[KevinH]

Your fonts were obfuscated as a way to prevent their theft or use outside that epub. The "key" to deobfuscate the font files back to working order depends on the value of the identifier pointed to by the book id attribute in the opf tag (that is how they tie it to the epub) and on the declared obfuscation method as specified in the encryption.xml file.

So somehow your epub has either improperly specified the font obfuscation method in the encryption.xml file or someone or something has created a new different unique identifier in the content.opf or edited the old one which breaks the de-obfuscation routine.

Both typically can happen when poor quality DeDRM software is used which plays with the book unique identifier to try to try to hide the fact this book was created by removing DRM.

Good quality DeDRM software knows enough to not change that book id, until all fonts have been deobfuscated and the encryption.xml file has been removed from the epub completely.