Encrypted TimeMachine external backup disk

[a_me]

I don't know how Time Machine backup are encrypted. If it's FileVault2 like you say you check https://github.com/libyal/libfvde. It provides an open source implementation for decrypting FileVault2. You will have to script something as it is not built to crack password only decrypt and mount.

Thank you so much, this looks like a promising start. 
Many greetings, Andreas