesar / shift cipher, not a transposition cipher.
• Characters outside the custom alphabet are preserved exactly as they appear.
• The project is intended as an educational Python exercise for text processing, file handling, and basic cryptography concepts.

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🧑‍💻 Author

Created by Avuii