Questions on HmacSecretExtension and spare key

[bibi21000]

Hi,
I am here to ask for your opinion and advice for file encryption.
I am developing a program to store my personal and confidential information in a digital safe.
I would then copy this file to a USB key and distribute it to my friends, so that I can retrieve them if needed (https://github.com/bibi21000/PyCoffer).
The biggest part of the work is done but i want to use FIDO2 Hmac secret keys to encrypt/decrypt my data.

The code is here : https://github.com/bibi21000/PyCoffer/blob/main/pycoffer/plugins/fido.py

I can encrypt/decrypt data using keys generated from FIDO2 key : https://github.com/bibi21000/PyCoffer/blob/57535707bf0a2f5ee84a71dcc91fda6922aa68af/tests/test_fido.py#L82

But i can't decrypt data using spare key :( https://github.com/bibi21000/PyCoffer/blob/57535707bf0a2f5ee84a71dcc91fda6922aa68af/tests/test_fido.py#L112

How can i make a 'spare key' for Hmac secret ?
TIA