How to check if the ePub DRM file is protected?

Question

Is there any software way to safely check if the epub file is protected by secure DRM (encrypted)?

I found that in the META-INF directory there may be .xml and encryption.xml permissions. Is the existence of either or both of these files meaning that the content is encrypted, and if not, is there another way to check?
(To be clear, I don't want to read ePubs with DRM at all, I just want to check if they are encrypted or not.)

Answer 1

FWIW epub article on Wikipedia

If present, the DRMed EPUB files should contain a file named rights.xml in the META-INF directory at the root level of the ZIP container.

It is reported that

epubcheck from version 1.0 detects DRM protection through the existence of encryption.xml and reports an error. epub3 spec says

An additional encryption.xml file in the META-INF directory at the root level of the container file system contains all encryption information about the contents of the container. This file is an XML document whose root element is encryption. The encryption element contains child elements of the type EncryptedKey and EncryptedData as defined by [XML ENC Core]. Each EncryptedData element describes how one or more files in the container are encrypted. Therefore, if any resource in the container is encrypted, encryption.xml should be in order to indicate that the resource is encrypted and provides information on how it is encrypted.

EPUB3 is undefined with respect to .xml rights, its meanings and whether it is mandatory for DRM'd files, so it is best not to depend on it.