How acme.sh deals with the private key file modes?

A short answer is: we deal with the file permission as little as possible.

A longer answer is: we almost don't change the file permissions, you set it yourself, and it will be kept forever.

Ok, let me give a longer answer:

1. By default, the key/cert files are saved in ~/.acme.sh, this folder is set to mode 700 by default. So, nobody else can read your private key.
2. When you use --install-cert command to copy the cert to the target locations, we use cat keyfile > target_key_file pattern, in which the target file permission is not changed, and you only need write permission to the target file. Yes, if the target file doesn't exist for the first time, it will be created with your default umask, which is umask 022 in most of the unix/linux systems. In this case, somebody else may read your private key file. But you can change the file mode manually, chmod 600 target_key_file. The reasons why we don't change the file mode are:
   1. We respect the users choice most. We trust you more than ourselves. you can change the file modes manually, you know your system best. We respect your choice.
   2. We can not use umask also. In webroot mode, we have to create validation file, if umask is set to 022, the webserver would not be able to read the validation file.

So, doing more doesn't mean doing better. Less is more.