🔐 W-MBus AES-128 Decryption – OMS Volume 2

📘 Overview

This project decrypts a Wireless M-Bus telegram using AES-128-CBC as per OMS Volume 2 (Security Mode 5). It extracts metadata, constructs the IV, decrypts the payload, and outputs human-readable data.

🧩 Telegram Structure

• Meter Address: 8 bytes (ID + version + device type)
• Access Number (ACC): 1 byte, repeated 8 times in IV
• Encrypted Payload: AES-128 encrypted data
• CI Field: Indicates encryption (e.g., 0x8C)

🔐 AES-128 Decryption Steps

1. Extract meter address and access number
2. Construct IV: IV = meterAddress + (ACC × 8)
3. Decrypt using AES-128-CBC with provided key
4. Validate: First two bytes should be 0x2F 0x2F
5. Parse TLV format for readings and status

🛠️ Build Instructions

Linux/macOS

sudo apt install libssl-dev g++
g++ main.cpp -o decrypt -lssl -lcrypto
./decrypt