Small, tested Python projects that explain security concepts through code.

flowchart LR
    A["User selects an operation"] --> B{"Encode or decode"}
    B -->|Encode| C["Apply substitution mapping"]
    B -->|Decode| D["Apply inverse mapping"]
    C --> E["Preserve case and punctuation"]
    D --> E
    E --> F["Display transformed message"]
    G["Unit tests"] --> C
    G --> D

A command-line monoalphabetic substitution cipher with reusable encode/decode functions, input validation, and automated tests.

The two CLI captures demonstrate both transformation directions while preserving capitalization, spaces, numbers, and punctuation.

Run it with:

python substitution-cipher/substitution_cipher.py
python -m unittest discover -s tests -v

Concepts demonstrated:

• Reversible character transformations
• Separation of reusable logic from the command-line interface
• Input validation and unit testing
• Why classical substitution is unsuitable for real security

This is an educational cryptography exercise, not secure encryption.

The test suite checks a known encoding, reversible round trips, preservation of case and punctuation, and rejection of invalid substitution keys.

Run the test suite with python -m unittest discover -s tests -v to verify the reversible transformation and input validation behavior.

Run the test suite with python -m unittest discover -s tests -v to verify the reversible transformation and input validation behavior.

Built by Ahmed Balde as part of a broader cybersecurity and Python engineering portfolio. See more work on GitHub.