Can metadata (like title, creator, ...) in package.opf be encrypted for DRMed epub or it's always available in clear? Or it depends on the drm technology used?
The EPUB Open Container Format specification ( http://www.idpf.org/epub/30/spec/epub30-ocf.html#sec-container-metainf-encryption.xml ) says:
The following files must never be encrypted, regardless of whether default or specific encryption is requested: [...] the EPUB rootfile (the Package Document)
(the Package Document is the OPF file). Hence the answer to your question is "never encrypted" if you just stick to the IDPF specification.