From HandWiki

ODTTF is an embedded font file type used in Microsoft's XML Paper Specification (XPS) and Microsoft Office 2007 formats. The file type refers to an obfuscated subsetted font based on the fonts used in the original document. The files can be extracted from the doc by renaming the files to ZIP, so the archive can be browsed and extracted (e.g.: rename cv.docx -> cv.zip)

The Microsoft assigned MIME type is application/vnd.ms-package.obfuscated-opentype.

According to the source code of Okular (see function parseGUID() and method XpsFile::loadFontByName())[1], the first 32 bytes of the font file are encoded using the font file name (GUID string) and a list of 32 magic numbers.