Add support for Xiaomi Body Composition Scale S400

[JPFrancoia]

Hi,

Thank you for maintaining openScale. I just bought the Xiaomi Body Composition Scale S400 and I noticed openScale didn't support it, so here we are.

I didn't reverse the bluetooth protocol myself, I leveraged the work done in this repo, so that sped things up.

This scale uses encryption in its bluetooth protocol, so the BLE key must be extracted. Here is how:

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1. Initial Scale Setup (if not done)

1. Install the Xiaomi Home app on your phone
2. Add your S400 scale as a new device
3. Complete at least one weighing to ensure the scale is registered with Xiaomi Cloud

2. Extract Tokens Using Xiaomi Cloud Tokens Extractor

# Clone the repository
git clone https://github.com/PiotrMachowski/Xiaomi-cloud-tokens-extractor.git
cd Xiaomi-cloud-tokens-extractor

# Install dependencies
pip install -r requirements.txt

# Run the extractor
python token_extractor.py

When prompted:

1. Enter your Xiaomi account email/username
2. Enter your password
3. Select your region (e.g., de for Germany, us for USA, cn for China)

3. Find Your Scale's Information

The extractor will output a list of all your Xiaomi devices. Look for your scale:

Device: Xiaomi Body Composition Scale S400
Model: xmtzc14hm (or similar)
MAC: AA:BB:CC:DD:EE:FF
BLE KEY: 0123456789abcdef0123456789abcdef

Save these two values:

• MAC: The Bluetooth address (format XX:XX:XX:XX:XX:XX)
• BLE KEY: A 32-character hexadecimal string

4. Kill the Xiaomi Home App

Kill the app or uninstall it.

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Once you have the key, this is pretty straightforward.

This is a measurement done with the Xiaomi Home App:

This is a measurement done with openScale (there is a difference because I did the measurement in the Xiaomi Home app the day after, while making this PR, to prove that it works):

Here are a few other screenshots:

To be clear: I built the app locally in debug mode, sideloaded it onto my phone and performed a real measurement.

I had to add a little bit of UI to add the BLE bind key in the setting. I couldn't find an existing example in openScale where this was done already, please let me know if I missed it.

Disclaimer: I did use AI to write most of the code. However I reviewed the code to the best of my abilities and I carefully tested the app. I compared all the metrics one by one with the official app to make sure that the values make sense in openScale.