@enuddleyarbl: Please note that the key for font encryption is very often the dc:identifier found in content.opf and anything that modifies it will disable font decryption. And yes. a calibre conversion can and will break the ability to decrypt fonts. One of the reasons I prefer to use KindleUnpack to generate epubs from a KF8/azw3 file.
See
Font obfuscation in EPUBs and why you shouldn't change the dc:identifier after exporting from InDesign and/or
Font Embedding for Open Container Format Files for more information.
Samples from a KindleUnpack and a calibre converted epub:
Kindle Unpack (note the epub v3.0 version):
Code:
<?xml version="1.0" encoding="utf-8"?>
<package version="3.0" unique-identifier="uid" prefix="calibre: https://calibre-ebook.com" xmlns="http://www.idpf.org/2007/opf">
<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">
<dc:identifier>uuid:53f2730e-fc4f-43ef-9be0-1f8dcd019e01</dc:identifier>
<dc:identifier id="uid">1607628343</dc:identifier>
calibre converted (note the epub v2.0 version):
Code:
<?xml version="1.0" encoding="utf-8"?>
<package version="2.0" unique-identifier="uuid_id" xmlns="http://www.idpf.org/2007/opf">
<metadata xmlns:calibre="http://calibre.kovidgoyal.net/2009/metadata" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:opf="http://www.idpf.org/2007/opf" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<dc:identifier id="uuid_id" opf:scheme="uuid">05ac5b14-e9b8-47ec-9ff6-96431094e93d</dc:identifier>