Add AES encryption implementation with CLI support

[Tarini-Ag]

Fixes #6

This PR implements AES-256 encryption/decryption functionality as requested in issue #6.

What's Included:

Core Encryption Functions (✓ Required)

• ✅ Secure 256-bit key generation using cryptographically secure random
• ✅ Proper padding/unpadding functions using PKCS7
• ✅ encrypt(text, key) function for text encryption
• ✅ decrypt(ciphertext, key, iv) function for text decryption
• ✅ Test block with comprehensive example usage

Stretch Tasks (✓ Completed)

• ✅ File-based encryption/decryption for .txt files
  • encrypt_file() and decrypt_file() functions
  • Support for any file type
• ✅ CLI-based interface using argparse
  • Text encryption mode
  • File encryption mode
  • File decryption mode
  • Optional key input via command line

Features:

• Uses AES-256 in CBC mode
• Proper initialization vector (IV) generation and handling
• Base64 encoding for easy key/IV/ciphertext storage
• Interactive test mode (runs when no CLI arguments provided)
• Comprehensive error handling
• Well-documented code with docstrings

Testing:

The script includes automatic testing when run without arguments, demonstrating:

• Key generation
• Text encryption/decryption
• File encryption/decryption
• Result verification

All required and stretch tasks have been completed successfully!