Ansible Tower credential store for "machine" (ssh) credentials

Hi,

I'm wondering how Ansible Tower stores the user's private key when adding a "machine" credential as a credential to use within Ansible Tower.

1. Does anybody know how Ansible Tower stores these keys in it's backend?
2. Is this a file or Database entry?
3. Where is it located?
4. Is it encrypted? If so, what type of encryption is used?
5. Where can I find more specific details on this?

I need to know this in order to be convinced whether it's safe to store precious private key's in Ansible Tower as "machine" credentials.

Thanks.
Kind regards.