BlackBox — Secure Dead Drop Communication System

A multi-component encrypted communication platform built for the ST5062CEM Programming and Algorithm 2 coursework.

What It Does

BlackBox is a secure dead drop system — users deposit encrypted messages that recipients can retrieve without sender and receiver ever communicating directly. Intelligence agencies use this technique to prevent interception.

Security Features

Feature	Implementation
Session Encryption	AES-256-CBC with HMAC-SHA256 (encrypt-then-MAC)
Key Exchange	Diffie-Hellman (RFC 3526 Group 14, 2048-bit prime) — custom implementation
Forward Secrecy	New DH keypair generated every connection
Identity	RSA-1024 keypair per user — custom Miller-Rabin prime generation
Replay Attack Prevention	Custom hash-table Nonce Registry (300s TTL)
DoS Prevention	Custom Token Bucket rate limiter (per-IP)
Tamper Detection	HMAC-SHA256 on every message
Audit Log	SHA-256 linked blockchain — tamper-evident
Data at Rest	All message bodies AES-encrypted before DB storage
Self-Destruct	Messages forensically wiped from DB after retrieval

Architecture

blackbox/
├── core/
│   ├── crypto_engine.py   # DH, AES, RSA, NonceRegistry, TokenBucket
│   └── database.py        # SQLite + AuditChain (blockchain log)
├── server/
│   └── server.py          # Multi-threaded TCP server
├── client/
│   └── client_lib.py      # Client library
├── tests/
│   └── test_blackbox.py   # 30+ unit + integration tests
├── blackbox_gui.py         # Main GUI (Tkinter)
└── requirements.txt

Setup

pip install cryptography pytest

Running

# Launch the full GUI (starts server internally)
python blackbox_gui.py

GUI Tabs

1. CONNECT — Start server, register/login
2. DEAD DROP — Compose and deposit encrypted messages
3. INBOX — List and retrieve drops
4. SERVER INTEL — Live stats, user list, rate limiter status
5. AUDIT CHAIN — View and verify blockchain audit log

Running Tests

python -m pytest tests/test_blackbox.py -v

Tests cover:

• DH key exchange (shared secret agreement, forward secrecy)
• AES encryption/decryption, tamper detection, wrong-key rejection
• RSA key generation, encrypt/decrypt, sign/verify
• Miller-Rabin primality test
• Nonce Registry replay attack detection (including thread-safety)
• Token Bucket rate limiting and refill
• Envelope (full crypto pipeline) round-trip
• Audit chain integrity and tamper detection
• Database CRUD + self-destruct behaviour
• Full client-server integration (register → login → drop → retrieve)

Custom Data Structures

• NonceRegistry: Custom polynomial hash table with chaining (not Python dict)
• TokenBucket: Custom per-IP rate limiter with time-based refill
• AuditChain: Custom linked-list blockchain with SHA-256 block hashing
• DiffieHellman: Full DH implementation over 2048-bit RFC 3526 prime
• RSAKeyPair: Custom Miller-Rabin prime generation, modular inverse via Extended Euclidean Algorithm

GitHub Branching Strategy

main
├── feature/crypto-engine
├── feature/server
├── feature/client
├── feature/gui
├── feature/database
└── feature/tests

References

• RFC 3526: More Modular Exponential (MODP) Diffie-Hellman groups
• NIST SP 800-38A: AES-CBC mode specification
• HMAC: RFC 2104
• Miller-Rabin: Probabilistic primality testing algorithm
• DoD 5220.22-M: Data sanitisation standard (forensic erasure)