W-MBus Decryptor (AES-128)

A C++ utility designed to decrypt raw Wireless M-Bus (W-MBus) telegrams using the AES-128 algorithm. I did take help of wmbusmeters code source and credits to them

Critical Note (OMS/WMBus Security Mode 5): As per section 9.3.5 of the W-MBus standard, Symmetric Encryption with Security Mode 5 uses AES-CBC with a persistent 128-bit key and a dynamic IV based on the Transport Layer's Access Number.

This tool processes internal, hardcoded W-MBus data to output the decrypted telegram.

W-MBus Telegram Structure

The W-MBus telegram is composed of unencrypted header fields followed by the encrypted payload.

Field Name	Description	Encryption Status
Header Fields	L, C, M, A, CI, and Transport Layer fields (e.g., Access Number)	Unencrypted
Encrypted Data	The data payload, containing application data and integrity check (CRC/MAC). Must be padded to a 16-byte block size for CBC mode.	AES-128 Encrypted

AES-128 Decryption Steps (CBC Mode - Security Mode 5)

This implementation uses AES-128 in Cipher Block Chaining (CBC) mode, as specified for W-MBus Security Mode 5.

1. Extract Header and Encrypted Data: Separate the unencrypted header from the encrypted payload.
2. Generate IV (Initialization Vector): A 128-bit IV is constructed dynamically, specifically based on the Access Number of the Transport Layer.
   • As currently implemented, the IV is: c5142785895070079d9d9d9d9d9d9d9d .
3. Decrypt (CBC Mode): The AES-128 CBC algorithm is applied to the Encrypted Data using the 128-bit Key and the derived IV.
   • Padding: After decryption, the padding (which depends on the application protocol) must be correctly stripped from the payload.
4. Construct Output: The final telegram is the concatenation of the original Unencrypted Header and the Decrypted Payload.

Build and Run Instructions

This project uses C++ (g++), the make utility, and the OpenSSL library for cryptographic operations.

Prerequisites

• C++ Compiler (g++ supporting C++17)
• make utility
• OpenSSL Development Libraries (required for libssl and libcrypto linking)

Building the Code

Use the provided Makefile to compile the project:

make

Running the decryptor

./wmbus_decrypt

Sample Input

AES-128 Key:
4255794d3dccfd46953146e701b7db68

Raw Encrypted Telegram:
a144c5142785895070078c20607a9d00902537ca231fa2da5889Be8df367
3ec136aeBfB80d4ce395Ba98f6B3844a115e4Be1B1c9f0a2d5ffBB92906aa388deaa 
82c929310e9e5c4c0922a784df89cf0ded833Be8da996eB5885409B6c9867978dea 
24001d68c603408d758a1e2B91c42eBad86a9B9d287880083BB0702850574d7B51
e9c209ed68e0374e9B01feBfd92B4cB9410fdeaf7fB526B742dc9a8d0682653

Actual Output

Constructed IV:
c5 14 27 85 89 50 70 07 9d 9d 9d 9d 9d 9d 9d 9d 

Decrypted payload (hex):
2f 2f 04 6d a4 30 3a 39 04 13 80 11 00 00 01 fd 17 00 42 6c ff ff 44 13 00 00 00 00 44 93 
3c 00 00 00 00 84 01 13 00 00 00 00 c4 01 13 00 00 00 00 84 02 13 12 00 00 00 c4 02 13 00 
00 00 00 84 03 13 ff ff ff ff c4 03 13 ff ff ff ff 84 04 13 ff ff ff ff c4 04 13 ff ff ff 
ff 84 05 13 ff ff ff ff c4 05 13 ff ff ff ff 84 06 13 ff ff ff ff c4 06 13 ff ff ff ff 84 
07 13 ff ff ff ff c4 07 13 ff ff ff ff 84 08 13 ff ff ff ff 2f 2f 2f 2f 

Decrypted telegram with header (Not parsed):
A144C5142785895070078C20607A9D0090252F2F046DA4303A3904138011000001FD
1700426CFFFF44130000000044933C0000000084011300000000C4011300000000840
21312000000C4021300000000840313FFFFFFFFC40313FFFFFFFF840413FFFFFFFFC40
413FFFFFFFF840513FFFFFFFFC40513FFFFFFFF840613FFFFFFFFC40613FFFFFFFF840
713FFFFFFFFC40713FFFFFFFF840813FFFFFFFF2F2F2F2F

Expected Raw telegram output

a144c5142785895070078c20607a9d00
90252f2f046da4303a39041380110000
01fd1700426cffff4413000000004493
3c0000000084011300000000c4011300
00000084021312000000c40213000000
00840313ffffffffc40313ffffffff84
0413ffffffffc40413ffffffff840513
ffffffffc40513ffffffff840613ffff
ffffc40613ffffffff840713ffffffff
c40713ffffffff840813ffffffff2f2f
2f2f

Vasanth Nayak