I am trying to store an RSA private key in Jenkins, and reference it using the Credentials Binding plugin.
I was able to upload the RSA private key as a Secret File, bound to a "domain".
(The "domain" is just some arbitrary text label).
This is the only way it appeared in the dropdown list in my project when I selected 'Use secret text(s) or file(s), then under Bindings select 'Secret text', and then it shows up in the dropdown list.
So then I selected it, bound it to an env var, and attempted to use it in my project.
I got this error:

+ rsync -auvz -e 'ssh -i /var/lib/jenkins/secretFiles/74ec48f8-ead9-4545-99ac-9a8c351cf19d/blah.id_rsa -p 12345' test_file someone@somewhere.net:/home/someuser/test_dir
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@ WARNING: UNPROTECTED PRIVATE KEY FILE! @
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Permissions 0644 for '/var/lib/jenkins/secretFiles/74ec48f8-ead9-4545-99ac-9a8c351cf19d/blah.id_rsa' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
bad permissions: ignore key: /var/lib/jenkins/secretFiles/74ec48f8-ead9-4545-99ac-9a8c351cf19d/blah.id_rsa