Format scope
Version 2 of the container is an authenticated encrypted binary wrapper around a versioned JSON envelope, currently envelope format 4. Tabla Focus for iOS is the reference reader and writer; Focus Studio, the browser reader and editor at tablafocus.com/compositions/open, is a second implementation and ships the same interchange key. The format is therefore open: encryption provides integrity, a version gate, and a clear signal that the payload is not plain text — it is not a licensing or access-control mechanism.
Interoperability considerations
The format is intended for interchange between Tabla Focus installations. Readers must verify the magic bytes, enforce a size limit, and check the envelope's minimum-reader field before processing a file — a file requiring a newer reader must be refused rather than partially interpreted. Files written before the encrypted container exist as plain JSON and remain readable; that legacy path is unauthenticated, so its contents are sanitised on import and any personal state it carries is discarded.
Security considerations
The format contains user-authored musical notation and annotations. It contains no executable content. Readers must authenticate the complete container before decoding, reject authentication failures, impose input-size limits, and treat all imported data as untrusted. The interchange key is compiled into every client that reads the format, so encryption is not confidentiality against a determined reader and is not DRM. A file carries only notation the sender chose to share: personal collection state — mastery, favourites, tempo marks and practice progress — has no representation in the payload and therefore cannot leak through it.
Change control and contact
The change controller is Tabla Focus. Questions about the format can be sent to hello@tablafocus.com.