Why is type of `box_purpose` string in `c2pa` box?

[ytakio]

Describe the issue
C2PA spec defines c2pa box to contain some data which box_purpose indicates. But type of box_pupose is string.
Type of string means to be variable length and be terminated with null. It makes implementation to scan sequentially to parsing for safety. And alignment will be unpredictable.

Fixed bitwidth (like unsigned int(64)?) may be easier to handle, implement and be quick for SW/HW implementation.

For instance...

aligned(8) class ContentProvenanceBox extends FullBox(`'c2pa'`, version, 0) {
    if (version==0) {
        string box_purpose;
    } else if (version==1) {
        unsigned int(64) box_purpose;
    }
    bit(8) data[];
}

64 bit can present manifest and merkle (or merkle ?).

[lrosenthol]

I believe that the improved text around our BMFF handling should address this, correct @swenkeratmicrosoft ?

[swenkeratmicrosoft]

@lrosenthol - Yes, I think so. We're going to let the box (expected to be uuid, see https://github.com/c2pa-org/specs-core/pull/641) continue to use string for now. Once we start validating our design with Dave at Apple and the mpeg4 ISO group and thus create an actual 'c2pa' box, we'll make a final decision regarding whether the purpose should be a string, an integer, a flag in the FullBox 'flags' field, or something else entirely for the new box at that time.