Allow to protect the opening of the integrity data map and
journal through a keyfile. For setting the key file two new
optional type attributes were added:
* integrity_keyfile
* integrity_metadata_key_description
The key file format must be correct according to the selected
integrity algorithm. As of now the kiwi default hmac-sha256
algorithm is used with the selected keyfile
The optional integrity_metadata_key_description attribute
allows to specify a custom description of an integrity key
as it is expected to be present in the kernel keyring. The
information is placed in the integrity metadata block. If
not specified kiwi creates a key argument string instead
which is based on the given integrity_keyfile filename.
The format of this key argument is:
:BASENAME_OF_integrity_keyfile_WITHOUT_FILE_EXTENSION
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