Comment 4 for bug 1847890

The obfuscation key has to come from the package uid. Not just any
identifier with scheme UUID. In the attached EPUB the package identifier
is

<dc:identifier id="uid">3500794507</dc:identifier>

you can tell what it is by looking at the unique-identifier attribute on
the <package> tag. This identifier is not touched by calibre during
metadata operations. Random other identifiers are changed and cannot be
used for font obfuscation anyway. To fix you issue, move id="uid" into
the UUID identifier.

 status invalid